Skip to content
Snippets Groups Projects
Select Git revision
  • 447f3ae7c193bf330419886d18e2da62fabab093
  • master default
  • petsc-uname
  • hdf5-uname
  • snakemake-7.21
  • unmerged-fixes
  • petsc-repro
  • base-for-issue-62196
  • v1.4.0
  • v1.4.0rc2
  • v1.4.0rc1
  • v1.3.0
  • v1.3.0rc2
  • v1.3.0rc1
  • v1.2.0
  • v1.2.0rc2
  • v1.2.0rc1
  • v1.1.0
  • v1.1.0rc2
  • v1.1.0rc1
  • bootstrap-20190815
  • v1.0.1
  • v1.0.0
  • v0.16.0
  • v0.15.0
  • v0.14.0
  • v0.13.0
27 results

guix.texi

Blame
  • Code owners
    Assign users and groups as approvers for specific file changes. Learn more.
    guix.texi 445.42 KiB
    \input texinfo
    @c -*-texinfo-*-
    
    @c %**start of header
    @setfilename guix.info
    @documentencoding UTF-8
    @settitle GNU Guix Reference Manual
    @c %**end of header
    
    @include version.texi
    
    @copying
    Copyright @copyright{} 2012, 2013, 2014, 2015, 2016 Ludovic Courtès@*
    Copyright @copyright{} 2013, 2014, 2016 Andreas Enge@*
    Copyright @copyright{} 2013 Nikita Karetnikov@*
    Copyright @copyright{} 2015, 2016 Mathieu Lirzin@*
    Copyright @copyright{} 2014 Pierre-Antoine Rault@*
    Copyright @copyright{} 2015 Taylan Ulrich Bayırlı/Kammer@*
    Copyright @copyright{} 2015, 2016 Leo Famulari@*
    Copyright @copyright{} 2016 Ben Woodcroft@*
    Copyright @copyright{} 2016 Chris Marusich@*
    Copyright @copyright{} 2016 Efraim Flashner
    
    Permission is granted to copy, distribute and/or modify this document
    under the terms of the GNU Free Documentation License, Version 1.3 or
    any later version published by the Free Software Foundation; with no
    Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
    copy of the license is included in the section entitled ``GNU Free
    Documentation License''.
    @end copying
    
    @dircategory System administration
    @direntry
    * Guix: (guix).       Manage installed software and system configuration.
    * guix package: (guix)Invoking guix package.  Installing, removing, and upgrading packages.
    * guix build: (guix)Invoking guix build.      Building packages.
    * guix gc: (guix)Invoking guix gc.            Reclaiming unused disk space.
    * guix pull: (guix)Invoking guix pull.        Update the list of available packages.
    * guix system: (guix)Invoking guix system.    Manage the operating system configuration.
    @end direntry
    
    @dircategory Software development
    @direntry
    * guix environment: (guix)Invoking guix environment. Building development environments with Guix.
    @end direntry
    
    @dircategory Emacs
    @direntry
    * Guix user interface: (guix)Emacs Interface. Package management from the comfort of Emacs.
    @end direntry
    
    
    @titlepage
    @title GNU Guix Reference Manual
    @subtitle Using the GNU Guix Functional Package Manager
    @author The GNU Guix Developers
    
    @page
    @vskip 0pt plus 1filll
    Edition @value{EDITION} @*
    @value{UPDATED} @*
    
    @insertcopying
    @end titlepage
    
    @contents
    
    @c *********************************************************************
    @node Top
    @top GNU Guix
    
    This document describes GNU Guix version @value{VERSION}, a functional
    package management tool written for the GNU system.
    
    @menu
    * Introduction::                What is Guix about?
    * Installation::                Installing Guix.
    * Package Management::          Package installation, upgrade, etc.
    * Emacs Interface::             Using Guix from Emacs.
    * Programming Interface::       Using Guix in Scheme.
    * Utilities::                   Package management commands.
    * GNU Distribution::            Software for your friendly GNU system.
    * Contributing::                Your help needed!
    
    * Acknowledgments::             Thanks!
    * GNU Free Documentation License::  The license of this manual.
    * Concept Index::               Concepts.
    * Programming Index::           Data types, functions, and variables.
    
    @detailmenu
     --- The Detailed Node Listing ---
    
    Installation
    
    * Binary Installation::         Getting Guix running in no time!
    * Requirements::                Software needed to build and run Guix.
    * Running the Test Suite::      Testing Guix.
    * Setting Up the Daemon::       Preparing the build daemon's environment.
    * Invoking guix-daemon::        Running the build daemon.
    * Application Setup::           Application-specific setup.
    
    Setting Up the Daemon
    
    * Build Environment Setup::     Preparing the isolated build environment.
    * Daemon Offload Setup::        Offloading builds to remote machines.
    
    Package Management
    
    * Features::                    How Guix will make your life brighter.
    * Invoking guix package::       Package installation, removal, etc.
    * Substitutes::                 Downloading pre-built binaries.
    * Packages with Multiple Outputs::  Single source package, multiple outputs.
    * Invoking guix gc::            Running the garbage collector.
    * Invoking guix pull::          Fetching the latest Guix and distribution.
    * Invoking guix archive::       Exporting and importing store files.
    
    Emacs Interface
    
    * Initial Setup: Emacs Initial Setup.	Preparing @file{~/.emacs}.
    * Package Management: Emacs Package Management.	Managing packages and generations.
    * Licenses: Emacs Licenses.		Interface for licenses of Guix packages.
    * Package Source Locations: Emacs Package Locations.	Interface for package location files.
    * Popup Interface: Emacs Popup Interface.	Magit-like interface for guix commands.
    * Prettify Mode: Emacs Prettify.	Abbreviating @file{/gnu/store/@dots{}} file names.
    * Build Log Mode: Emacs Build Log.	Highlighting Guix build logs.
    * Completions: Emacs Completions.	Completing @command{guix} shell command.
    * Development: Emacs Development.	Tools for Guix developers.
    * Hydra: Emacs Hydra.			Interface for Guix build farm.
    
    Programming Interface
    
    * Defining Packages::           Defining new packages.
    * Build Systems::               Specifying how packages are built.
    * The Store::                   Manipulating the package store.
    * Derivations::                 Low-level interface to package derivations.
    * The Store Monad::             Purely functional interface to the store.
    * G-Expressions::               Manipulating build expressions.
    
    Defining Packages
    
    * package Reference::           The package data type.
    * origin Reference::            The origin data type.
    
    Utilities
    
    * Invoking guix build::         Building packages from the command line.
    * Invoking guix edit::          Editing package definitions.
    * Invoking guix download::      Downloading a file and printing its hash.
    * Invoking guix hash::          Computing the cryptographic hash of a file.
    * Invoking guix import::        Importing package definitions.
    * Invoking guix refresh::       Updating package definitions.
    * Invoking guix lint::          Finding errors in package definitions.
    * Invoking guix size::          Profiling disk usage.
    * Invoking guix graph::         Visualizing the graph of packages.
    * Invoking guix environment::   Setting up development environments.
    * Invoking guix publish::       Sharing substitutes.
    * Invoking guix challenge::     Challenging substitute servers.
    * Invoking guix container::     Process isolation.
    
    Invoking @command{guix build}
    
    * Common Build Options::        Build options for most commands.
    * Package Transformation Options::    Creating variants of packages.
    * Additional Build Options::    Options specific to 'guix build'.
    
    GNU Distribution
    
    * System Installation::         Installing the whole operating system.
    * System Configuration::        Configuring the operating system.
    * Installing Debugging Files::  Feeding the debugger.
    * Security Updates::            Deploying security fixes quickly.
    * Package Modules::             Packages from the programmer's viewpoint.
    * Packaging Guidelines::        Growing the distribution.
    * Bootstrapping::               GNU/Linux built from scratch.
    * Porting::                     Targeting another platform or kernel.
    
    System Installation
    
    * Limitations::                 What you can expect.
    * Hardware Considerations::     Supported hardware.
    * USB Stick Installation::      Preparing the installation medium.
    * Preparing for Installation::  Networking, partitioning, etc.
    * Proceeding with the Installation::  The real thing.
    * Building the Installation Image::  How this comes to be.
    
    System Configuration
    
    * Using the Configuration System::  Customizing your GNU system.
    * operating-system Reference::  Detail of operating-system declarations.
    * File Systems::                Configuring file system mounts.
    * Mapped Devices::              Block device extra processing.
    * User Accounts::               Specifying user accounts.
    * Locales::                     Language and cultural convention settings.
    * Services::                    Specifying system services.
    * Setuid Programs::             Programs running with root privileges.
    * X.509 Certificates::          Authenticating HTTPS servers.
    * Name Service Switch::         Configuring libc's name service switch.
    * Initial RAM Disk::            Linux-Libre bootstrapping.
    * GRUB Configuration::          Configuring the boot loader.
    * Invoking guix system::        Instantiating a system configuration.
    * Running GuixSD in a VM::      How to run GuixSD in a virtual machine.
    * Defining Services::           Adding new service definitions.
    
    Services
    
    * Base Services::               Essential system services.
    * Scheduled Job Execution::     The mcron service.
    * Networking Services::         Network setup, SSH daemon, etc.
    * X Window::                    Graphical display.
    * Desktop Services::            D-Bus and desktop services.
    * Database Services::           SQL databases.
    * Mail Services::               IMAP, POP3, SMTP, and all that.
    * Web Services::                Web servers.
    * Various Services::            Other services.
    
    Defining Services
    
    * Service Composition::         The model for composing services.
    * Service Types and Services::  Types and services.
    * Service Reference::           API reference.
    * Shepherd Services::           A particular type of service.
    
    Packaging Guidelines
    
    * Software Freedom::            What may go into the distribution.
    * Package Naming::              What's in a name?
    * Version Numbers::             When the name is not enough.
    * Synopses and Descriptions::   Helping users find the right package.
    * Python Modules::              Taming the snake.
    * Perl Modules::                Little pearls.
    * Java Packages::               Coffee break.
    * Fonts::                       Fond of fonts.
    
    Contributing
    
    * Building from Git::           The latest and greatest.
    * Running Guix Before It Is Installed::  Hacker tricks.
    * The Perfect Setup::           The right tools.
    * Coding Style::                Hygiene of the contributor.
    * Submitting Patches::          Share your work.
    
    Coding Style
    
    * Programming Paradigm::        How to compose your elements.
    * Modules::                     Where to store your code?
    * Data Types and Pattern Matching::  Implementing data structures.
    * Formatting Code::             Writing conventions.
    
    @end detailmenu
    @end menu
    
    @c *********************************************************************
    @node Introduction
    @chapter Introduction
    
    @cindex purpose
    GNU Guix@footnote{``Guix'' is pronounced like ``geeks'', or ``ɡiːks''
    using the international phonetic alphabet (IPA).} is a package
    management tool for the GNU system.  Guix makes it easy for unprivileged
    users to install, upgrade, or remove packages, to roll back to a
    previous package set, to build packages from source, and generally
    assists with the creation and maintenance of software environments.
    
    @cindex user interfaces
    Guix provides a command-line package management interface
    (@pxref{Invoking guix package}), a set of command-line utilities
    (@pxref{Utilities}), a visual user interface in Emacs (@pxref{Emacs
    Interface}), as well as Scheme programming interfaces
    (@pxref{Programming Interface}).
    @cindex build daemon
    Its @dfn{build daemon} is responsible for building packages on behalf of
    users (@pxref{Setting Up the Daemon}) and for downloading pre-built
    binaries from authorized sources (@pxref{Substitutes}).
    
    @cindex extensibility of the distribution
    @cindex customization of packages
    Guix includes package definitions for many GNU and non-GNU packages, all
    of which @uref{https://www.gnu.org/philosophy/free-sw.html, respect the
    user's computing freedom}.  It is @emph{extensible}: users can write
    their own package definitions (@pxref{Defining Packages}) and make them
    available as independent package modules (@pxref{Package Modules}).  It
    is also @emph{customizable}: users can @emph{derive} specialized package
    definitions from existing ones, including from the command line
    (@pxref{Package Transformation Options}).
    
    @cindex Guix System Distribution
    @cindex GuixSD
    You can install GNU@tie{}Guix on top of an existing GNU/Linux system
    where it complements the available tools without interference
    (@pxref{Installation}), or you can use it as part of the standalone
    @dfn{Guix System Distribution} or GuixSD (@pxref{GNU Distribution}).
    With GNU@tie{}GuixSD, you @emph{declare} all aspects of the operating
    system configuration and Guix takes care of instantiating the
    configuration in a transactional, reproducible, and stateless fashion
    (@pxref{System Configuration}).
    
    @cindex functional package management
    Under the hood, Guix implements the @dfn{functional package management}
    discipline pioneered by Nix (@pxref{Acknowledgments}).
    In Guix, the package build and installation process is seen
    as a @emph{function}, in the mathematical sense.  That function takes inputs,
    such as build scripts, a compiler, and libraries, and
    returns an installed package.  As a pure function, its result depends
    solely on its inputs---for instance, it cannot refer to software or
    scripts that were not explicitly passed as inputs.  A build function
    always produces the same result when passed a given set of inputs.  It
    cannot alter the environment of the running system in
    any way; for instance, it cannot create, modify, or delete files outside
    of its build and installation directories.  This is achieved by running
    build processes in isolated environments (or @dfn{containers}), where only their
    explicit inputs are visible.
    
    @cindex store
    The result of package build functions is @dfn{cached} in the file
    system, in a special directory called @dfn{the store} (@pxref{The
    Store}).  Each package is installed in a directory of its own in the
    store---by default under @file{/gnu/store}.  The directory name contains
    a hash of all the inputs used to build that package; thus, changing an
    input yields a different directory name.
    
    This approach is the foundation for the salient features of Guix: support
    for transactional package upgrade and rollback, per-user installation, and
    garbage collection of packages (@pxref{Features}).
    
    
    @c *********************************************************************
    @node Installation
    @chapter Installation
    
    GNU Guix is available for download from its website at
    @url{http://www.gnu.org/software/guix/}.  This section describes the
    software requirements of Guix, as well as how to install it and get
    ready to use it.
    
    Note that this section is concerned with the installation of the package
    manager, which can be done on top of a running GNU/Linux system.  If,
    instead, you want to install the complete GNU operating system,
    @pxref{System Installation}.
    
    @cindex foreign distro
    When installed on a running GNU/Linux system---thereafter called a
    @dfn{foreign distro}---GNU@tie{}Guix complements the available tools
    without interference.  Its data lives exclusively in two directories,
    usually @file{/gnu/store} and @file{/var/guix}; other files on your
    system, such as @file{/etc}, are left untouched.
    
    @menu
    * Binary Installation::         Getting Guix running in no time!
    * Requirements::                Software needed to build and run Guix.
    * Running the Test Suite::      Testing Guix.
    * Setting Up the Daemon::       Preparing the build daemon's environment.
    * Invoking guix-daemon::        Running the build daemon.
    * Application Setup::           Application-specific setup.
    @end menu
    
    @node Binary Installation
    @section Binary Installation
    
    This section describes how to install Guix on an arbitrary system from a
    self-contained tarball providing binaries for Guix and for all its
    dependencies.  This is often quicker than installing from source, which
    is described in the next sections.  The only requirement is to have
    GNU@tie{}tar and Xz.
    
    Installing goes along these lines:
    
    @enumerate
    @item
    Download the binary tarball from
    @indicateurl{ftp://alpha.gnu.org/gnu/guix/guix-binary-@value{VERSION}.@var{system}.tar.xz},
    where @var{system} is @code{x86_64-linux} for an @code{x86_64} machine
    already running the kernel Linux, and so on.
    
    Make sure to download the associated @file{.sig} file and to verify the
    authenticity of the tarball against it, along these lines:
    
    @example
    $ wget ftp://alpha.gnu.org/gnu/guix/guix-binary-@value{VERSION}.@var{system}.tar.xz.sig
    $ gpg --verify guix-binary-@value{VERSION}.@var{system}.tar.xz.sig
    @end example
    
    If that command fails because you do not have the required public key,
    then run this command to import it:
    
    @example
    $ gpg --keyserver pgp.mit.edu --recv-keys 090B11993D9AEBB5
    @end example
    
    @noindent
    and rerun the @code{gpg --verify} command.
    
    @item
    As @code{root}, run:
    
    @example
    # cd /tmp
    # tar --warning=no-timestamp -xf \
         guix-binary-@value{VERSION}.@var{system}.tar.xz
    # mv var/guix /var/ && mv gnu /
    @end example
    
    This creates @file{/gnu/store} (@pxref{The Store}) and @file{/var/guix}.
    The latter contains a ready-to-use profile for @code{root} (see next
    step.)
    
    Do @emph{not} unpack the tarball on a working Guix system since that
    would overwrite its own essential files.
    
    The @code{--warning=no-timestamp} option makes sure GNU@tie{}tar does
    not emit warnings about ``implausibly old time stamps'' (such
    warnings were triggered by GNU@tie{}tar 1.26 and older; recent
    versions are fine.)
    They stem from the fact that all the
    files in the archive have their modification time set to zero (which
    means January 1st, 1970.)  This is done on purpose to make sure the
    archive content is independent of its creation time, thus making it
    reproducible.
    
    @item
    Make @code{root}'s profile available under @file{~/.guix-profile}:
    
    @example
    # ln -sf /var/guix/profiles/per-user/root/guix-profile \
             ~root/.guix-profile
    @end example
    
    @item
    Create the group and user accounts for build users as explained below
    (@pxref{Build Environment Setup}).
    
    @item
    Run the daemon, and set it to automatically start on boot.
    
    If your host distro uses the systemd init system, this can be achieved
    with these commands:
    
    @example
    # cp ~root/.guix-profile/lib/systemd/system/guix-daemon.service \
            /etc/systemd/system/
    # systemctl start guix-daemon && systemctl enable guix-daemon
    @end example
    
    If your host distro uses the Upstart init system:
    
    @example
    # cp ~root/.guix-profile/lib/upstart/system/guix-daemon.conf /etc/init/
    # start guix-daemon
    @end example
    
    Otherwise, you can still start the daemon manually with:
    
    @example
    # ~root/.guix-profile/bin/guix-daemon --build-users-group=guixbuild
    @end example
    
    @item
    Make the @command{guix} command available to other users on the machine,
    for instance with:
    
    @example
    # mkdir -p /usr/local/bin
    # cd /usr/local/bin
    # ln -s /var/guix/profiles/per-user/root/guix-profile/bin/guix
    @end example
    
    It is also a good idea to make the Info version of this manual available
    there:
    
    @example
    # mkdir -p /usr/local/share/info
    # cd /usr/local/share/info
    # for i in /var/guix/profiles/per-user/root/guix-profile/share/info/* ;
      do ln -s $i ; done
    @end example
    
    That way, assuming @file{/usr/local/share/info} is in the search path,
    running @command{info guix} will open this manual (@pxref{Other Info
    Directories,,, texinfo, GNU Texinfo}, for more details on changing the
    Info search path.)
    
    @item
    To use substitutes from @code{hydra.gnu.org} or one of its mirrors
    (@pxref{Substitutes}), authorize them:
    
    @example
    # guix archive --authorize < ~root/.guix-profile/share/guix/hydra.gnu.org.pub
    @end example
    @end enumerate
    
    This completes root-level install of Guix.  Each user will need to
    perform additional steps to make their Guix envionment ready for use,
    @pxref{Application Setup}.
    
    You can confirm that Guix is working by installing a sample package into
    the root profile:
    
    @example
    # guix package -i hello
    @end example
    
    The @code{guix} package must remain available in @code{root}'s profile,
    or it would become subject to garbage collection---in which case you
    would find yourself badly handicapped by the lack of the @command{guix}
    command.  In other words, do not remove @code{guix} by running
    @code{guix package -r guix}.
    
    The binary installation tarball can be (re)produced and verified simply
    by running the following command in the Guix source tree:
    
    @example
    make guix-binary.@var{system}.tar.xz
    @end example
    
    
    @node Requirements
    @section Requirements
    
    This section lists requirements when building Guix from source.  The
    build procedure for Guix is the same as for other GNU software, and is
    not covered here.  Please see the files @file{README} and @file{INSTALL}
    in the Guix source tree for additional details.
    
    GNU Guix depends on the following packages:
    
    @itemize
    @item @url{http://gnu.org/software/guile/, GNU Guile}, version 2.0.7 or later;
    @item @url{http://gnupg.org/, GNU libgcrypt};
    @item @url{http://www.gnu.org/software/make/, GNU Make}.
    @end itemize
    
    The following dependencies are optional:
    
    @itemize
    @item
    Installing @uref{http://gnutls.org/, GnuTLS-Guile} will allow you to
    access @code{https} URLs for substitutes, which is highly recommended
    (@pxref{Substitutes}).  It also allows you to access HTTPS URLs with the
    @command{guix download} command (@pxref{Invoking guix download}), the
    @command{guix import pypi} command, and the @command{guix import cpan}
    command.  @xref{Guile Preparations, how to install the GnuTLS bindings
    for Guile,, gnutls-guile, GnuTLS-Guile}.
    
    @item
    Installing
    @url{http://savannah.nongnu.org/projects/guile-json/, Guile-JSON} will
    allow you to use the @command{guix import pypi} command (@pxref{Invoking
    guix import}).  It is of
    interest primarily for developers and not for casual users.
    @end itemize
    
    Unless @code{--disable-daemon} was passed to @command{configure}, the
    following packages are also needed:
    
    @itemize
    @item @url{http://sqlite.org, SQLite 3};
    @item @url{http://www.bzip.org, libbz2};
    @item @url{http://gcc.gnu.org, GCC's g++}, with support for the
    C++11 standard.
    @end itemize
    
    When configuring Guix on a system that already has a Guix installation,
    be sure to specify the same state directory as the existing installation
    using the @code{--localstatedir} option of the @command{configure}
    script (@pxref{Directory Variables, @code{localstatedir},, standards,
    GNU Coding Standards}).  The @command{configure} script protects against
    unintended misconfiguration of @var{localstatedir} so you do not
    inadvertently corrupt your store (@pxref{The Store}).
    
    When a working installation of @url{http://nixos.org/nix/, the Nix package
    manager} is available, you
    can instead configure Guix with @code{--disable-daemon}.  In that case,
    Nix replaces the three dependencies above.
    
    Guix is compatible with Nix, so it is possible to share the same store
    between both.  To do so, you must pass @command{configure} not only the
    same @code{--with-store-dir} value, but also the same
    @code{--localstatedir} value.  The latter is essential because it
    specifies where the database that stores metadata about the store is
    located, among other things.  The default values for Nix are
    @code{--with-store-dir=/nix/store} and @code{--localstatedir=/nix/var}.
    Note that @code{--disable-daemon} is not required if
    your goal is to share the store with Nix.
    
    @node Running the Test Suite
    @section Running the Test Suite
    
    After a successful @command{configure} and @code{make} run, it is a good
    idea to run the test suite.  It can help catch issues with the setup or
    environment, or bugs in Guix itself---and really, reporting test
    failures is a good way to help improve the software.  To run the test
    suite, type:
    
    @example
    make check
    @end example
    
    Test cases can run in parallel: you can use the @code{-j} option of
    GNU@tie{}make to speed things up.  The first run may take a few minutes
    on a recent machine; subsequent runs will be faster because the store
    that is created for test purposes will already have various things in
    cache.
    
    It is also possible to run a subset of the tests by defining the
    @code{TESTS} makefile variable as in this example:
    
    @example
    make check TESTS="tests/store.scm tests/cpio.scm"
    @end example
    
    By default, tests results are displayed at a file level.  In order to
    see the details of every individual test cases, it is possible to define
    the @code{SCM_LOG_DRIVER_FLAGS} makefile variable as in this example:
    
    @example
    make check TESTS="tests/base64.scm" SCM_LOG_DRIVER_FLAGS="--brief=no"
    @end example
    
    Upon failure, please email @email{bug-guix@@gnu.org} and attach the
    @file{test-suite.log} file.  Please specify the Guix version being used
    as well as version numbers of the dependencies (@pxref{Requirements}) in
    your message.
    
    Guix also comes with a whole-system test suite that tests complete
    GuixSD operating system instances.  It can only run on systems where
    Guix is already installed, using:
    
    @example
    make check-system
    @end example
    
    @noindent
    or, again, by defining @code{TESTS} to select a subset of tests to run:
    
    @example
    make check-system TESTS="basic mcron"
    @end example
    
    These system tests are defined in the @code{(gnu tests @dots{})}
    modules.  They work by running the operating systems under test with
    lightweight instrumentation in a virtual machine (VM).  They can be
    computationally intensive or rather cheap, depending on whether
    substitutes are available for their dependencies (@pxref{Substitutes}).
    Some of them require a lot of storage space to hold VM images.
    
    Again in case of test failures, please send @email{bug-guix@@gnu.org}
    all the details.
    
    @node Setting Up the Daemon
    @section Setting Up the Daemon
    
    @cindex daemon
    Operations such as building a package or running the garbage collector
    are all performed by a specialized process, the @dfn{build daemon}, on
    behalf of clients.  Only the daemon may access the store and its
    associated database.  Thus, any operation that manipulates the store
    goes through the daemon.  For instance, command-line tools such as
    @command{guix package} and @command{guix build} communicate with the
    daemon (@i{via} remote procedure calls) to instruct it what to do.
    
    The following sections explain how to prepare the build daemon's
    environment.  See also @ref{Substitutes}, for information on how to allow
    the daemon to download pre-built binaries.
    
    @menu
    * Build Environment Setup::     Preparing the isolated build environment.
    * Daemon Offload Setup::        Offloading builds to remote machines.
    @end menu
    
    @node Build Environment Setup
    @subsection Build Environment Setup
    
    In a standard multi-user setup, Guix and its daemon---the
    @command{guix-daemon} program---are installed by the system
    administrator; @file{/gnu/store} is owned by @code{root} and
    @command{guix-daemon} runs as @code{root}.  Unprivileged users may use
    Guix tools to build packages or otherwise access the store, and the
    daemon will do it on their behalf, ensuring that the store is kept in a
    consistent state, and allowing built packages to be shared among users.
    
    @cindex build users
    When @command{guix-daemon} runs as @code{root}, you may not want package
    build processes themselves to run as @code{root} too, for obvious
    security reasons.  To avoid that, a special pool of @dfn{build users}
    should be created for use by build processes started by the daemon.
    These build users need not have a shell and a home directory: they will
    just be used when the daemon drops @code{root} privileges in build
    processes.  Having several such users allows the daemon to launch
    distinct build processes under separate UIDs, which guarantees that they
    do not interfere with each other---an essential feature since builds are
    regarded as pure functions (@pxref{Introduction}).
    
    On a GNU/Linux system, a build user pool may be created like this (using
    Bash syntax and the @code{shadow} commands):
    
    @c See http://lists.gnu.org/archive/html/bug-guix/2013-01/msg00239.html
    @c for why `-G' is needed.
    @example
    # groupadd --system guixbuild
    # for i in `seq -w 1 10`;
      do
        useradd -g guixbuild -G guixbuild           \
                -d /var/empty -s `which nologin`    \
                -c "Guix build user $i" --system    \
                guixbuilder$i;
      done
    @end example
    
    @noindent
    The number of build users determines how many build jobs may run in
    parallel, as specified by the @option{--max-jobs} option
    (@pxref{Invoking guix-daemon, @option{--max-jobs}}).  To use
    @command{guix system vm} and related commands, you may need to add the
    build users to the @code{kvm} group so they can access @file{/dev/kvm},
    using @code{-G guixbuild,kvm} instead of @code{-G guixbuild}
    (@pxref{Invoking guix system}).
    
    The @code{guix-daemon} program may then be run as @code{root} with the
    following command@footnote{If your machine uses the systemd init system,
    dropping the @file{@var{prefix}/lib/systemd/system/guix-daemon.service}
    file in @file{/etc/systemd/system} will ensure that
    @command{guix-daemon} is automatically started.  Similarly, if your
    machine uses the Upstart init system, drop the
    @file{@var{prefix}/lib/upstart/system/guix-daemon.conf}
    file in @file{/etc/init}.}:
    
    @example
    # guix-daemon --build-users-group=guixbuild
    @end example
    
    @cindex chroot
    @noindent
    This way, the daemon starts build processes in a chroot, under one of
    the @code{guixbuilder} users.  On GNU/Linux, by default, the chroot
    environment contains nothing but:
    
    @c Keep this list in sync with libstore/build.cc! -----------------------
    @itemize
    @item
    a minimal @code{/dev} directory, created mostly independently from the
    host @code{/dev}@footnote{``Mostly'', because while the set of files
    that appear in the chroot's @code{/dev} is fixed, most of these files
    can only be created if the host has them.};
    
    @item
    the @code{/proc} directory; it only shows the processes of the container
    since a separate PID name space is used;
    
    @item
    @file{/etc/passwd} with an entry for the current user and an entry for
    user @file{nobody};
    
    @item
    @file{/etc/group} with an entry for the user's group;
    
    @item
    @file{/etc/hosts} with an entry that maps @code{localhost} to
    @code{127.0.0.1};
    
    @item
    a writable @file{/tmp} directory.
    @end itemize
    
    You can influence the directory where the daemon stores build trees
    @i{via} the @code{TMPDIR} environment variable.  However, the build tree
    within the chroot is always called @file{/tmp/guix-build-@var{name}.drv-0},
    where @var{name} is the derivation name---e.g., @code{coreutils-8.24}.
    This way, the value of @code{TMPDIR} does not leak inside build
    environments, which avoids discrepancies in cases where build processes
    capture the name of their build tree.
    
    @vindex http_proxy
    The daemon also honors the @code{http_proxy} environment variable for
    HTTP downloads it performs, be it for fixed-output derivations
    (@pxref{Derivations}) or for substitutes (@pxref{Substitutes}).
    
    If you are installing Guix as an unprivileged user, it is still possible
    to run @command{guix-daemon} provided you pass @code{--disable-chroot}.
    However, build processes will not be isolated from one another, and not
    from the rest of the system.  Thus, build processes may interfere with
    each other, and may access programs, libraries, and other files
    available on the system---making it much harder to view them as
    @emph{pure} functions.
    
    
    @node Daemon Offload Setup
    @subsection Using the Offload Facility
    
    @cindex offloading
    @cindex build hook
    When desired, the build daemon can @dfn{offload}
    derivation builds to other machines
    running Guix, using the @code{offload} @dfn{build hook}.  When that
    feature is enabled, a list of user-specified build machines is read from
    @file{/etc/guix/machines.scm}; every time a build is requested, for
    instance via @code{guix build}, the daemon attempts to offload it to one
    of the machines that satisfy the constraints of the derivation, in
    particular its system type---e.g., @file{x86_64-linux}.  Missing
    prerequisites for the build are copied over SSH to the target machine,
    which then proceeds with the build; upon success the output(s) of the
    build are copied back to the initial machine.
    
    The @file{/etc/guix/machines.scm} file typically looks like this:
    
    @example
    (list (build-machine
            (name "eightysix.example.org")
            (system "x86_64-linux")
            (user "bob")
            (speed 2.))    ; incredibly fast!
    
          (build-machine
            (name "meeps.example.org")
            (system "mips64el-linux")
            (user "alice")
            (private-key
             (string-append (getenv "HOME")
                            "/.lsh/identity-for-guix"))))
    @end example
    
    @noindent
    In the example above we specify a list of two build machines, one for
    the @code{x86_64} architecture and one for the @code{mips64el}
    architecture.
    
    In fact, this file is---not surprisingly!---a Scheme file that is
    evaluated when the @code{offload} hook is started.  Its return value
    must be a list of @code{build-machine} objects.  While this example
    shows a fixed list of build machines, one could imagine, say, using
    DNS-SD to return a list of potential build machines discovered in the
    local network (@pxref{Introduction, Guile-Avahi,, guile-avahi, Using
    Avahi in Guile Scheme Programs}).  The @code{build-machine} data type is
    detailed below.
    
    @deftp {Data Type} build-machine
    This data type represents build machines to which the daemon may offload
    builds.  The important fields are:
    
    @table @code
    
    @item name
    The host name of the remote machine.
    
    @item system
    The system type of the remote machine---e.g., @code{"x86_64-linux"}.
    
    @item user
    The user account to use when connecting to the remote machine over SSH.
    Note that the SSH key pair must @emph{not} be passphrase-protected, to
    allow non-interactive logins.
    
    @end table
    
    A number of optional fields may be specified:
    
    @table @code
    
    @item port
    Port number of SSH server on the machine (default: 22).
    
    @item private-key
    The SSH private key file to use when connecting to the machine.
    
    Currently offloading uses GNU@tie{}lsh as its SSH client
    (@pxref{Invoking lsh,,, GNU lsh Manual}).  Thus, the key file here must
    be an lsh key file.  This may change in the future, though.
    
    @item parallel-builds
    The number of builds that may run in parallel on the machine (1 by
    default.)
    
    @item speed
    A ``relative speed factor''.  The offload scheduler will tend to prefer
    machines with a higher speed factor.
    
    @item features
    A list of strings denoting specific features supported by the machine.
    An example is @code{"kvm"} for machines that have the KVM Linux modules
    and corresponding hardware support.  Derivations can request features by
    name, and they will be scheduled on matching build machines.
    
    @end table
    @end deftp
    
    The @code{guix} command must be in the search path on the build
    machines, since offloading works by invoking the @code{guix archive} and
    @code{guix build} commands.  In addition, the Guix modules must be in
    @code{$GUILE_LOAD_PATH} on the build machine---you can check whether
    this is the case by running:
    
    @example
    lsh build-machine guile -c "'(use-modules (guix config))'"
    @end example
    
    There is one last thing to do once @file{machines.scm} is in place.  As
    explained above, when offloading, files are transferred back and forth
    between the machine stores.  For this to work, you first need to
    generate a key pair on each machine to allow the daemon to export signed
    archives of files from the store (@pxref{Invoking guix archive}):
    
    @example
    # guix archive --generate-key
    @end example
    
    @noindent
    Each build machine must authorize the key of the master machine so that
    it accepts store items it receives from the master:
    
    @example
    # guix archive --authorize < master-public-key.txt
    @end example
    
    @noindent
    Likewise, the master machine must authorize the key of each build machine.
    
    All the fuss with keys is here to express pairwise mutual trust
    relations between the master and the build machines.  Concretely, when
    the master receives files from a build machine (and @i{vice versa}), its
    build daemon can make sure they are genuine, have not been tampered
    with, and that they are signed by an authorized key.
    
    
    @node Invoking guix-daemon
    @section Invoking @command{guix-daemon}
    
    The @command{guix-daemon} program implements all the functionality to
    access the store.  This includes launching build processes, running the
    garbage collector, querying the availability of a build result, etc.  It
    is normally run as @code{root} like this:
    
    @example
    # guix-daemon --build-users-group=guixbuild
    @end example
    
    @noindent
    For details on how to set it up, @pxref{Setting Up the Daemon}.
    
    @cindex chroot
    @cindex container, build environment
    @cindex build environment
    @cindex reproducible builds
    By default, @command{guix-daemon} launches build processes under
    different UIDs, taken from the build group specified with
    @code{--build-users-group}.  In addition, each build process is run in a
    chroot environment that only contains the subset of the store that the
    build process depends on, as specified by its derivation
    (@pxref{Programming Interface, derivation}), plus a set of specific
    system directories.  By default, the latter contains @file{/dev} and
    @file{/dev/pts}.  Furthermore, on GNU/Linux, the build environment is a
    @dfn{container}: in addition to having its own file system tree, it has
    a separate mount name space, its own PID name space, network name space,
    etc.  This helps achieve reproducible builds (@pxref{Features}).
    
    When the daemon performs a build on behalf of the user, it creates a
    build directory under @file{/tmp} or under the directory specified by
    its @code{TMPDIR} environment variable; this directory is shared with
    the container for the duration of the build.  Be aware that using a
    directory other than @file{/tmp} can affect build results---for example,
    with a longer directory name, a build process that uses Unix-domain
    sockets might hit the name length limitation for @code{sun_path}, which
    it would otherwise not hit.
    
    The build directory is automatically deleted upon completion, unless the
    build failed and the client specified @option{--keep-failed}
    (@pxref{Invoking guix build, @option{--keep-failed}}).
    
    The following command-line options are supported:
    
    @table @code
    @item --build-users-group=@var{group}
    Take users from @var{group} to run build processes (@pxref{Setting Up
    the Daemon, build users}).
    
    @item --no-substitutes
    @cindex substitutes
    Do not use substitutes for build products.  That is, always build things
    locally instead of allowing downloads of pre-built binaries
    (@pxref{Substitutes}).
    
    By default substitutes are used, unless the client---such as the
    @command{guix package} command---is explicitly invoked with
    @code{--no-substitutes}.
    
    When the daemon runs with @code{--no-substitutes}, clients can still
    explicitly enable substitution @i{via} the @code{set-build-options}
    remote procedure call (@pxref{The Store}).
    
    @item --substitute-urls=@var{urls}
    @anchor{daemon-substitute-urls}
    Consider @var{urls} the default whitespace-separated list of substitute
    source URLs.  When this option is omitted,
    @indicateurl{https://mirror.hydra.gnu.org https://hydra.gnu.org} is used
    (@code{mirror.hydra.gnu.org} is a mirror of @code{hydra.gnu.org}).
    
    This means that substitutes may be downloaded from @var{urls}, as long
    as they are signed by a trusted signature (@pxref{Substitutes}).
    
    @cindex build hook
    @item --no-build-hook
    Do not use the @dfn{build hook}.
    
    The build hook is a helper program that the daemon can start and to
    which it submits build requests.  This mechanism is used to offload
    builds to other machines (@pxref{Daemon Offload Setup}).
    
    @item --cache-failures
    Cache build failures.  By default, only successful builds are cached.
    
    When this option is used, @command{guix gc --list-failures} can be used
    to query the set of store items marked as failed; @command{guix gc
    --clear-failures} removes store items from the set of cached failures.
    @xref{Invoking guix gc}.
    
    @item --cores=@var{n}
    @itemx -c @var{n}
    Use @var{n} CPU cores to build each derivation; @code{0} means as many
    as available.
    
    The default value is @code{0}, but it may be overridden by clients, such
    as the @code{--cores} option of @command{guix build} (@pxref{Invoking
    guix build}).
    
    The effect is to define the @code{NIX_BUILD_CORES} environment variable
    in the build process, which can then use it to exploit internal
    parallelism---for instance, by running @code{make -j$NIX_BUILD_CORES}.
    
    @item --max-jobs=@var{n}
    @itemx -M @var{n}
    Allow at most @var{n} build jobs in parallel.  The default value is
    @code{1}.  Setting it to @code{0} means that no builds will be performed
    locally; instead, the daemon will offload builds (@pxref{Daemon Offload
    Setup}), or simply fail.
    
    @item --rounds=@var{N}
    Build each derivation @var{n} times in a row, and raise an error if
    consecutive build results are not bit-for-bit identical.  Note that this
    setting can be overridden by clients such as @command{guix build}
    (@pxref{Invoking guix build}).
    
    When used in conjunction with @option{--keep-failed}, the differing
    output is kept in the store, under @file{/gnu/store/@dots{}-check}.
    This makes it easy to look for differences between the two results.
    
    @item --debug
    Produce debugging output.
    
    This is useful to debug daemon start-up issues, but then it may be
    overridden by clients, for example the @code{--verbosity} option of
    @command{guix build} (@pxref{Invoking guix build}).
    
    @item --chroot-directory=@var{dir}
    Add @var{dir} to the build chroot.
    
    Doing this may change the result of build processes---for instance if
    they use optional dependencies found in @var{dir} when it is available,
    and not otherwise.  For that reason, it is not recommended to do so.
    Instead, make sure that each derivation declares all the inputs that it
    needs.
    
    @item --disable-chroot
    Disable chroot builds.
    
    Using this option is not recommended since, again, it would allow build
    processes to gain access to undeclared dependencies.  It is necessary,
    though, when @command{guix-daemon} is running under an unprivileged user
    account.
    
    @item --disable-log-compression
    Disable compression of the build logs.
    
    Unless @code{--lose-logs} is used, all the build logs are kept in the
    @var{localstatedir}.  To save space, the daemon automatically compresses
    them with bzip2 by default.  This option disables that.
    
    @item --disable-deduplication
    @cindex deduplication
    Disable automatic file ``deduplication'' in the store.
    
    By default, files added to the store are automatically ``deduplicated'':
    if a newly added file is identical to another one found in the store,
    the daemon makes the new file a hard link to the other file.  This can
    noticeably reduce disk usage, at the expense of slightly increased
    input/output load at the end of a build process.  This option disables
    this optimization.
    
    @item --gc-keep-outputs[=yes|no]
    Tell whether the garbage collector (GC) must keep outputs of live
    derivations.
    
    When set to ``yes'', the GC will keep the outputs of any live derivation
    available in the store---the @code{.drv} files.  The default is ``no'',
    meaning that derivation outputs are kept only if they are GC roots.
    
    @item --gc-keep-derivations[=yes|no]
    Tell whether the garbage collector (GC) must keep derivations
    corresponding to live outputs.
    
    When set to ``yes'', as is the case by default, the GC keeps
    derivations---i.e., @code{.drv} files---as long as at least one of their
    outputs is live.  This allows users to keep track of the origins of
    items in their store.  Setting it to ``no'' saves a bit of disk space.
    
    Note that when both @code{--gc-keep-derivations} and
    @code{--gc-keep-outputs} are used, the effect is to keep all the build
    prerequisites (the sources, compiler, libraries, and other build-time
    tools) of live objects in the store, regardless of whether these
    prerequisites are live.  This is convenient for developers since it
    saves rebuilds or downloads.
    
    @item --impersonate-linux-2.6
    On Linux-based systems, impersonate Linux 2.6.  This means that the
    kernel's @code{uname} system call will report 2.6 as the release number.
    
    This might be helpful to build programs that (usually wrongfully) depend
    on the kernel version number.
    
    @item --lose-logs
    Do not keep build logs.  By default they are kept under
    @code{@var{localstatedir}/guix/log}.
    
    @item --system=@var{system}
    Assume @var{system} as the current system type.  By default it is the
    architecture/kernel pair found at configure time, such as
    @code{x86_64-linux}.
    
    @item --listen=@var{socket}
    Listen for connections on @var{socket}, the file name of a Unix-domain
    socket.  The default socket is
    @file{@var{localstatedir}/daemon-socket/socket}.  This option is only
    useful in exceptional circumstances, such as if you need to run several
    daemons on the same machine.
    @end table
    
    
    @node Application Setup
    @section Application Setup
    
    @cindex foreign distro
    When using Guix on top of GNU/Linux distribution other than GuixSD---a
    so-called @dfn{foreign distro}---a few additional steps are needed to
    get everything in place.  Here are some of them.
    
    @subsection Locales
    
    @anchor{locales-and-locpath}
    @cindex locales, when not on GuixSD
    @vindex LOCPATH
    @vindex GUIX_LOCPATH
    Packages installed @i{via} Guix will not use the locale data of the
    host system.  Instead, you must first install one of the locale packages
    available with Guix and then define the @code{GUIX_LOCPATH} environment
    variable:
    
    @example
    $ guix package -i glibc-locales
    $ export GUIX_LOCPATH=$HOME/.guix-profile/lib/locale
    @end example
    
    Note that the @code{glibc-locales} package contains data for all the
    locales supported by the GNU@tie{}libc and weighs in at around
    110@tie{}MiB.  Alternatively, the @code{glibc-utf8-locales} is smaller but
    limited to a few UTF-8 locales.
    
    The @code{GUIX_LOCPATH} variable plays a role similar to @code{LOCPATH}
    (@pxref{Locale Names, @code{LOCPATH},, libc, The GNU C Library Reference
    Manual}).  There are two important differences though:
    
    @enumerate
    @item
    @code{GUIX_LOCPATH} is honored only by the libc in Guix, and not by the libc
    provided by foreign distros.  Thus, using @code{GUIX_LOCPATH} allows you
    to make sure the programs of the foreign distro will not end up loading
    incompatible locale data.
    
    @item
    libc suffixes each entry of @code{GUIX_LOCPATH} with @code{/X.Y}, where
    @code{X.Y} is the libc version---e.g., @code{2.22}.  This means that,
    should your Guix profile contain a mixture of programs linked against
    different libc version, each libc version will only try to load locale
    data in the right format.
    @end enumerate
    
    This is important because the locale data format used by different libc
    versions may be incompatible.
    
    @subsection X11 Fonts
    
    The majority of graphical applications use Fontconfig to locate and
    load fonts and perform X11-client-side rendering.  The @code{fontconfig}
    package in Guix looks for fonts in @file{$HOME/.guix-profile}
    by default.  Thus, to allow graphical applications installed with Guix
    to display fonts, you have to install fonts with Guix as well.
    Essential font packages include @code{gs-fonts}, @code{font-dejavu}, and
    @code{font-gnu-freefont-ttf}.
    
    To display text written in Chinese languages, Japanese, or Korean in
    graphical applications, consider installing
    @code{font-adobe-source-han-sans} or @code{font-wqy-zenhei}.  The former
    has multiple outputs, one per language family (@pxref{Packages with
    Multiple Outputs}).  For instance, the following command installs fonts
    for Chinese languages:
    
    @example
    guix package -i font-adobe-source-han-sans:cn
    @end example
    
    @subsection X.509 Certificates
    
    The @code{nss-certs} package provides X.509 certificates, which allow
    programs to authenticate Web servers accessed over HTTPS.
    
    When using Guix on a foreign distro, you can install this package and
    define the relevant environment variables so that packages know where to
    look for certificates.  @pxref{X.509 Certificates}, for detailed
    information.
    
    @subsection Emacs Packages
    
    When you install Emacs packages with Guix, the elisp files may be placed
    either in @file{$HOME/.guix-profile/share/emacs/site-lisp/} or in
    sub-directories of
    @file{$HOME/.guix-profile/share/emacs/site-lisp/guix.d/}.  The latter
    directory exists because potentially there may exist thousands of Emacs
    packages and storing all their files in a single directory may be not
    reliable (because of name conflicts).  So we think using a separate
    directory for each package is a good idea.  It is very similar to how
    the Emacs package system organizes the file structure (@pxref{Package
    Files,,, emacs, The GNU Emacs Manual}).
    
    By default, Emacs (installed with Guix) ``knows'' where these packages
    are placed, so you do not need to perform any configuration.  If, for
    some reason, you want to avoid auto-loading Emacs packages installed
    with Guix, you can do so by running Emacs with @code{--no-site-file}
    option (@pxref{Init File,,, emacs, The GNU Emacs Manual}).
    
    @c TODO What else?
    
    @c *********************************************************************
    @node Package Management
    @chapter Package Management
    
    The purpose of GNU Guix is to allow users to easily install, upgrade, and
    remove software packages, without having to know about their build
    procedures or dependencies.  Guix also goes beyond this obvious set of
    features.
    
    This chapter describes the main features of Guix, as well as the package
    management tools it provides.  Two user interfaces are provided for
    routine package management tasks: A command-line interface described below
    (@pxref{Invoking guix package, @code{guix package}}), as well as a visual user
    interface in Emacs described in a subsequent chapter (@pxref{Emacs Interface}).
    
    @menu
    * Features::                    How Guix will make your life brighter.
    * Invoking guix package::       Package installation, removal, etc.
    * Substitutes::                 Downloading pre-built binaries.
    * Packages with Multiple Outputs::  Single source package, multiple outputs.
    * Invoking guix gc::            Running the garbage collector.
    * Invoking guix pull::          Fetching the latest Guix and distribution.
    * Invoking guix archive::       Exporting and importing store files.
    @end menu
    
    @node Features
    @section Features
    
    When using Guix, each package ends up in the @dfn{package store}, in its
    own directory---something that resembles
    @file{/gnu/store/xxx-package-1.2}, where @code{xxx} is a base32 string
    (note that Guix comes with an Emacs extension to shorten those file
    names, @pxref{Emacs Prettify}.)
    
    Instead of referring to these directories, users have their own
    @dfn{profile}, which points to the packages that they actually want to
    use.  These profiles are stored within each user's home directory, at
    @code{$HOME/.guix-profile}.
    
    For example, @code{alice} installs GCC 4.7.2.  As a result,
    @file{/home/alice/.guix-profile/bin/gcc} points to
    @file{/gnu/store/@dots{}-gcc-4.7.2/bin/gcc}.  Now, on the same machine,
    @code{bob} had already installed GCC 4.8.0.  The profile of @code{bob}
    simply continues to point to
    @file{/gnu/store/@dots{}-gcc-4.8.0/bin/gcc}---i.e., both versions of GCC
    coexist on the same system without any interference.
    
    The @command{guix package} command is the central tool to manage
    packages (@pxref{Invoking guix package}).  It operates on the per-user
    profiles, and can be used @emph{with normal user privileges}.
    
    The command provides the obvious install, remove, and upgrade
    operations.  Each invocation is actually a @emph{transaction}: either
    the specified operation succeeds, or nothing happens.  Thus, if the
    @command{guix package} process is terminated during the transaction,
    or if a power outage occurs during the transaction, then the user's
    profile remains in its previous state, and remains usable.
    
    In addition, any package transaction may be @emph{rolled back}.  So, if,
    for example, an upgrade installs a new version of a package that turns
    out to have a serious bug, users may roll back to the previous instance
    of their profile, which was known to work well.  Similarly, the global
    system configuration on GuixSD is subject to
    transactional upgrades and roll-back
    (@pxref{Using the Configuration System}).
    
    All packages in the package store may be @emph{garbage-collected}.
    Guix can determine which packages are still referenced by user
    profiles, and remove those that are provably no longer referenced
    (@pxref{Invoking guix gc}).  Users may also explicitly remove old
    generations of their profile so that the packages they refer to can be
    collected.
    
    @cindex reproducibility
    @cindex reproducible builds
    Finally, Guix takes a @dfn{purely functional} approach to package
    management, as described in the introduction (@pxref{Introduction}).
    Each @file{/gnu/store} package directory name contains a hash of all the
    inputs that were used to build that package---compiler, libraries, build
    scripts, etc.  This direct correspondence allows users to make sure a
    given package installation matches the current state of their
    distribution.  It also helps maximize @dfn{build reproducibility}:
    thanks to the isolated build environments that are used, a given build
    is likely to yield bit-identical files when performed on different
    machines (@pxref{Invoking guix-daemon, container}).
    
    @cindex substitutes
    This foundation allows Guix to support @dfn{transparent binary/source
    deployment}.  When a pre-built binary for a @file{/gnu/store} item is
    available from an external source---a @dfn{substitute}, Guix just
    downloads it and unpacks it;
    otherwise, it builds the package from source, locally
    (@pxref{Substitutes}).  Because build results are usually bit-for-bit
    reproducible, users do not have to trust servers that provide
    substitutes: they can force a local build and @emph{challenge} providers
    (@pxref{Invoking guix challenge}).
    
    Control over the build environment is a feature that is also useful for
    developers.  The @command{guix environment} command allows developers of
    a package to quickly set up the right development environment for their
    package, without having to manually install the dependencies of the
    package into their profile (@pxref{Invoking guix environment}).
    
    @node Invoking guix package
    @section Invoking @command{guix package}
    
    The @command{guix package} command is the tool that allows users to
    install, upgrade, and remove packages, as well as rolling back to
    previous configurations.  It operates only on the user's own profile,
    and works with normal user privileges (@pxref{Features}).  Its syntax
    is:
    
    @example
    guix package @var{options}
    @end example
    
    Primarily, @var{options} specifies the operations to be performed during
    the transaction.  Upon completion, a new profile is created, but
    previous @dfn{generations} of the profile remain available, should the user
    want to roll back.
    
    For example, to remove @code{lua} and install @code{guile} and
    @code{guile-cairo} in a single transaction:
    
    @example
    guix package -r lua -i guile guile-cairo
    @end example
    
    @command{guix package} also supports a @dfn{declarative approach}
    whereby the user specifies the exact set of packages to be available and
    passes it @i{via} the @option{--manifest} option
    (@pxref{profile-manifest, @option{--manifest}}).
    
    For each user, a symlink to the user's default profile is automatically
    created in @file{$HOME/.guix-profile}.  This symlink always points to the
    current generation of the user's default profile.  Thus, users can add
    @file{$HOME/.guix-profile/bin} to their @code{PATH} environment
    variable, and so on.
    @cindex search paths
    If you are not using the Guix System Distribution, consider adding the
    following lines to your @file{~/.bash_profile} (@pxref{Bash Startup
    Files,,, bash, The GNU Bash Reference Manual}) so that newly-spawned
    shells get all the right environment variable definitions:
    
    @example
    GUIX_PROFILE="$HOME/.guix-profile" \
    source "$HOME/.guix-profile/etc/profile"
    @end example
    
    In a multi-user setup, user profiles are stored in a place registered as
    a @dfn{garbage-collector root}, which @file{$HOME/.guix-profile} points
    to (@pxref{Invoking guix gc}).  That directory is normally
    @code{@var{localstatedir}/profiles/per-user/@var{user}}, where
    @var{localstatedir} is the value passed to @code{configure} as
    @code{--localstatedir}, and @var{user} is the user name.  The
    @file{per-user} directory is created when @command{guix-daemon} is
    started, and the @var{user} sub-directory is created by @command{guix
    package}.
    
    The @var{options} can be among the following:
    
    @table @code
    
    @item --install=@var{package} @dots{}
    @itemx -i @var{package} @dots{}
    Install the specified @var{package}s.
    
    Each @var{package} may specify either a simple package name, such as
    @code{guile}, or a package name followed by an at-sign and version number,
    such as @code{guile@@1.8.8} or simply @code{guile@@1.8} (in the latter
    case, the newest version prefixed by @code{1.8} is selected.)
    
    If no version number is specified, the
    newest available version will be selected.  In addition, @var{package}
    may contain a colon, followed by the name of one of the outputs of the
    package, as in @code{gcc:doc} or @code{binutils@@2.22:lib}
    (@pxref{Packages with Multiple Outputs}).  Packages with a corresponding
    name (and optionally version) are searched for among the GNU
    distribution modules (@pxref{Package Modules}).
    
    @cindex propagated inputs
    Sometimes packages have @dfn{propagated inputs}: these are dependencies
    that automatically get installed along with the required package
    (@pxref{package-propagated-inputs, @code{propagated-inputs} in
    @code{package} objects}, for information about propagated inputs in
    package definitions).
    
    @anchor{package-cmd-propagated-inputs}
    An example is the GNU MPC library: its C header files refer to those of
    the GNU MPFR library, which in turn refer to those of the GMP library.
    Thus, when installing MPC, the MPFR and GMP libraries also get installed
    in the profile; removing MPC also removes MPFR and GMP---unless they had
    also been explicitly installed by the user.
    
    Besides, packages sometimes rely on the definition of environment
    variables for their search paths (see explanation of
    @code{--search-paths} below).  Any missing or possibly incorrect
    environment variable definitions are reported here.
    
    @item --install-from-expression=@var{exp}
    @itemx -e @var{exp}
    Install the package @var{exp} evaluates to.
    
    @var{exp} must be a Scheme expression that evaluates to a
    @code{<package>} object.  This option is notably useful to disambiguate
    between same-named variants of a package, with expressions such as
    @code{(@@ (gnu packages base) guile-final)}.
    
    Note that this option installs the first output of the specified
    package, which may be insufficient when needing a specific output of a
    multiple-output package.
    
    @item --install-from-file=@var{file}
    @itemx -f @var{file}
    Install the package that the code within @var{file} evaluates to.
    
    As an example, @var{file} might contain a definition like this
    (@pxref{Defining Packages}):
    
    @example
    @verbatiminclude package-hello.scm
    @end example
    
    Developers may find it useful to include such a @file{guix.scm} file
    in the root of their project source tree that can be used to test
    development snapshots and create reproducible development environments
    (@pxref{Invoking guix environment}).
    
    @item --remove=@var{package} @dots{}
    @itemx -r @var{package} @dots{}
    Remove the specified @var{package}s.
    
    As for @code{--install}, each @var{package} may specify a version number
    and/or output name in addition to the package name.  For instance,
    @code{-r glibc:debug} would remove the @code{debug} output of
    @code{glibc}.
    
    @item --upgrade[=@var{regexp} @dots{}]
    @itemx -u [@var{regexp} @dots{}]
    Upgrade all the installed packages.  If one or more @var{regexp}s are
    specified, upgrade only installed packages whose name matches a
    @var{regexp}.  Also see the @code{--do-not-upgrade} option below.
    
    Note that this upgrades package to the latest version of packages found
    in the distribution currently installed.  To update your distribution,
    you should regularly run @command{guix pull} (@pxref{Invoking guix
    pull}).
    
    @item --do-not-upgrade[=@var{regexp} @dots{}]
    When used together with the @code{--upgrade} option, do @emph{not}
    upgrade any packages whose name matches a @var{regexp}.  For example, to
    upgrade all packages in the current profile except those containing the
    substring ``emacs'':
    
    @example
    $ guix package --upgrade . --do-not-upgrade emacs
    @end example
    
    @item @anchor{profile-manifest}--manifest=@var{file}
    @itemx -m @var{file}
    @cindex profile declaration
    @cindex profile manifest
    Create a new generation of the profile from the manifest object
    returned by the Scheme code in @var{file}.
    
    This allows you to @emph{declare} the profile's contents rather than
    constructing it through a sequence of @code{--install} and similar
    commands.  The advantage is that @var{file} can be put under version
    control, copied to different machines to reproduce the same profile, and
    so on.
    
    @c FIXME: Add reference to (guix profile) documentation when available.
    @var{file} must return a @dfn{manifest} object, which is roughly a list
    of packages:
    
    @findex packages->manifest
    @example
    (use-package-modules guile emacs)
    
    (packages->manifest
     (list emacs
           guile-2.0
           ;; Use a specific package output.
           (list guile-2.0 "debug")))
    @end example
    
    @item --roll-back
    Roll back to the previous @dfn{generation} of the profile---i.e., undo
    the last transaction.
    
    When combined with options such as @code{--install}, roll back occurs
    before any other actions.
    
    When rolling back from the first generation that actually contains
    installed packages, the profile is made to point to the @dfn{zeroth
    generation}, which contains no files apart from its own metadata.
    
    After having rolled back, installing, removing, or upgrading packages
    overwrites previous future generations.  Thus, the history of the
    generations in a profile is always linear.
    
    @item --switch-generation=@var{pattern}
    @itemx -S @var{pattern}
    Switch to a particular generation defined by @var{pattern}.
    
    @var{pattern} may be either a generation number or a number prefixed
    with ``+'' or ``-''.  The latter means: move forward/backward by a
    specified number of generations.  For example, if you want to return to
    the latest generation after @code{--roll-back}, use
    @code{--switch-generation=+1}.
    
    The difference between @code{--roll-back} and
    @code{--switch-generation=-1} is that @code{--switch-generation} will
    not make a zeroth generation, so if a specified generation does not
    exist, the current generation will not be changed.
    
    @item --search-paths[=@var{kind}]
    @cindex search paths
    Report environment variable definitions, in Bash syntax, that may be
    needed in order to use the set of installed packages.  These environment
    variables are used to specify @dfn{search paths} for files used by some
    of the installed packages.
    
    For example, GCC needs the @code{CPATH} and @code{LIBRARY_PATH}
    environment variables to be defined so it can look for headers and
    libraries in the user's profile (@pxref{Environment Variables,,, gcc,
    Using the GNU Compiler Collection (GCC)}).  If GCC and, say, the C
    library are installed in the profile, then @code{--search-paths} will
    suggest setting these variables to @code{@var{profile}/include} and
    @code{@var{profile}/lib}, respectively.
    
    The typical use case is to define these environment variables in the
    shell:
    
    @example
    $ eval `guix package --search-paths`
    @end example
    
    @var{kind} may be one of @code{exact}, @code{prefix}, or @code{suffix},
    meaning that the returned environment variable definitions will either
    be exact settings, or prefixes or suffixes of the current value of these
    variables.  When omitted, @var{kind} defaults to @code{exact}.
    
    This option can also be used to compute the @emph{combined} search paths
    of several profiles.  Consider this example:
    
    @example
    $ guix package -p foo -i guile
    $ guix package -p bar -i guile-json
    $ guix package -p foo -p bar --search-paths
    @end example
    
    The last command above reports about the @code{GUILE_LOAD_PATH}
    variable, even though, taken individually, neither @file{foo} nor
    @file{bar} would lead to that recommendation.
    
    
    @item --profile=@var{profile}
    @itemx -p @var{profile}
    Use @var{profile} instead of the user's default profile.
    
    @item --verbose
    Produce verbose output.  In particular, emit the build log of the
    environment on the standard error port.
    
    @item --bootstrap
    Use the bootstrap Guile to build the profile.  This option is only
    useful to distribution developers.
    
    @end table
    
    In addition to these actions, @command{guix package} supports the
    following options to query the current state of a profile, or the
    availability of packages:
    
    @table @option
    
    @item --search=@var{regexp}
    @itemx -s @var{regexp}
    @cindex searching for packages
    List the available packages whose name, synopsis, or description matches
    @var{regexp}.  Print all the metadata of matching packages in
    @code{recutils} format (@pxref{Top, GNU recutils databases,, recutils,
    GNU recutils manual}).
    
    This allows specific fields to be extracted using the @command{recsel}
    command, for instance:
    
    @example
    $ guix package -s malloc | recsel -p name,version
    name: glibc
    version: 2.17
    
    name: libgc
    version: 7.2alpha6
    @end example
    
    Similarly, to show the name of all the packages available under the
    terms of the GNU@tie{}LGPL version 3:
    
    @example
    $ guix package -s "" | recsel -p name -e 'license ~ "LGPL 3"'
    name: elfutils
    
    name: gmp
    @dots{}
    @end example
    
    It is also possible to refine search results using several @code{-s}
    flags.  For example, the following command returns a list of board
    games:
    
    @example
    $ guix package -s '\<board\>' -s game | recsel -p name
    name: gnubg
    @dots{}
    @end example
    
    If we were to omit @code{-s game}, we would also get software packages
    that deal with printed circuit boards; removing the angle brackets
    around @code{board} would further add packages that have to do with
    keyboards.
    
    And now for a more elaborate example.  The following command searches
    for cryptographic libraries, filters out Haskell, Perl, Python, and Ruby
    libraries, and prints the name and synopsis of the matching packages:
    
    @example
    $ guix package -s crypto -s library | \
        recsel -e '! (name ~ "^(ghc|perl|python|ruby)")' -p name,synopsis
    @end example
    
    @noindent
    @xref{Selection Expressions,,, recutils, GNU recutils manual}, for more
    information on @dfn{selection expressions} for @code{recsel -e}.
    
    @item --show=@var{package}
    Show details about @var{package}, taken from the list of available packages, in
    @code{recutils} format (@pxref{Top, GNU recutils databases,, recutils, GNU
    recutils manual}).
    
    @example
    $ guix package --show=python | recsel -p name,version
    name: python
    version: 2.7.6
    
    name: python
    version: 3.3.5
    @end example
    
    You may also specify the full name of a package to only get details about a
    specific version of it:
    @example
    $ guix package --show=python-3.3.5 | recsel -p name,version
    name: python
    version: 3.3.5
    @end example
    
    
    
    @item --list-installed[=@var{regexp}]
    @itemx -I [@var{regexp}]
    List the currently installed packages in the specified profile, with the
    most recently installed packages shown last.  When @var{regexp} is
    specified, list only installed packages whose name matches @var{regexp}.
    
    For each installed package, print the following items, separated by
    tabs: the package name, its version string, the part of the package that
    is installed (for instance, @code{out} for the default output,
    @code{include} for its headers, etc.), and the path of this package in
    the store.
    
    @item --list-available[=@var{regexp}]
    @itemx -A [@var{regexp}]
    List packages currently available in the distribution for this system
    (@pxref{GNU Distribution}).  When @var{regexp} is specified, list only
    installed packages whose name matches @var{regexp}.
    
    For each package, print the following items separated by tabs: its name,
    its version string, the parts of the package (@pxref{Packages with
    Multiple Outputs}), and the source location of its definition.
    
    @item --list-generations[=@var{pattern}]
    @itemx -l [@var{pattern}]
    Return a list of generations along with their creation dates; for each
    generation, show the installed packages, with the most recently
    installed packages shown last.  Note that the zeroth generation is never
    shown.
    
    For each installed package, print the following items, separated by
    tabs: the name of a package, its version string, the part of the package
    that is installed (@pxref{Packages with Multiple Outputs}), and the
    location of this package in the store.
    
    When @var{pattern} is used, the command returns only matching
    generations.  Valid patterns include:
    
    @itemize
    @item @emph{Integers and comma-separated integers}.  Both patterns denote
    generation numbers.  For instance, @code{--list-generations=1} returns
    the first one.
    
    And @code{--list-generations=1,8,2} outputs three generations in the
    specified order.  Neither spaces nor trailing commas are allowed.
    
    @item @emph{Ranges}.  @code{--list-generations=2..9} prints the
    specified generations and everything in between.  Note that the start of
    a range must be smaller than its end.
    
    It is also possible to omit the endpoint.  For example,
    @code{--list-generations=2..}, returns all generations starting from the
    second one.
    
    @item @emph{Durations}.  You can also get the last @emph{N}@tie{}days, weeks,
    or months by passing an integer along with the first letter of the
    duration.  For example, @code{--list-generations=20d} lists generations
    that are up to 20 days old.
    @end itemize
    
    @item --delete-generations[=@var{pattern}]
    @itemx -d [@var{pattern}]
    When @var{pattern} is omitted, delete all generations except the current
    one.
    
    This command accepts the same patterns as @option{--list-generations}.
    When @var{pattern} is specified, delete the matching generations.  When
    @var{pattern} specifies a duration, generations @emph{older} than the
    specified duration match.  For instance, @code{--delete-generations=1m}
    deletes generations that are more than one month old.
    
    If the current generation matches, it is @emph{not} deleted.  Also, the
    zeroth generation is never deleted.
    
    Note that deleting generations prevents rolling back to them.
    Consequently, this command must be used with care.
    
    @end table
    
    Finally, since @command{guix package} may actually start build
    processes, it supports all the common build options (@pxref{Common Build
    Options}).  It also supports package transformation options, such as
    @option{--with-source} (@pxref{Package Transformation Options}).
    However, note that package transformations are lost when upgrading; to
    preserve transformations across upgrades, you should define your own
    package variant in a Guile module and add it to @code{GUIX_PACKAGE_PATH}
    (@pxref{Defining Packages}).
    
    
    @node Substitutes
    @section Substitutes
    
    @cindex substitutes
    @cindex pre-built binaries
    Guix supports transparent source/binary deployment, which means that it
    can either build things locally, or download pre-built items from a
    server.  We call these pre-built items @dfn{substitutes}---they are
    substitutes for local build results.  In many cases, downloading a
    substitute is much faster than building things locally.
    
    Substitutes can be anything resulting from a derivation build
    (@pxref{Derivations}).  Of course, in the common case, they are
    pre-built package binaries, but source tarballs, for instance, which
    also result from derivation builds, can be available as substitutes.
    
    The @code{hydra.gnu.org} server is a front-end to a build farm that
    builds packages from the GNU distribution continuously for some
    architectures, and makes them available as substitutes (@pxref{Emacs
    Hydra}, for information on how to query the continuous integration
    server).  This is the
    default source of substitutes; it can be overridden by passing the
    @option{--substitute-urls} option either to @command{guix-daemon}
    (@pxref{daemon-substitute-urls,, @code{guix-daemon --substitute-urls}})
    or to client tools such as @command{guix package}
    (@pxref{client-substitute-urls,, client @option{--substitute-urls}
    option}).
    
    Substitute URLs can be either HTTP or HTTPS@footnote{For HTTPS access,
    the Guile bindings of GnuTLS must be installed.  @xref{Requirements}.}
    HTTPS is recommended because communications are encrypted; conversely,
    using HTTP makes all communications visible to an eavesdropper, who
    could use the information gathered to determine, for instance, whether
    your system has unpatched security vulnerabilities.
    
    @cindex security
    @cindex digital signatures
    To allow Guix to download substitutes from @code{hydra.gnu.org} or a
    mirror thereof, you
    must add its public key to the access control list (ACL) of archive
    imports, using the @command{guix archive} command (@pxref{Invoking guix
    archive}).  Doing so implies that you trust @code{hydra.gnu.org} to not
    be compromised and to serve genuine substitutes.
    
    This public key is installed along with Guix, in
    @code{@var{prefix}/share/guix/hydra.gnu.org.pub}, where @var{prefix} is
    the installation prefix of Guix.  If you installed Guix from source,
    make sure you checked the GPG signature of
    @file{guix-@value{VERSION}.tar.gz}, which contains this public key file.
    Then, you can run something like this:
    
    @example
    # guix archive --authorize < hydra.gnu.org.pub
    @end example
    
    Once this is in place, the output of a command like @code{guix build}
    should change from something like:
    
    @example
    $ guix build emacs --dry-run
    The following derivations would be built:
       /gnu/store/yr7bnx8xwcayd6j95r2clmkdl1qh688w-emacs-24.3.drv
       /gnu/store/x8qsh1hlhgjx6cwsjyvybnfv2i37z23w-dbus-1.6.4.tar.gz.drv
       /gnu/store/1ixwp12fl950d15h2cj11c73733jay0z-alsa-lib-1.0.27.1.tar.bz2.drv
       /gnu/store/nlma1pw0p603fpfiqy7kn4zm105r5dmw-util-linux-2.21.drv
    @dots{}
    @end example
    
    @noindent
    to something like:
    
    @example
    $ guix build emacs --dry-run
    The following files would be downloaded:
       /gnu/store/pk3n22lbq6ydamyymqkkz7i69wiwjiwi-emacs-24.3
       /gnu/store/2ygn4ncnhrpr61rssa6z0d9x22si0va3-libjpeg-8d
       /gnu/store/71yz6lgx4dazma9dwn2mcjxaah9w77jq-cairo-1.12.16
       /gnu/store/7zdhgp0n1518lvfn8mb96sxqfmvqrl7v-libxrender-0.9.7
    @dots{}
    @end example
    
    @noindent
    This indicates that substitutes from @code{hydra.gnu.org} are usable and
    will be downloaded, when possible, for future builds.
    
    Guix ignores substitutes that are not signed, or that are not signed by
    one of the keys listed in the ACL.  It also detects and raises an error
    when attempting to use a substitute that has been tampered with.
    
    @vindex http_proxy
    Substitutes are downloaded over HTTP or HTTPS.
    The @code{http_proxy} environment
    variable can be set in the environment of @command{guix-daemon} and is
    honored for downloads of substitutes.  Note that the value of
    @code{http_proxy} in the environment where @command{guix build},
    @command{guix package}, and other client commands are run has
    @emph{absolutely no effect}.
    
    When using HTTPS, the server's X.509 certificate is @emph{not} validated
    (in other words, the server is not authenticated), contrary to what
    HTTPS clients such as Web browsers usually do.  This is because Guix
    authenticates substitute information itself, as explained above, which
    is what we care about (whereas X.509 certificates are about
    authenticating bindings between domain names and public keys.)
    
    The substitute mechanism can be disabled globally by running
    @code{guix-daemon} with @code{--no-substitutes} (@pxref{Invoking
    guix-daemon}).  It can also be disabled temporarily by passing the
    @code{--no-substitutes} option to @command{guix package}, @command{guix
    build}, and other command-line tools.
    
    
    @unnumberedsubsec On Trusting Binaries
    
    Today, each individual's control over their own computing is at the
    mercy of institutions, corporations, and groups with enough power and
    determination to subvert the computing infrastructure and exploit its
    weaknesses.  While using @code{hydra.gnu.org} substitutes can be
    convenient, we encourage users to also build on their own, or even run
    their own build farm, such that @code{hydra.gnu.org} is less of an
    interesting target.  One way to help is by publishing the software you
    build using @command{guix publish} so that others have one more choice
    of server to download substitutes from (@pxref{Invoking guix publish}).
    
    Guix has the foundations to maximize build reproducibility
    (@pxref{Features}).  In most cases, independent builds of a given
    package or derivation should yield bit-identical results.  Thus, through
    a diverse set of independent package builds, we can strengthen the
    integrity of our systems.  The @command{guix challenge} command aims to
    help users assess substitute servers, and to assist developers in
    finding out about non-deterministic package builds (@pxref{Invoking guix
    challenge}).  Similarly, the @option{--check} option of @command{guix
    build} allows users to check whether previously-installed substitutes
    are genuine by rebuilding them locally (@pxref{build-check,
    @command{guix build --check}}).
    
    In the future, we want Guix to have support to publish and retrieve
    binaries to/from other users, in a peer-to-peer fashion.  If you would
    like to discuss this project, join us on @email{guix-devel@@gnu.org}.
    
    
    @node Packages with Multiple Outputs
    @section Packages with Multiple Outputs
    
    @cindex multiple-output packages
    @cindex package outputs
    
    Often, packages defined in Guix have a single @dfn{output}---i.e., the
    source package leads to exactly one directory in the store.  When running
    @command{guix package -i glibc}, one installs the default output of the
    GNU libc package; the default output is called @code{out}, but its name
    can be omitted as shown in this command.  In this particular case, the
    default output of @code{glibc} contains all the C header files, shared
    libraries, static libraries, Info documentation, and other supporting
    files.
    
    Sometimes it is more appropriate to separate the various types of files
    produced from a single source package into separate outputs.  For
    instance, the GLib C library (used by GTK+ and related packages)
    installs more than 20 MiB of reference documentation as HTML pages.
    To save space for users who do not need it, the documentation goes to a
    separate output, called @code{doc}.  To install the main GLib output,
    which contains everything but the documentation, one would run:
    
    @example
    guix package -i glib
    @end example
    
    The command to install its documentation is:
    
    @example
    guix package -i glib:doc
    @end example
    
    Some packages install programs with different ``dependency footprints''.
    For instance, the WordNet package installs both command-line tools and
    graphical user interfaces (GUIs).  The former depend solely on the C
    library, whereas the latter depend on Tcl/Tk and the underlying X
    libraries.  In this case, we leave the command-line tools in the default
    output, whereas the GUIs are in a separate output.  This allows users
    who do not need the GUIs to save space.  The @command{guix size} command
    can help find out about such situations (@pxref{Invoking guix size}).
    @command{guix graph} can also be helpful (@pxref{Invoking guix graph}).
    
    There are several such multiple-output packages in the GNU distribution.
    Other conventional output names include @code{lib} for libraries and
    possibly header files, @code{bin} for stand-alone programs, and
    @code{debug} for debugging information (@pxref{Installing Debugging
    Files}).  The outputs of a packages are listed in the third column of
    the output of @command{guix package --list-available} (@pxref{Invoking
    guix package}).
    
    
    @node Invoking guix gc
    @section Invoking @command{guix gc}
    
    @cindex garbage collector
    Packages that are installed, but not used, may be @dfn{garbage-collected}.
    The @command{guix gc} command allows users to explicitly run the garbage
    collector to reclaim space from the @file{/gnu/store} directory.  It is
    the @emph{only} way to remove files from @file{/gnu/store}---removing
    files or directories manually may break it beyond repair!
    
    The garbage collector has a set of known @dfn{roots}: any file under
    @file{/gnu/store} reachable from a root is considered @dfn{live} and
    cannot be deleted; any other file is considered @dfn{dead} and may be
    deleted.  The set of garbage collector roots includes default user
    profiles, and may be augmented with @command{guix build --root}, for
    example (@pxref{Invoking guix build}).
    
    Prior to running @code{guix gc --collect-garbage} to make space, it is
    often useful to remove old generations from user profiles; that way, old
    package builds referenced by those generations can be reclaimed.  This
    is achieved by running @code{guix package --delete-generations}
    (@pxref{Invoking guix package}).
    
    The @command{guix gc} command has three modes of operation: it can be
    used to garbage-collect any dead files (the default), to delete specific
    files (the @code{--delete} option), to print garbage-collector
    information, or for more advanced queries.  The garbage collection
    options are as follows:
    
    @table @code
    @item --collect-garbage[=@var{min}]
    @itemx -C [@var{min}]
    Collect garbage---i.e., unreachable @file{/gnu/store} files and
    sub-directories.  This is the default operation when no option is
    specified.
    
    When @var{min} is given, stop once @var{min} bytes have been collected.
    @var{min} may be a number of bytes, or it may include a unit as a
    suffix, such as @code{MiB} for mebibytes and @code{GB} for gigabytes
    (@pxref{Block size, size specifications,, coreutils, GNU Coreutils}).
    
    When @var{min} is omitted, collect all the garbage.
    
    @item --free-space=@var{free}
    @itemx -F @var{free}
    Collect garbage until @var{free} space is available under
    @file{/gnu/store}, if possible; @var{free} denotes storage space, such
    as @code{500MiB}, as described above.
    
    When @var{free} or more is already available in @file{/gnu/store}, do
    nothing and exit immediately.
    
    @item --delete
    @itemx -d
    Attempt to delete all the store files and directories specified as
    arguments.  This fails if some of the files are not in the store, or if
    they are still live.
    
    @item --list-failures
    List store items corresponding to cached build failures.
    
    This prints nothing unless the daemon was started with
    @option{--cache-failures} (@pxref{Invoking guix-daemon,
    @option{--cache-failures}}).
    
    @item --clear-failures
    Remove the specified store items from the failed-build cache.
    
    Again, this option only makes sense when the daemon is started with
    @option{--cache-failures}.  Otherwise, it does nothing.
    
    @item --list-dead
    Show the list of dead files and directories still present in the
    store---i.e., files and directories no longer reachable from any root.
    
    @item --list-live
    Show the list of live store files and directories.
    
    @end table
    
    In addition, the references among existing store files can be queried:
    
    @table @code
    
    @item --references
    @itemx --referrers
    List the references (respectively, the referrers) of store files given
    as arguments.
    
    @item --requisites
    @itemx -R
    @cindex closure
    List the requisites of the store files passed as arguments.  Requisites
    include the store files themselves, their references, and the references
    of these, recursively.  In other words, the returned list is the
    @dfn{transitive closure} of the store files.
    
    @xref{Invoking guix size}, for a tool to profile the size of the closure
    of an element.  @xref{Invoking guix graph}, for a tool to visualize
    the graph of references.
    
    @end table
    
    Lastly, the following options allow you to check the integrity of the
    store and to control disk usage.
    
    @table @option
    
    @item --verify[=@var{options}]
    @cindex integrity, of the store
    @cindex integrity checking
    Verify the integrity of the store.
    
    By default, make sure that all the store items marked as valid in the
    database of the daemon actually exist in @file{/gnu/store}.
    
    When provided, @var{options} must be a comma-separated list containing one
    or more of @code{contents} and @code{repair}.
    
    When passing @option{--verify=contents}, the daemon computse the
    content hash of each store item and compares it against its hash in the
    database.  Hash mismatches are reported as data corruptions.  Because it
    traverses @emph{all the files in the store}, this command can take a
    long time, especially on systems with a slow disk drive.
    
    @cindex repairing the store
    Using @option{--verify=repair} or @option{--verify=contents,repair}
    causes the daemon to try to repair corrupt store items by fetching
    substitutes for them (@pxref{Substitutes}).  Because repairing is not
    atomic, and thus potentially dangerous, it is available only to the
    system administrator.
    
    @item --optimize
    @cindex deduplication
    Optimize the store by hard-linking identical files---this is
    @dfn{deduplication}.
    
    The daemon performs deduplication after each successful build or archive
    import, unless it was started with @code{--disable-deduplication}
    (@pxref{Invoking guix-daemon, @code{--disable-deduplication}}).  Thus,
    this option is primarily useful when the daemon was running with
    @code{--disable-deduplication}.
    
    @end table
    
    @node Invoking guix pull
    @section Invoking @command{guix pull}
    
    Packages are installed or upgraded to the latest version available in
    the distribution currently available on your local machine.  To update
    that distribution, along with the Guix tools, you must run @command{guix
    pull}: the command downloads the latest Guix source code and package
    descriptions, and deploys it.
    
    On completion, @command{guix package} will use packages and package
    versions from this just-retrieved copy of Guix.  Not only that, but all
    the Guix commands and Scheme modules will also be taken from that latest
    version.  New @command{guix} sub-commands added by the update also
    become available.
    
    Any user can update their Guix copy using @command{guix pull}, and the
    effect is limited to the user who run @command{guix pull}.  For
    instance, when user @code{root} runs @command{guix pull}, this has no
    effect on the version of Guix that user @code{alice} sees, and vice
    versa@footnote{Under the hood, @command{guix pull} updates the
    @file{~/.config/guix/latest} symbolic link to point to the latest Guix,
    and the @command{guix} command loads code from there.}.
    
    The @command{guix pull} command is usually invoked with no arguments,
    but it supports the following options:
    
    @table @code
    @item --verbose
    Produce verbose output, writing build logs to the standard error output.
    
    @item --url=@var{url}
    Download the source tarball of Guix from @var{url}.
    
    By default, the tarball is taken from its canonical address at
    @code{gnu.org}, for the stable branch of Guix.
    
    @item --bootstrap
    Use the bootstrap Guile to build the latest Guix.  This option is only
    useful to Guix developers.
    @end table
    
    
    @node Invoking guix archive
    @section Invoking @command{guix archive}
    
    The @command{guix archive} command allows users to @dfn{export} files
    from the store into a single archive, and to later @dfn{import} them.
    In particular, it allows store files to be transferred from one machine
    to the store on another machine.
    
    To export store files as an archive to standard output, run:
    
    @example
    guix archive --export @var{options} @var{specifications}...
    @end example
    
    @var{specifications} may be either store file names or package
    specifications, as for @command{guix package} (@pxref{Invoking guix
    package}).  For instance, the following command creates an archive
    containing the @code{gui} output of the @code{git} package and the main
    output of @code{emacs}:
    
    @example
    guix archive --export git:gui /gnu/store/...-emacs-24.3 > great.nar
    @end example
    
    If the specified packages are not built yet, @command{guix archive}
    automatically builds them.  The build process may be controlled with the
    common build options (@pxref{Common Build Options}).
    
    To transfer the @code{emacs} package to a machine connected over SSH,
    one would run:
    
    @example
    guix archive --export -r emacs | ssh the-machine guix archive --import
    @end example
    
    @noindent
    Similarly, a complete user profile may be transferred from one machine
    to another like this:
    
    @example
    guix archive --export -r $(readlink -f ~/.guix-profile) | \
      ssh the-machine guix-archive --import
    @end example
    
    @noindent
    However, note that, in both examples, all of @code{emacs} and the
    profile as well as all of their dependencies are transferred (due to
    @code{-r}), regardless of what is already available in the store on the
    target machine.  The @code{--missing} option can help figure out which
    items are missing from the target store.
    
    Archives are stored in the ``Nix archive'' or ``Nar'' format, which is
    comparable in spirit to `tar', but with a few noteworthy differences
    that make it more appropriate for our purposes.  First, rather than
    recording all Unix metadata for each file, the Nar format only mentions
    the file type (regular, directory, or symbolic link); Unix permissions
    and owner/group are dismissed.  Second, the order in which directory
    entries are stored always follows the order of file names according to
    the C locale collation order.  This makes archive production fully
    deterministic.
    
    When exporting, the daemon digitally signs the contents of the archive,
    and that digital signature is appended.  When importing, the daemon
    verifies the signature and rejects the import in case of an invalid
    signature or if the signing key is not authorized.
    @c FIXME: Add xref to daemon doc about signatures.
    
    The main options are:
    
    @table @code
    @item --export
    Export the specified store files or packages (see below.)  Write the
    resulting archive to the standard output.
    
    Dependencies are @emph{not} included in the output, unless
    @code{--recursive} is passed.
    
    @item -r
    @itemx --recursive
    When combined with @code{--export}, this instructs @command{guix
    archive} to include dependencies of the given items in the archive.
    Thus, the resulting archive is self-contained: it contains the closure
    of the exported store items.
    
    @item --import
    Read an archive from the standard input, and import the files listed
    therein into the store.  Abort if the archive has an invalid digital
    signature, or if it is signed by a public key not among the authorized
    keys (see @code{--authorize} below.)
    
    @item --missing
    Read a list of store file names from the standard input, one per line,
    and write on the standard output the subset of these files missing from
    the store.
    
    @item --generate-key[=@var{parameters}]
    @cindex signing, archives
    Generate a new key pair for the daemon.  This is a prerequisite before
    archives can be exported with @code{--export}.  Note that this operation
    usually takes time, because it needs to gather enough entropy to
    generate the key pair.
    
    The generated key pair is typically stored under @file{/etc/guix}, in
    @file{signing-key.pub} (public key) and @file{signing-key.sec} (private
    key, which must be kept secret.)  When @var{parameters} is omitted,
    an ECDSA key using the Ed25519 curve is generated, or, for Libgcrypt
    versions before 1.6.0, it is a 4096-bit RSA key.
    Alternatively, @var{parameters} can specify
    @code{genkey} parameters suitable for Libgcrypt (@pxref{General
    public-key related Functions, @code{gcry_pk_genkey},, gcrypt, The
    Libgcrypt Reference Manual}).
    
    @item --authorize
    @cindex authorizing, archives
    Authorize imports signed by the public key passed on standard input.
    The public key must be in ``s-expression advanced format''---i.e., the
    same format as the @file{signing-key.pub} file.
    
    The list of authorized keys is kept in the human-editable file
    @file{/etc/guix/acl}.  The file contains
    @url{http://people.csail.mit.edu/rivest/Sexp.txt, ``advanced-format
    s-expressions''} and is structured as an access-control list in the
    @url{http://theworld.com/~cme/spki.txt, Simple Public-Key Infrastructure
    (SPKI)}.
    
    @item --extract=@var{directory}
    @itemx -x @var{directory}
    Read a single-item archive as served by substitute servers
    (@pxref{Substitutes}) and extract it to @var{directory}.  This is a
    low-level operation needed in only very narrow use cases; see below.
    
    For example, the following command extracts the substitute for Emacs
    served by @code{hydra.gnu.org} to @file{/tmp/emacs}:
    
    @example
    $ wget -O - \
      https://hydra.gnu.org/nar/@dots{}-emacs-24.5 \
      | bunzip2 | guix archive -x /tmp/emacs
    @end example
    
    Single-item archives are different from multiple-item archives produced
    by @command{guix archive --export}; they contain a single store item,
    and they do @emph{not} embed a signature.  Thus this operation does
    @emph{no} signature verification and its output should be considered
    unsafe.
    
    The primary purpose of this operation is to facilitate inspection of
    archive contents coming from possibly untrusted substitute servers.
    
    @end table
    
    @c *********************************************************************
    @include emacs.texi
    
    @c *********************************************************************
    @node Programming Interface
    @chapter Programming Interface
    
    GNU Guix provides several Scheme programming interfaces (APIs) to
    define, build, and query packages.  The first interface allows users to
    write high-level package definitions.  These definitions refer to
    familiar packaging concepts, such as the name and version of a package,
    its build system, and its dependencies.  These definitions can then be
    turned into concrete build actions.
    
    Build actions are performed by the Guix daemon, on behalf of users.  In a
    standard setup, the daemon has write access to the store---the
    @file{/gnu/store} directory---whereas users do not.  The recommended
    setup also has the daemon perform builds in chroots, under a specific
    build users, to minimize interference with the rest of the system.
    
    @cindex derivation
    Lower-level APIs are available to interact with the daemon and the
    store.  To instruct the daemon to perform a build action, users actually
    provide it with a @dfn{derivation}.  A derivation is a low-level
    representation of the build actions to be taken, and the environment in
    which they should occur---derivations are to package definitions what
    assembly is to C programs.  The term ``derivation'' comes from the fact
    that build results @emph{derive} from them.
    
    This chapter describes all these APIs in turn, starting from high-level
    package definitions.
    
    @menu
    * Defining Packages::           Defining new packages.
    * Build Systems::               Specifying how packages are built.
    * The Store::                   Manipulating the package store.
    * Derivations::                 Low-level interface to package derivations.
    * The Store Monad::             Purely functional interface to the store.
    * G-Expressions::               Manipulating build expressions.
    @end menu
    
    @node Defining Packages
    @section Defining Packages
    
    The high-level interface to package definitions is implemented in the
    @code{(guix packages)} and @code{(guix build-system)} modules.  As an
    example, the package definition, or @dfn{recipe}, for the GNU Hello
    package looks like this:
    
    @example
    (define-module (gnu packages hello)
      #:use-module (guix packages)
      #:use-module (guix download)
      #:use-module (guix build-system gnu)
      #:use-module (guix licenses)
      #:use-module (gnu packages gawk))
    
    (define-public hello
      (package
        (name "hello")
        (version "2.10")
        (source (origin
                  (method url-fetch)
                  (uri (string-append "mirror://gnu/hello/hello-" version
                                      ".tar.gz"))
                  (sha256
                   (base32
                    "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
        (build-system gnu-build-system)
        (arguments `(#:configure-flags '("--enable-silent-rules")))
        (inputs `(("gawk" ,gawk)))
        (synopsis "Hello, GNU world: An example GNU package")
        (description "Guess what GNU Hello prints!")
        (home-page "http://www.gnu.org/software/hello/")
        (license gpl3+)))
    @end example
    
    @noindent
    Without being a Scheme expert, the reader may have guessed the meaning
    of the various fields here.  This expression binds the variable
    @code{hello} to a @code{<package>} object, which is essentially a record
    (@pxref{SRFI-9, Scheme records,, guile, GNU Guile Reference Manual}).
    This package object can be inspected using procedures found in the
    @code{(guix packages)} module; for instance, @code{(package-name hello)}
    returns---surprise!---@code{"hello"}.
    
    With luck, you may be able to import part or all of the definition of
    the package you are interested in from another repository, using the
    @code{guix import} command (@pxref{Invoking guix import}).
    
    In the example above, @var{hello} is defined in a module of its own,
    @code{(gnu packages hello)}.  Technically, this is not strictly
    necessary, but it is convenient to do so: all the packages defined in
    modules under @code{(gnu packages @dots{})} are automatically known to
    the command-line tools (@pxref{Package Modules}).
    
    There are a few points worth noting in the above package definition:
    
    @itemize
    @item
    The @code{source} field of the package is an @code{<origin>} object
    (@pxref{origin Reference}, for the complete reference).
    Here, the @code{url-fetch} method from @code{(guix download)} is used,
    meaning that the source is a file to be downloaded over FTP or HTTP.
    
    The @code{mirror://gnu} prefix instructs @code{url-fetch} to use one of
    the GNU mirrors defined in @code{(guix download)}.
    
    The @code{sha256} field specifies the expected SHA256 hash of the file
    being downloaded.  It is mandatory, and allows Guix to check the
    integrity of the file.  The @code{(base32 @dots{})} form introduces the
    base32 representation of the hash.  You can obtain this information with
    @code{guix download} (@pxref{Invoking guix download}) and @code{guix
    hash} (@pxref{Invoking guix hash}).
    
    @cindex patches
    When needed, the @code{origin} form can also have a @code{patches} field
    listing patches to be applied, and a @code{snippet} field giving a
    Scheme expression to modify the source code.
    
    @item
    @cindex GNU Build System
    The @code{build-system} field specifies the procedure to build the
    package (@pxref{Build Systems}).  Here, @var{gnu-build-system}
    represents the familiar GNU Build System, where packages may be
    configured, built, and installed with the usual @code{./configure &&
    make && make check && make install} command sequence.
    
    @item
    The @code{arguments} field specifies options for the build system
    (@pxref{Build Systems}).  Here it is interpreted by
    @var{gnu-build-system} as a request run @file{configure} with the
    @code{--enable-silent-rules} flag.
    
    @item
    The @code{inputs} field specifies inputs to the build process---i.e.,
    build-time or run-time dependencies of the package.  Here, we define an
    input called @code{"gawk"} whose value is that of the @var{gawk}
    variable; @var{gawk} is itself bound to a @code{<package>} object.
    
    Note that GCC, Coreutils, Bash, and other essential tools do not need to
    be specified as inputs here.  Instead, @var{gnu-build-system} takes care
    of ensuring that they are present (@pxref{Build Systems}).
    
    However, any other dependencies need to be specified in the
    @code{inputs} field.  Any dependency not specified here will simply be
    unavailable to the build process, possibly leading to a build failure.
    @end itemize
    
    @xref{package Reference}, for a full description of possible fields.
    
    Once a package definition is in place, the
    package may actually be built using the @code{guix build} command-line
    tool (@pxref{Invoking guix build}).  You can easily jump back to the
    package definition using the @command{guix edit} command
    (@pxref{Invoking guix edit}).
    @xref{Packaging Guidelines}, for
    more information on how to test package definitions, and
    @ref{Invoking guix lint}, for information on how to check a definition
    for style conformance.
    
    Finally, updating the package definition to a new upstream version
    can be partly automated by the @command{guix refresh} command
    (@pxref{Invoking guix refresh}).
    
    Behind the scenes, a derivation corresponding to the @code{<package>}
    object is first computed by the @code{package-derivation} procedure.
    That derivation is stored in a @code{.drv} file under @file{/gnu/store}.
    The build actions it prescribes may then be realized by using the
    @code{build-derivations} procedure (@pxref{The Store}).
    
    @deffn {Scheme Procedure} package-derivation @var{store} @var{package} [@var{system}]
    Return the @code{<derivation>} object of @var{package} for @var{system}
    (@pxref{Derivations}).
    
    @var{package} must be a valid @code{<package>} object, and @var{system}
    must be a string denoting the target system type---e.g.,
    @code{"x86_64-linux"} for an x86_64 Linux-based GNU system.  @var{store}
    must be a connection to the daemon, which operates on the store
    (@pxref{The Store}).
    @end deffn
    
    @noindent
    @cindex cross-compilation
    Similarly, it is possible to compute a derivation that cross-builds a
    package for some other system:
    
    @deffn {Scheme Procedure} package-cross-derivation @var{store} @
                @var{package} @var{target} [@var{system}]
    Return the @code{<derivation>} object of @var{package} cross-built from
    @var{system} to @var{target}.
    
    @var{target} must be a valid GNU triplet denoting the target hardware
    and operating system, such as @code{"mips64el-linux-gnu"}
    (@pxref{Configuration Names, GNU configuration triplets,, configure, GNU
    Configure and Build System}).
    @end deffn
    
    @menu
    * package Reference ::          The package data type.
    * origin Reference::            The origin data type.
    @end menu
    
    
    @node package Reference
    @subsection @code{package} Reference
    
    This section summarizes all the options available in @code{package}
    declarations (@pxref{Defining Packages}).
    
    @deftp {Data Type} package
    This is the data type representing a package recipe.
    
    @table @asis
    @item @code{name}
    The name of the package, as a string.
    
    @item @code{version}
    The version of the package, as a string.
    
    @item @code{source}
    An object telling how the source code for the package should be
    acquired.  Most of the time, this is an @code{origin} object, which
    denotes a file fetched from the Internet (@pxref{origin Reference}).  It
    can also be any other ``file-like'' object such as a @code{local-file},
    which denotes a file from the local file system (@pxref{G-Expressions,
    @code{local-file}}).
    
    @item @code{build-system}
    The build system that should be used to build the package (@pxref{Build
    Systems}).
    
    @item @code{arguments} (default: @code{'()})
    The arguments that should be passed to the build system.  This is a
    list, typically containing sequential keyword-value pairs.
    
    @item @code{inputs} (default: @code{'()})
    @itemx @code{native-inputs} (default: @code{'()})
    @itemx @code{propagated-inputs} (default: @code{'()})
    @cindex inputs, of packages
    These fields list dependencies of the package.  Each one is a list of
    tuples, where each tuple has a label for the input (a string) as its
    first element, a package, origin, or derivation as its second element,
    and optionally the name of the output thereof that should be used, which
    defaults to @code{"out"} (@pxref{Packages with Multiple Outputs}, for
    more on package outputs).  For example, the list below specifies three
    inputs:
    
    @example
    `(("libffi" ,libffi)
      ("libunistring" ,libunistring)
      ("glib:bin" ,glib "bin"))  ;the "bin" output of Glib
    @end example
    
    @cindex cross compilation, package dependencies
    The distinction between @code{native-inputs} and @code{inputs} is
    necessary when considering cross-compilation.  When cross-compiling,
    dependencies listed in @code{inputs} are built for the @emph{target}
    architecture; conversely, dependencies listed in @code{native-inputs}
    are built for the architecture of the @emph{build} machine.
    
    @code{native-inputs} is typically used to list tools needed at
    build time, but not at run time, such as Autoconf, Automake, pkg-config,
    Gettext, or Bison.  @command{guix lint} can report likely mistakes in
    this area (@pxref{Invoking guix lint}).
    
    @anchor{package-propagated-inputs}
    Lastly, @code{propagated-inputs} is similar to @code{inputs}, but the
    specified packages will be automatically installed alongside the package
    they belong to (@pxref{package-cmd-propagated-inputs, @command{guix
    package}}, for information on how @command{guix package} deals with
    propagated inputs.)
    
    For example this is necessary when a C/C++ library needs headers of
    another library to compile, or when a pkg-config file refers to another
    one @i{via} its @code{Requires} field.
    
    Another example where @code{propagated-inputs} is useful is for languages
    that lack a facility to record the run-time search path akin to the
    @code{RUNPATH} of ELF files; this includes Guile, Python, Perl, GHC, and
    more.  To ensure that libraries written in those languages can find
    library code they depend on at run time, run-time dependencies must be
    listed in @code{propagated-inputs} rather than @code{inputs}.
    
    @item @code{self-native-input?} (default: @code{#f})
    This is a Boolean field telling whether the package should use itself as
    a native input when cross-compiling.
    
    @item @code{outputs} (default: @code{'("out")})
    The list of output names of the package.  @xref{Packages with Multiple
    Outputs}, for typical uses of additional outputs.
    
    @item @code{native-search-paths} (default: @code{'()})
    @itemx @code{search-paths} (default: @code{'()})
    A list of @code{search-path-specification} objects describing
    search-path environment variables honored by the package.
    
    @item @code{replacement} (default: @code{#f})
    This must be either @code{#f} or a package object that will be used as a
    @dfn{replacement} for this package.  @xref{Security Updates, grafts},
    for details.
    
    @item @code{synopsis}
    A one-line description of the package.
    
    @item @code{description}
    A more elaborate description of the package.
    
    @item @code{license}
    The license of the package; a value from @code{(guix licenses)},
    or a list of such values.
    
    @item @code{home-page}
    The URL to the home-page of the package, as a string.
    
    @item @code{supported-systems} (default: @var{%supported-systems})
    The list of systems supported by the package, as strings of the form
    @code{architecture-kernel}, for example @code{"x86_64-linux"}.
    
    @item @code{maintainers} (default: @code{'()})
    The list of maintainers of the package, as @code{maintainer} objects.
    
    @item @code{location} (default: source location of the @code{package} form)
    The source location of the package.  It is useful to override this when
    inheriting from another package, in which case this field is not
    automatically corrected.
    @end table
    @end deftp
    
    
    @node origin Reference
    @subsection @code{origin} Reference
    
    This section summarizes all the options available in @code{origin}
    declarations (@pxref{Defining Packages}).
    
    @deftp {Data Type} origin
    This is the data type representing a source code origin.
    
    @table @asis
    @item @code{uri}
    An object containing the URI of the source.  The object type depends on
    the @code{method} (see below).  For example, when using the
    @var{url-fetch} method of @code{(guix download)}, the valid @code{uri}
    values are: a URL represented as a string, or a list thereof.
    
    @item @code{method}
    A procedure that handles the URI.
    
    Examples include:
    
    @table @asis
    @item @var{url-fetch} from @code{(guix download)}
    download a file from the HTTP, HTTPS, or FTP URL specified in the
    @code{uri} field;
    
    @item @var{git-fetch} from @code{(guix git-download)}
    clone the Git version control repository, and check out the revision
    specified in the @code{uri} field as a @code{git-reference} object; a
    @code{git-reference} looks like this:
    
    @example
    (git-reference
      (url "git://git.debian.org/git/pkg-shadow/shadow")
      (commit "v4.1.5.1"))
    @end example
    @end table
    
    @item @code{sha256}
    A bytevector containing the SHA-256 hash of the source.  Typically the
    @code{base32} form is used here to generate the bytevector from a
    base-32 string.
    
    @item @code{file-name} (default: @code{#f})
    The file name under which the source code should be saved.  When this is
    @code{#f}, a sensible default value will be used in most cases.  In case
    the source is fetched from a URL, the file name from the URL will be
    used.  For version control checkouts, it is recommended to provide the
    file name explicitly because the default is not very descriptive.
    
    @item @code{patches} (default: @code{'()})
    A list of file names containing patches to be applied to the source.
    
    @item @code{snippet} (default: @code{#f})
    A G-expression (@pxref{G-Expressions}) or S-expression that will be run
    in the source directory.  This is a convenient way to modify the source,
    sometimes more convenient than a patch.
    
    @item @code{patch-flags} (default: @code{'("-p1")})
    A list of command-line flags that should be passed to the @code{patch}
    command.
    
    @item @code{patch-inputs} (default: @code{#f})
    Input packages or derivations to the patching process.  When this is
    @code{#f}, the usual set of inputs necessary for patching are provided,
    such as GNU@tie{}Patch.
    
    @item @code{modules} (default: @code{'()})
    A list of Guile modules that should be loaded during the patching
    process and while running the code in the @code{snippet} field.
    
    @item @code{patch-guile} (default: @code{#f})
    The Guile package that should be used in the patching process.  When
    this is @code{#f}, a sensible default is used.
    @end table
    @end deftp
    
    
    @node Build Systems
    @section Build Systems
    
    @cindex build system
    Each package definition specifies a @dfn{build system} and arguments for
    that build system (@pxref{Defining Packages}).  This @code{build-system}
    field represents the build procedure of the package, as well as implicit
    dependencies of that build procedure.
    
    Build systems are @code{<build-system>} objects.  The interface to
    create and manipulate them is provided by the @code{(guix build-system)}
    module, and actual build systems are exported by specific modules.
    
    @cindex bag (low-level package representation)
    Under the hood, build systems first compile package objects to
    @dfn{bags}.  A @dfn{bag} is like a package, but with less
    ornamentation---in other words, a bag is a lower-level representation of
    a package, which includes all the inputs of that package, including some
    that were implicitly added by the build system.  This intermediate
    representation is then compiled to a derivation (@pxref{Derivations}).
    
    Build systems accept an optional list of @dfn{arguments}.  In package
    definitions, these are passed @i{via} the @code{arguments} field
    (@pxref{Defining Packages}).  They are typically keyword arguments
    (@pxref{Optional Arguments, keyword arguments in Guile,, guile, GNU
    Guile Reference Manual}).  The value of these arguments is usually
    evaluated in the @dfn{build stratum}---i.e., by a Guile process launched
    by the daemon (@pxref{Derivations}).
    
    The main build system is @var{gnu-build-system}, which implements the
    standard build procedure for GNU and many other packages.  It
    is provided by the @code{(guix build-system gnu)} module.
    
    @defvr {Scheme Variable} gnu-build-system
    @var{gnu-build-system} represents the GNU Build System, and variants
    thereof (@pxref{Configuration, configuration and makefile conventions,,
    standards, GNU Coding Standards}).
    
    @cindex build phases
    In a nutshell, packages using it are configured, built, and installed with
    the usual @code{./configure && make && make check && make install}
    command sequence.  In practice, a few additional steps are often needed.
    All these steps are split up in separate @dfn{phases},
    notably@footnote{Please see the @code{(guix build gnu-build-system)}
    modules for more details about the build phases.}:
    
    @table @code
    @item unpack
    Unpack the source tarball, and change the current directory to the
    extracted source tree.  If the source is actually a directory, copy it
    to the build tree, and enter that directory.
    
    @item patch-source-shebangs
    Patch shebangs encountered in source files so they refer to the right
    store file names.  For instance, this changes @code{#!/bin/sh} to
    @code{#!/gnu/store/@dots{}-bash-4.3/bin/sh}.
    
    @item configure
    Run the @file{configure} script with a number of default options, such
    as @code{--prefix=/gnu/store/@dots{}}, as well as the options specified
    by the @code{#:configure-flags} argument.
    
    @item build
    Run @code{make} with the list of flags specified with
    @code{#:make-flags}.  If the @code{#:parallel-build?} argument is true
    (the default), build with @code{make -j}.
    
    @item check
    Run @code{make check}, or some other target specified with
    @code{#:test-target}, unless @code{#:tests? #f} is passed.  If the
    @code{#:parallel-tests?} argument is true (the default), run @code{make
    check -j}.
    
    @item install
    Run @code{make install} with the flags listed in @code{#:make-flags}.
    
    @item patch-shebangs
    Patch shebangs on the installed executable files.
    
    @item strip
    Strip debugging symbols from ELF files (unless @code{#:strip-binaries?}
    is false), copying them to the @code{debug} output when available
    (@pxref{Installing Debugging Files}).
    @end table
    
    @vindex %standard-phases
    The build-side module @code{(guix build gnu-build-system)} defines
    @var{%standard-phases} as the default list of build phases.
    @var{%standard-phases} is a list of symbol/procedure pairs, where the
    procedure implements the actual phase.
    
    The list of phases used for a particular package can be changed with the
    @code{#:phases} parameter.  For instance, passing:
    
    @example
    #:phases (modify-phases %standard-phases (delete 'configure))
    @end example
    
    means that all the phases described above will be used, except the
    @code{configure} phase.
    
    In addition, this build system ensures that the ``standard'' environment
    for GNU packages is available.  This includes tools such as GCC, libc,
    Coreutils, Bash, Make, Diffutils, grep, and sed (see the @code{(guix
    build-system gnu)} module for a complete list).  We call these the
    @dfn{implicit inputs} of a package, because package definitions do not
    have to mention them.
    @end defvr
    
    Other @code{<build-system>} objects are defined to support other
    conventions and tools used by free software packages.  They inherit most
    of @var{gnu-build-system}, and differ mainly in the set of inputs
    implicitly added to the build process, and in the list of phases
    executed.  Some of these build systems are listed below.
    
    @defvr {Scheme Variable} ant-build-system
    This variable is exported by @code{(guix build-system ant)}.  It
    implements the build procedure for Java packages that can be built with
    @url{http://ant.apache.org/, Ant build tool}.
    
    It adds both @code{ant} and the @dfn{Java Development Kit} (JDK) as
    provided by the @code{icedtea} package to the set of inputs.  Different
    packages can be specified with the @code{#:ant} and @code{#:jdk}
    parameters, respectively.
    
    When the original package does not provide a suitable Ant build file,
    the parameter @code{#:jar-name} can be used to generate a minimal Ant
    build file @file{build.xml} with tasks to build the specified jar
    archive.
    
    The parameter @code{#:build-target} can be used to specify the Ant task
    that should be run during the @code{build} phase.  By default the
    ``jar'' task will be run.
    
    @end defvr
    
    @defvr {Scheme Variable} cmake-build-system
    This variable is exported by @code{(guix build-system cmake)}.  It
    implements the build procedure for packages using the
    @url{http://www.cmake.org, CMake build tool}.
    
    It automatically adds the @code{cmake} package to the set of inputs.
    Which package is used can be specified with the @code{#:cmake}
    parameter.
    
    The @code{#:configure-flags} parameter is taken as a list of flags
    passed to the @command{cmake} command.  The @code{#:build-type}
    parameter specifies in abstract terms the flags passed to the compiler;
    it defaults to @code{"RelWithDebInfo"} (short for ``release mode with
    debugging information''), which roughly means that code is compiled with
    @code{-O2 -g}, as is the case for Autoconf-based packages by default.
    @end defvr
    
    @defvr {Scheme Variable} glib-or-gtk-build-system
    This variable is exported by @code{(guix build-system glib-or-gtk)}.  It
    is intended for use with packages making use of GLib or GTK+.
    
    This build system adds the following two phases to the ones defined by
    @var{gnu-build-system}:
    
    @table @code
    @item glib-or-gtk-wrap
    The phase @code{glib-or-gtk-wrap} ensures that programs in
    @file{bin/} are able to find GLib ``schemas'' and
    @uref{https://developer.gnome.org/gtk3/stable/gtk-running.html, GTK+
    modules}.  This is achieved by wrapping the programs in launch scripts
    that appropriately set the @code{XDG_DATA_DIRS} and @code{GTK_PATH}
    environment variables.
    
    It is possible to exclude specific package outputs from that wrapping
    process by listing their names in the
    @code{#:glib-or-gtk-wrap-excluded-outputs} parameter.  This is useful
    when an output is known not to contain any GLib or GTK+ binaries, and
    where wrapping would gratuitously add a dependency of that output on
    GLib and GTK+.
    
    @item glib-or-gtk-compile-schemas
    The phase @code{glib-or-gtk-compile-schemas} makes sure that all
    @uref{https://developer.gnome.org/gio/stable/glib-compile-schemas.html,
    GSettings schemas} of GLib are compiled.  Compilation is performed by the
    @command{glib-compile-schemas} program.  It is provided by the package
    @code{glib:bin} which is automatically imported by the build system.
    The @code{glib} package providing @command{glib-compile-schemas} can be
    specified with the @code{#:glib} parameter.
    @end table
    
    Both phases are executed after the @code{install} phase.
    @end defvr
    
    @defvr {Scheme Variable} python-build-system
    This variable is exported by @code{(guix build-system python)}.  It
    implements the more or less standard build procedure used by Python
    packages, which consists in running @code{python setup.py build} and
    then @code{python setup.py install --prefix=/gnu/store/@dots{}}.
    
    For packages that install stand-alone Python programs under @code{bin/},
    it takes care of wrapping these programs so that their @code{PYTHONPATH}
    environment variable points to all the Python libraries they depend on.
    
    Which Python package is used to perform the build can be specified with
    the @code{#:python} parameter.  This is a useful way to force a package
    to be built for a specific version of the Python interpreter, which
    might be necessary if the package is only compatible with a single
    interpreter version.
    @end defvr
    
    @defvr {Scheme Variable} perl-build-system
    This variable is exported by @code{(guix build-system perl)}.  It
    implements the standard build procedure for Perl packages, which either
    consists in running @code{perl Build.PL --prefix=/gnu/store/@dots{}},
    followed by @code{Build} and @code{Build install}; or in running
    @code{perl Makefile.PL PREFIX=/gnu/store/@dots{}}, followed by
    @code{make} and @code{make install}, depending on which of
    @code{Build.PL} or @code{Makefile.PL} is present in the package
    distribution.  Preference is given to the former if both @code{Build.PL}
    and @code{Makefile.PL} exist in the package distribution.  This
    preference can be reversed by specifying @code{#t} for the
    @code{#:make-maker?} parameter.
    
    The initial @code{perl Makefile.PL} or @code{perl Build.PL} invocation
    passes flags specified by the @code{#:make-maker-flags} or
    @code{#:module-build-flags} parameter, respectively.
    
    Which Perl package is used can be specified with @code{#:perl}.
    @end defvr
    
    @defvr {Scheme Variable} r-build-system
    This variable is exported by @code{(guix build-system r)}.  It
    implements the build procedure used by @uref{http://r-project.org, R}
    packages, which essentially is little more than running @code{R CMD
    INSTALL --library=/gnu/store/@dots{}} in an environment where
    @code{R_LIBS_SITE} contains the paths to all R package inputs.  Tests
    are run after installation using the R function
    @code{tools::testInstalledPackage}.
    @end defvr
    
    @defvr {Scheme Variable} ruby-build-system
    This variable is exported by @code{(guix build-system ruby)}.  It
    implements the RubyGems build procedure used by Ruby packages, which
    involves running @code{gem build} followed by @code{gem install}.
    
    The @code{source} field of a package that uses this build system
    typically references a gem archive, since this is the format that Ruby
    developers use when releasing their software.  The build system unpacks
    the gem archive, potentially patches the source, runs the test suite,
    repackages the gem, and installs it.  Additionally, directories and
    tarballs may be referenced to allow building unreleased gems from Git or
    a traditional source release tarball.
    
    Which Ruby package is used can be specified with the @code{#:ruby}
    parameter.  A list of additional flags to be passed to the @command{gem}
    command can be specified with the @code{#:gem-flags} parameter.
    @end defvr
    
    @defvr {Scheme Variable} waf-build-system
    This variable is exported by @code{(guix build-system waf)}.  It
    implements a build procedure around the @code{waf} script.  The common
    phases---@code{configure}, @code{build}, and @code{install}---are
    implemented by passing their names as arguments to the @code{waf}
    script.
    
    The @code{waf} script is executed by the Python interpreter.  Which
    Python package is used to run the script can be specified with the
    @code{#:python} parameter.
    @end defvr
    
    @defvr {Scheme Variable} haskell-build-system
    This variable is exported by @code{(guix build-system haskell)}.  It
    implements the Cabal build procedure used by Haskell packages, which
    involves running @code{runhaskell Setup.hs configure
    --prefix=/gnu/store/@dots{}} and @code{runhaskell Setup.hs build}.
    Instead of installing the package by running @code{runhaskell Setup.hs
    install}, to avoid trying to register libraries in the read-only
    compiler store directory, the build system uses @code{runhaskell
    Setup.hs copy}, followed by @code{runhaskell Setup.hs register}.  In
    addition, the build system generates the package documentation by
    running @code{runhaskell Setup.hs haddock}, unless @code{#:haddock? #f}
    is passed.  Optional Haddock parameters can be passed with the help of
    the @code{#:haddock-flags} parameter.  If the file @code{Setup.hs} is
    not found, the build system looks for @code{Setup.lhs} instead.
    
    Which Haskell compiler is used can be specified with the @code{#:haskell}
    parameter which defaults to @code{ghc}.
    @end defvr
    
    @defvr {Scheme Variable} emacs-build-system
    This variable is exported by @code{(guix build-system emacs)}.  It
    implements an installation procedure similar to the packaging system
    of Emacs itself (@pxref{Packages,,, emacs, The GNU Emacs Manual}).
    
    It first creates the @code{@var{package}-autoloads.el} file, then it
    byte compiles all Emacs Lisp files.  Differently from the Emacs
    packaging system, the Info documentation files are moved to the standard
    documentation directory and the @file{dir} file is deleted.  Each
    package is installed in its own directory under
    @file{share/emacs/site-lisp/guix.d}.
    @end defvr
    
    Lastly, for packages that do not need anything as sophisticated, a
    ``trivial'' build system is provided.  It is trivial in the sense that
    it provides basically no support: it does not pull any implicit inputs,
    and does not have a notion of build phases.
    
    @defvr {Scheme Variable} trivial-build-system
    This variable is exported by @code{(guix build-system trivial)}.
    
    This build system requires a @code{#:builder} argument.  This argument
    must be a Scheme expression that builds the package output(s)---as
    with @code{build-expression->derivation} (@pxref{Derivations,
    @code{build-expression->derivation}}).
    @end defvr
    
    @node The Store
    @section The Store
    
    @cindex store
    @cindex store items
    @cindex store paths
    
    Conceptually, the @dfn{store} is the place where derivations that have
    been built successfully are stored---by default, @file{/gnu/store}.
    Sub-directories in the store are referred to as @dfn{store items} or
    sometimes @dfn{store paths}.  The store has an associated database that
    contains information such as the store paths referred to by each store
    path, and the list of @emph{valid} store items---results of successful
    builds.  This database resides in @file{@var{localstatedir}/guix/db},
    where @var{localstatedir} is the state directory specified @i{via}
    @option{--localstatedir} at configure time, usually @file{/var}.
    
    The store is @emph{always} accessed by the daemon on behalf of its clients
    (@pxref{Invoking guix-daemon}).  To manipulate the store, clients
    connect to the daemon over a Unix-domain socket, send requests to it,
    and read the result---these are remote procedure calls, or RPCs.
    
    @quotation Note
    Users must @emph{never} modify files under @file{/gnu/store} directly.
    This would lead to inconsistencies and break the immutability
    assumptions of Guix's functional model (@pxref{Introduction}).
    
    @xref{Invoking guix gc, @command{guix gc --verify}}, for information on
    how to check the integrity of the store and attempt recovery from
    accidental modifications.
    @end quotation
    
    The @code{(guix store)} module provides procedures to connect to the
    daemon, and to perform RPCs.  These are described below.
    
    @deffn {Scheme Procedure} open-connection [@var{file}] [#:reserve-space? #t]
    Connect to the daemon over the Unix-domain socket at @var{file}.  When
    @var{reserve-space?} is true, instruct it to reserve a little bit of
    extra space on the file system so that the garbage collector can still
    operate should the disk become full.  Return a server object.
    
    @var{file} defaults to @var{%default-socket-path}, which is the normal
    location given the options that were passed to @command{configure}.
    @end deffn
    
    @deffn {Scheme Procedure} close-connection @var{server}
    Close the connection to @var{server}.
    @end deffn
    
    @defvr {Scheme Variable} current-build-output-port
    This variable is bound to a SRFI-39 parameter, which refers to the port
    where build and error logs sent by the daemon should be written.
    @end defvr
    
    Procedures that make RPCs all take a server object as their first
    argument.
    
    @deffn {Scheme Procedure} valid-path? @var{server} @var{path}
    @cindex invalid store items
    Return @code{#t} when @var{path} designates a valid store item and
    @code{#f} otherwise (an invalid item may exist on disk but still be
    invalid, for instance because it is the result of an aborted or failed
    build.)
    
    A @code{&nix-protocol-error} condition is raised if @var{path} is not
    prefixed by the store directory (@file{/gnu/store}).
    @end deffn
    
    @deffn {Scheme Procedure} add-text-to-store @var{server} @var{name} @var{text} [@var{references}]
    Add @var{text} under file @var{name} in the store, and return its store
    path.  @var{references} is the list of store paths referred to by the
    resulting store path.
    @end deffn
    
    @deffn {Scheme Procedure} build-derivations @var{server} @var{derivations}
    Build @var{derivations} (a list of @code{<derivation>} objects or
    derivation paths), and return when the worker is done building them.
    Return @code{#t} on success.
    @end deffn
    
    Note that the @code{(guix monads)} module provides a monad as well as
    monadic versions of the above procedures, with the goal of making it
    more convenient to work with code that accesses the store (@pxref{The
    Store Monad}).
    
    @c FIXME
    @i{This section is currently incomplete.}
    
    @node Derivations
    @section Derivations
    
    @cindex derivations
    Low-level build actions and the environment in which they are performed
    are represented by @dfn{derivations}.  A derivation contains the
    following pieces of information:
    
    @itemize
    @item
    The outputs of the derivation---derivations produce at least one file or
    directory in the store, but may produce more.
    
    @item
    The inputs of the derivations, which may be other derivations or plain
    files in the store (patches, build scripts, etc.)
    
    @item
    The system type targeted by the derivation---e.g., @code{x86_64-linux}.
    
    @item
    The file name of a build script in the store, along with the arguments
    to be passed.
    
    @item
    A list of environment variables to be defined.
    
    @end itemize
    
    @cindex derivation path
    Derivations allow clients of the daemon to communicate build actions to
    the store.  They exist in two forms: as an in-memory representation,
    both on the client- and daemon-side, and as files in the store whose
    name end in @code{.drv}---these files are referred to as @dfn{derivation
    paths}.  Derivations paths can be passed to the @code{build-derivations}
    procedure to perform the build actions they prescribe (@pxref{The
    Store}).
    
    The @code{(guix derivations)} module provides a representation of
    derivations as Scheme objects, along with procedures to create and
    otherwise manipulate derivations.  The lowest-level primitive to create
    a derivation is the @code{derivation} procedure:
    
    @deffn {Scheme Procedure} derivation @var{store} @var{name} @var{builder} @
      @var{args} [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
      [#:recursive? #f] [#:inputs '()] [#:env-vars '()] @
      [#:system (%current-system)] [#:references-graphs #f] @
      [#:allowed-references #f] [#:disallowed-references #f] @
      [#:leaked-env-vars #f] [#:local-build? #f] @
      [#:substitutable? #t]
    Build a derivation with the given arguments, and return the resulting
    @code{<derivation>} object.
    
    When @var{hash} and @var{hash-algo} are given, a
    @dfn{fixed-output derivation} is created---i.e., one whose result is
    known in advance, such as a file download.  If, in addition,
    @var{recursive?} is true, then that fixed output may be an executable
    file or a directory and @var{hash} must be the hash of an archive
    containing this output.
    
    When @var{references-graphs} is true, it must be a list of file
    name/store path pairs.  In that case, the reference graph of each store
    path is exported in the build environment in the corresponding file, in
    a simple text format.
    
    When @var{allowed-references} is true, it must be a list of store items
    or outputs that the derivation's output may refer to.  Likewise,
    @var{disallowed-references}, if true, must be a list of things the
    outputs may @emph{not} refer to.
    
    When @var{leaked-env-vars} is true, it must be a list of strings
    denoting environment variables that are allowed to ``leak'' from the
    daemon's environment to the build environment.  This is only applicable
    to fixed-output derivations---i.e., when @var{hash} is true.  The main
    use is to allow variables such as @code{http_proxy} to be passed to
    derivations that download files.
    
    When @var{local-build?} is true, declare that the derivation is not a
    good candidate for offloading and should rather be built locally
    (@pxref{Daemon Offload Setup}).  This is the case for small derivations
    where the costs of data transfers would outweigh the benefits.
    
    When @var{substitutable?} is false, declare that substitutes of the
    derivation's output should not be used (@pxref{Substitutes}).  This is
    useful, for instance, when building packages that capture details of the
    host CPU instruction set.
    @end deffn
    
    @noindent
    Here's an example with a shell script as its builder, assuming
    @var{store} is an open connection to the daemon, and @var{bash} points
    to a Bash executable in the store:
    
    @lisp
    (use-modules (guix utils)
                 (guix store)
                 (guix derivations))
    
    (let ((builder   ; add the Bash script to the store
            (add-text-to-store store "my-builder.sh"
                               "echo hello world > $out\n" '())))
      (derivation store "foo"
                  bash `("-e" ,builder)
                  #:inputs `((,bash) (,builder))
                  #:env-vars '(("HOME" . "/homeless"))))
    @result{} #<derivation /gnu/store/@dots{}-foo.drv => /gnu/store/@dots{}-foo>
    @end lisp
    
    As can be guessed, this primitive is cumbersome to use directly.  A
    better approach is to write build scripts in Scheme, of course!  The
    best course of action for that is to write the build code as a
    ``G-expression'', and to pass it to @code{gexp->derivation}.  For more
    information, @pxref{G-Expressions}.
    
    Once upon a time, @code{gexp->derivation} did not exist and constructing
    derivations with build code written in Scheme was achieved with
    @code{build-expression->derivation}, documented below.  This procedure
    is now deprecated in favor of the much nicer @code{gexp->derivation}.
    
    @deffn {Scheme Procedure} build-expression->derivation @var{store} @
           @var{name} @var{exp} @
           [#:system (%current-system)] [#:inputs '()] @
           [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
           [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
           [#:references-graphs #f] [#:allowed-references #f] @
           [#:disallowed-references #f] @
           [#:local-build? #f] [#:substitutable? #t] [#:guile-for-build #f]
    Return a derivation that executes Scheme expression @var{exp} as a
    builder for derivation @var{name}.  @var{inputs} must be a list of
    @code{(name drv-path sub-drv)} tuples; when @var{sub-drv} is omitted,
    @code{"out"} is assumed.  @var{modules} is a list of names of Guile
    modules from the current search path to be copied in the store,
    compiled, and made available in the load path during the execution of
    @var{exp}---e.g., @code{((guix build utils) (guix build
    gnu-build-system))}.
    
    @var{exp} is evaluated in an environment where @code{%outputs} is bound
    to a list of output/path pairs, and where @code{%build-inputs} is bound
    to a list of string/output-path pairs made from @var{inputs}.
    Optionally, @var{env-vars} is a list of string pairs specifying the name
    and value of environment variables visible to the builder.  The builder
    terminates by passing the result of @var{exp} to @code{exit}; thus, when
    @var{exp} returns @code{#f}, the build is considered to have failed.
    
    @var{exp} is built using @var{guile-for-build} (a derivation).  When
    @var{guile-for-build} is omitted or is @code{#f}, the value of the
    @code{%guile-for-build} fluid is used instead.
    
    See the @code{derivation} procedure for the meaning of
    @var{references-graphs}, @var{allowed-references},
    @var{disallowed-references}, @var{local-build?}, and
    @var{substitutable?}.
    @end deffn
    
    @noindent
    Here's an example of a single-output derivation that creates a directory
    containing one file:
    
    @lisp
    (let ((builder '(let ((out (assoc-ref %outputs "out")))
                      (mkdir out)    ; create /gnu/store/@dots{}-goo
                      (call-with-output-file (string-append out "/test")
                        (lambda (p)
                          (display '(hello guix) p))))))
      (build-expression->derivation store "goo" builder))
    
    @result{} #<derivation /gnu/store/@dots{}-goo.drv => @dots{}>
    @end lisp
    
    
    @node The Store Monad
    @section The Store Monad
    
    @cindex monad
    
    The procedures that operate on the store described in the previous
    sections all take an open connection to the build daemon as their first
    argument.  Although the underlying model is functional, they either have
    side effects or depend on the current state of the store.
    
    The former is inconvenient: the connection to the build daemon has to be
    carried around in all those functions, making it impossible to compose
    functions that do not take that parameter with functions that do.  The
    latter can be problematic: since store operations have side effects
    and/or depend on external state, they have to be properly sequenced.
    
    @cindex monadic values
    @cindex monadic functions
    This is where the @code{(guix monads)} module comes in.  This module
    provides a framework for working with @dfn{monads}, and a particularly
    useful monad for our uses, the @dfn{store monad}.  Monads are a
    construct that allows two things: associating ``context'' with values
    (in our case, the context is the store), and building sequences of
    computations (here computations include accesses to the store).  Values
    in a monad---values that carry this additional context---are called
    @dfn{monadic values}; procedures that return such values are called
    @dfn{monadic procedures}.
    
    Consider this ``normal'' procedure:
    
    @example
    (define (sh-symlink store)
      ;; Return a derivation that symlinks the 'bash' executable.
      (let* ((drv (package-derivation store bash))
             (out (derivation->output-path drv))
             (sh  (string-append out "/bin/bash")))
        (build-expression->derivation store "sh"
                                      `(symlink ,sh %output))))
    @end example
    
    Using @code{(guix monads)} and @code{(guix gexp)}, it may be rewritten
    as a monadic function:
    
    @example
    (define (sh-symlink)
      ;; Same, but return a monadic value.
      (mlet %store-monad ((drv (package->derivation bash)))
        (gexp->derivation "sh"
                          #~(symlink (string-append #$drv "/bin/bash")
                                     #$output))))
    @end example
    
    There are several things to note in the second version: the @code{store}
    parameter is now implicit and is ``threaded'' in the calls to the
    @code{package->derivation} and @code{gexp->derivation} monadic
    procedures, and the monadic value returned by @code{package->derivation}
    is @dfn{bound} using @code{mlet} instead of plain @code{let}.
    
    As it turns out, the call to @code{package->derivation} can even be
    omitted since it will take place implicitly, as we will see later
    (@pxref{G-Expressions}):
    
    @example
    (define (sh-symlink)
      (gexp->derivation "sh"
                        #~(symlink (string-append #$bash "/bin/bash")
                                   #$output)))
    @end example
    
    @c See
    @c <https://syntaxexclamation.wordpress.com/2014/06/26/escaping-continuations/>
    @c for the funny quote.
    Calling the monadic @code{sh-symlink} has no effect.  As someone once
    said, ``you exit a monad like you exit a building on fire: by running''.
    So, to exit the monad and get the desired effect, one must use
    @code{run-with-store}:
    
    @example
    (run-with-store (open-connection) (sh-symlink))
    @result{} /gnu/store/...-sh-symlink
    @end example
    
    Note that the @code{(guix monad-repl)} module extends the Guile REPL with
    new ``meta-commands'' to make it easier to deal with monadic procedures:
    @code{run-in-store}, and @code{enter-store-monad}.  The former is used
    to ``run'' a single monadic value through the store:
    
    @example
    scheme@@(guile-user)> ,run-in-store (package->derivation hello)
    $1 = #<derivation /gnu/store/@dots{}-hello-2.9.drv => @dots{}>
    @end example
    
    The latter enters a recursive REPL, where all the return values are
    automatically run through the store:
    
    @example
    scheme@@(guile-user)> ,enter-store-monad
    store-monad@@(guile-user) [1]> (package->derivation hello)
    $2 = #<derivation /gnu/store/@dots{}-hello-2.9.drv => @dots{}>
    store-monad@@(guile-user) [1]> (text-file "foo" "Hello!")
    $3 = "/gnu/store/@dots{}-foo"
    store-monad@@(guile-user) [1]> ,q
    scheme@@(guile-user)>
    @end example
    
    @noindent
    Note that non-monadic values cannot be returned in the
    @code{store-monad} REPL.
    
    The main syntactic forms to deal with monads in general are provided by
    the @code{(guix monads)} module and are described below.
    
    @deffn {Scheme Syntax} with-monad @var{monad} @var{body} ...
    Evaluate any @code{>>=} or @code{return} forms in @var{body} as being
    in @var{monad}.
    @end deffn
    
    @deffn {Scheme Syntax} return @var{val}
    Return a monadic value that encapsulates @var{val}.
    @end deffn
    
    @deffn {Scheme Syntax} >>= @var{mval} @var{mproc} ...
    @dfn{Bind} monadic value @var{mval}, passing its ``contents'' to monadic
    procedures @var{mproc}@dots{}@footnote{This operation is commonly
    referred to as ``bind'', but that name denotes an unrelated procedure in
    Guile.  Thus we use this somewhat cryptic symbol inherited from the
    Haskell language.}.  There can be one @var{mproc} or several of them, as
    in this example:
    
    @example
    (run-with-state
        (with-monad %state-monad
          (>>= (return 1)
               (lambda (x) (return (+ 1 x)))
               (lambda (x) (return (* 2 x)))))
      'some-state)
    
    @result{} 4
    @result{} some-state
    @end example
    @end deffn
    
    @deffn {Scheme Syntax} mlet @var{monad} ((@var{var} @var{mval}) ...) @
           @var{body} ...
    @deffnx {Scheme Syntax} mlet* @var{monad} ((@var{var} @var{mval}) ...) @
           @var{body} ...
    Bind the variables @var{var} to the monadic values @var{mval} in
    @var{body}.  The form (@var{var} -> @var{val}) binds @var{var} to the
    ``normal'' value @var{val}, as per @code{let}.
    
    @code{mlet*} is to @code{mlet} what @code{let*} is to @code{let}
    (@pxref{Local Bindings,,, guile, GNU Guile Reference Manual}).
    @end deffn
    
    @deffn {Scheme System} mbegin @var{monad} @var{mexp} ...
    Bind @var{mexp} and the following monadic expressions in sequence,
    returning the result of the last expression.
    
    This is akin to @code{mlet}, except that the return values of the
    monadic expressions are ignored.  In that sense, it is analogous to
    @code{begin}, but applied to monadic expressions.
    @end deffn
    
    @cindex state monad
    The @code{(guix monads)} module provides the @dfn{state monad}, which
    allows an additional value---the state---to be @emph{threaded} through
    monadic procedure calls.
    
    @defvr {Scheme Variable} %state-monad
    The state monad.  Procedures in the state monad can access and change
    the state that is threaded.
    
    Consider the example below.  The @code{square} procedure returns a value
    in the state monad.  It returns the square of its argument, but also
    increments the current state value:
    
    @example
    (define (square x)
      (mlet %state-monad ((count (current-state)))
        (mbegin %state-monad
          (set-current-state (+ 1 count))
          (return (* x x)))))
    
    (run-with-state (sequence %state-monad (map square (iota 3))) 0)
    @result{} (0 1 4)
    @result{} 3
    @end example
    
    When ``run'' through @var{%state-monad}, we obtain that additional state
    value, which is the number of @code{square} calls.
    @end defvr
    
    @deffn {Monadic Procedure} current-state
    Return the current state as a monadic value.
    @end deffn
    
    @deffn {Monadic Procedure} set-current-state @var{value}
    Set the current state to @var{value} and return the previous state as a
    monadic value.
    @end deffn
    
    @deffn {Monadic Procedure} state-push @var{value}
    Push @var{value} to the current state, which is assumed to be a list,
    and return the previous state as a monadic value.
    @end deffn
    
    @deffn {Monadic Procedure} state-pop
    Pop a value from the current state and return it as a monadic value.
    The state is assumed to be a list.
    @end deffn
    
    @deffn {Scheme Procedure} run-with-state @var{mval} [@var{state}]
    Run monadic value @var{mval} starting with @var{state} as the initial
    state.  Return two values: the resulting value, and the resulting state.
    @end deffn
    
    The main interface to the store monad, provided by the @code{(guix
    store)} module, is as follows.
    
    @defvr {Scheme Variable} %store-monad
    The store monad---an alias for @var{%state-monad}.
    
    Values in the store monad encapsulate accesses to the store.  When its
    effect is needed, a value of the store monad must be ``evaluated'' by
    passing it to the @code{run-with-store} procedure (see below.)
    @end defvr
    
    @deffn {Scheme Procedure} run-with-store @var{store} @var{mval} [#:guile-for-build] [#:system (%current-system)]
    Run @var{mval}, a monadic value in the store monad, in @var{store}, an
    open store connection.
    @end deffn
    
    @deffn {Monadic Procedure} text-file @var{name} @var{text} [@var{references}]
    Return as a monadic value the absolute file name in the store of the file
    containing @var{text}, a string.  @var{references} is a list of store items that the
    resulting text file refers to; it defaults to the empty list.
    @end deffn
    
    @deffn {Monadic Procedure} interned-file @var{file} [@var{name}] @
             [#:recursive? #t] [#:select? (const #t)]
    Return the name of @var{file} once interned in the store.  Use
    @var{name} as its store name, or the basename of @var{file} if
    @var{name} is omitted.
    
    When @var{recursive?} is true, the contents of @var{file} are added
    recursively; if @var{file} designates a flat file and @var{recursive?}
    is true, its contents are added, and its permission bits are kept.
    
    When @var{recursive?} is true, call @code{(@var{select?} @var{file}
    @var{stat})} for each directory entry, where @var{file} is the entry's
    absolute file name and @var{stat} is the result of @code{lstat}; exclude
    entries for which @var{select?} does not return true.
    
    The example below adds a file to the store, under two different names:
    
    @example
    (run-with-store (open-connection)
      (mlet %store-monad ((a (interned-file "README"))
                          (b (interned-file "README" "LEGU-MIN")))
        (return (list a b))))
    
    @result{} ("/gnu/store/rwm@dots{}-README" "/gnu/store/44i@dots{}-LEGU-MIN")
    @end example
    
    @end deffn
    
    The @code{(guix packages)} module exports the following package-related
    monadic procedures:
    
    @deffn {Monadic Procedure} package-file @var{package} [@var{file}] @
           [#:system (%current-system)] [#:target #f] @
           [#:output "out"]
    Return as a monadic
    value in the absolute file name of @var{file} within the @var{output}
    directory of @var{package}.  When @var{file} is omitted, return the name
    of the @var{output} directory of @var{package}.  When @var{target} is
    true, use it as a cross-compilation target triplet.
    @end deffn
    
    @deffn {Monadic Procedure} package->derivation @var{package} [@var{system}]
    @deffnx {Monadic Procedure} package->cross-derivation @var{package} @
              @var{target} [@var{system}]
    Monadic version of @code{package-derivation} and
    @code{package-cross-derivation} (@pxref{Defining Packages}).
    @end deffn
    
    
    @node G-Expressions
    @section G-Expressions
    
    @cindex G-expression
    @cindex build code quoting
    So we have ``derivations'', which represent a sequence of build actions
    to be performed to produce an item in the store (@pxref{Derivations}).
    These build actions are performed when asking the daemon to actually
    build the derivations; they are run by the daemon in a container
    (@pxref{Invoking guix-daemon}).
    
    @cindex strata of code
    It should come as no surprise that we like to write these build actions
    in Scheme.  When we do that, we end up with two @dfn{strata} of Scheme
    code@footnote{The term @dfn{stratum} in this context was coined by
    Manuel Serrano et al.@: in the context of their work on Hop.  Oleg
    Kiselyov, who has written insightful
    @url{http://okmij.org/ftp/meta-programming/#meta-scheme, essays and code
    on this topic}, refers to this kind of code generation as
    @dfn{staging}.}: the ``host code''---code that defines packages, talks
    to the daemon, etc.---and the ``build code''---code that actually
    performs build actions, such as making directories, invoking
    @command{make}, etc.
    
    To describe a derivation and its build actions, one typically needs to
    embed build code inside host code.  It boils down to manipulating build
    code as data, and the homoiconicity of Scheme---code has a direct
    representation as data---comes in handy for that.  But we need more than
    the normal @code{quasiquote} mechanism in Scheme to construct build
    expressions.
    
    The @code{(guix gexp)} module implements @dfn{G-expressions}, a form of
    S-expressions adapted to build expressions.  G-expressions, or
    @dfn{gexps}, consist essentially of three syntactic forms: @code{gexp},
    @code{ungexp}, and @code{ungexp-splicing} (or simply: @code{#~},
    @code{#$}, and @code{#$@@}), which are comparable to
    @code{quasiquote}, @code{unquote}, and @code{unquote-splicing},
    respectively (@pxref{Expression Syntax, @code{quasiquote},, guile,
    GNU Guile Reference Manual}).  However, there are major differences:
    
    @itemize
    @item
    Gexps are meant to be written to a file and run or manipulated by other
    processes.
    
    @item
    When a high-level object such as a package or derivation is unquoted
    inside a gexp, the result is as if its output file name had been
    introduced.
    
    @item
    Gexps carry information about the packages or derivations they refer to,
    and these dependencies are automatically added as inputs to the build
    processes that use them.
    @end itemize
    
    @cindex lowering, of high-level objects in gexps
    This mechanism is not limited to package and derivation
    objects: @dfn{compilers} able to ``lower'' other high-level objects to
    derivations or files in the store can be defined,
    such that these objects can also be inserted
    into gexps.  For example, a useful type of high-level objects that can be
    inserted in a gexp is ``file-like objects'', which make it easy to
    add files to the store and to refer to them in
    derivations and such (see @code{local-file} and @code{plain-file}
    below.)
    
    To illustrate the idea, here is an example of a gexp:
    
    @example
    (define build-exp
      #~(begin
          (mkdir #$output)
          (chdir #$output)
          (symlink (string-append #$coreutils "/bin/ls")
                   "list-files")))
    @end example
    
    This gexp can be passed to @code{gexp->derivation}; we obtain a
    derivation that builds a directory containing exactly one symlink to
    @file{/gnu/store/@dots{}-coreutils-8.22/bin/ls}:
    
    @example
    (gexp->derivation "the-thing" build-exp)
    @end example
    
    As one would expect, the @code{"/gnu/store/@dots{}-coreutils-8.22"} string is
    substituted to the reference to the @var{coreutils} package in the
    actual build code, and @var{coreutils} is automatically made an input to
    the derivation.  Likewise, @code{#$output} (equivalent to @code{(ungexp
    output)}) is replaced by a string containing the directory name of the
    output of the derivation.
    
    @cindex cross compilation
    In a cross-compilation context, it is useful to distinguish between
    references to the @emph{native} build of a package---that can run on the
    host---versus references to cross builds of a package.  To that end, the
    @code{#+} plays the same role as @code{#$}, but is a reference to a
    native package build:
    
    @example
    (gexp->derivation "vi"
       #~(begin
           (mkdir #$output)
           (system* (string-append #+coreutils "/bin/ln")
                    "-s"
                    (string-append #$emacs "/bin/emacs")
                    (string-append #$output "/bin/vi")))
       #:target "mips64el-linux")
    @end example
    
    @noindent
    In the example above, the native build of @var{coreutils} is used, so
    that @command{ln} can actually run on the host; but then the
    cross-compiled build of @var{emacs} is referenced.
    
    @cindex imported modules, for gexps
    @findex with-imported-modules
    Another gexp feature is @dfn{imported modules}: sometimes you want to be
    able to use certain Guile modules from the ``host environment'' in the
    gexp, so those modules should be imported in the ``build environment''.
    The @code{with-imported-modules} form allows you to express that:
    
    @example
    (let ((build (with-imported-modules '((guix build utils))
                   #~(begin
                       (use-modules (guix build utils))
                       (mkdir-p (string-append #$output "/bin"))))))
      (gexp->derivation "empty-dir"
                        #~(begin
                            #$build
                            (display "success!\n")
                            #t)))
    @end example
    
    @noindent
    In this example, the @code{(guix build utils)} module is automatically
    pulled into the isolated build environment of our gexp, such that
    @code{(use-modules (guix build utils))} works as expected.
    
    The syntactic form to construct gexps is summarized below.
    
    @deffn {Scheme Syntax} #~@var{exp}
    @deffnx {Scheme Syntax} (gexp @var{exp})
    Return a G-expression containing @var{exp}.  @var{exp} may contain one
    or more of the following forms:
    
    @table @code
    @item #$@var{obj}
    @itemx (ungexp @var{obj})
    Introduce a reference to @var{obj}.  @var{obj} may have one of the
    supported types, for example a package or a
    derivation, in which case the @code{ungexp} form is replaced by its
    output file name---e.g., @code{"/gnu/store/@dots{}-coreutils-8.22}.
    
    If @var{obj} is a list, it is traversed and references to supported
    objects are substituted similarly.
    
    If @var{obj} is another gexp, its contents are inserted and its
    dependencies are added to those of the containing gexp.
    
    If @var{obj} is another kind of object, it is inserted as is.
    
    @item #$@var{obj}:@var{output}
    @itemx (ungexp @var{obj} @var{output})
    This is like the form above, but referring explicitly to the
    @var{output} of @var{obj}---this is useful when @var{obj} produces
    multiple outputs (@pxref{Packages with Multiple Outputs}).
    
    @item #+@var{obj}
    @itemx #+@var{obj}:output
    @itemx (ungexp-native @var{obj})
    @itemx (ungexp-native @var{obj} @var{output})
    Same as @code{ungexp}, but produces a reference to the @emph{native}
    build of @var{obj} when used in a cross compilation context.
    
    @item #$output[:@var{output}]
    @itemx (ungexp output [@var{output}])
    Insert a reference to derivation output @var{output}, or to the main
    output when @var{output} is omitted.
    
    This only makes sense for gexps passed to @code{gexp->derivation}.
    
    @item #$@@@var{lst}
    @itemx (ungexp-splicing @var{lst})
    Like the above, but splices the contents of @var{lst} inside the
    containing list.
    
    @item #+@@@var{lst}
    @itemx (ungexp-native-splicing @var{lst})
    Like the above, but refers to native builds of the objects listed in
    @var{lst}.
    
    @end table
    
    G-expressions created by @code{gexp} or @code{#~} are run-time objects
    of the @code{gexp?} type (see below.)
    @end deffn
    
    @deffn {Scheme Syntax} with-imported-modules @var{modules} @var{body}@dots{}
    Mark the gexps defined in @var{body}@dots{} as requiring @var{modules}
    in their execution environment.  @var{modules} must be a list of Guile
    module names, such as @code{'((guix build utils) (guix build gremlin))}.
    
    This form has @emph{lexical} scope: it has an effect on the gexps
    directly defined in @var{body}@dots{}, but not on those defined, say, in
    procedures called from @var{body}@dots{}.
    @end deffn
    
    @deffn {Scheme Procedure} gexp? @var{obj}
    Return @code{#t} if @var{obj} is a G-expression.
    @end deffn
    
    G-expressions are meant to be written to disk, either as code building
    some derivation, or as plain files in the store.  The monadic procedures
    below allow you to do that (@pxref{The Store Monad}, for more
    information about monads.)
    
    @deffn {Monadic Procedure} gexp->derivation @var{name} @var{exp} @
           [#:system (%current-system)] [#:target #f] [#:graft? #t] @
           [#:hash #f] [#:hash-algo #f] @
           [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
           [#:module-path @var{%load-path}] @
           [#:references-graphs #f] [#:allowed-references #f] @
           [#:disallowed-references #f] @
           [#:leaked-env-vars #f] @
           [#:script-name (string-append @var{name} "-builder")] @
           [#:local-build? #f] [#:substitutable? #t] [#:guile-for-build #f]
    Return a derivation @var{name} that runs @var{exp} (a gexp) with
    @var{guile-for-build} (a derivation) on @var{system}; @var{exp} is
    stored in a file called @var{script-name}.  When @var{target} is true,
    it is used as the cross-compilation target triplet for packages referred
    to by @var{exp}.
    
    @var{modules} is deprecated in favor of @code{with-imported-modules}.
    Its meaning is to
    make @var{modules} available in the evaluation context of @var{exp};
    @var{modules} is a list of names of Guile modules searched in
    @var{module-path} to be copied in the store, compiled, and made available in
    the load path during the execution of @var{exp}---e.g., @code{((guix
    build utils) (guix build gnu-build-system))}.
    
    @var{graft?} determines whether packages referred to by @var{exp} should be grafted when
    applicable.
    
    When @var{references-graphs} is true, it must be a list of tuples of one of the
    following forms:
    
    @example
    (@var{file-name} @var{package})
    (@var{file-name} @var{package} @var{output})
    (@var{file-name} @var{derivation})
    (@var{file-name} @var{derivation} @var{output})
    (@var{file-name} @var{store-item})
    @end example
    
    The right-hand-side of each element of @var{references-graphs} is automatically made
    an input of the build process of @var{exp}.  In the build environment, each
    @var{file-name} contains the reference graph of the corresponding item, in a simple
    text format.
    
    @var{allowed-references} must be either @code{#f} or a list of output names and packages.
    In the latter case, the list denotes store items that the result is allowed to
    refer to.  Any reference to another store item will lead to a build error.
    Similarly for @var{disallowed-references}, which can list items that must not be
    referenced by the outputs.
    
    The other arguments are as for @code{derivation} (@pxref{Derivations}).
    @end deffn
    
    @cindex file-like objects
    The @code{local-file}, @code{plain-file}, @code{computed-file},
    @code{program-file}, and @code{scheme-file} procedures below return
    @dfn{file-like objects}.  That is, when unquoted in a G-expression,
    these objects lead to a file in the store.  Consider this G-expression:
    
    @example
    #~(system* (string-append #$glibc "/sbin/nscd") "-f"
               #$(local-file "/tmp/my-nscd.conf"))
    @end example
    
    The effect here is to ``intern'' @file{/tmp/my-nscd.conf} by copying it
    to the store.  Once expanded, for instance @i{via}
    @code{gexp->derivation}, the G-expression refers to that copy under
    @file{/gnu/store}; thus, modifying or removing the file in @file{/tmp}
    does not have any effect on what the G-expression does.
    @code{plain-file} can be used similarly; it differs in that the file
    content is directly passed as a string.
    
    @deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
       [#:recursive? #f] [#:select? (const #t)]
    Return an object representing local file @var{file} to add to the store; this
    object can be used in a gexp.  If @var{file} is a relative file name, it is looked
    up relative to the source file where this form appears.  @var{file} will be added to
    the store under @var{name}--by default the base name of @var{file}.
    
    When @var{recursive?} is true, the contents of @var{file} are added recursively; if @var{file}
    designates a flat file and @var{recursive?} is true, its contents are added, and its
    permission bits are kept.
    
    When @var{recursive?} is true, call @code{(@var{select?} @var{file}
    @var{stat})} for each directory entry, where @var{file} is the entry's
    absolute file name and @var{stat} is the result of @code{lstat}; exclude
    entries for which @var{select?} does not return true.
    
    This is the declarative counterpart of the @code{interned-file} monadic
    procedure (@pxref{The Store Monad, @code{interned-file}}).
    @end deffn
    
    @deffn {Scheme Procedure} plain-file @var{name} @var{content}
    Return an object representing a text file called @var{name} with the given
    @var{content} (a string) to be added to the store.
    
    This is the declarative counterpart of @code{text-file}.
    @end deffn
    
    @deffn {Scheme Procedure} computed-file @var{name} @var{gexp} @
              [#:options '(#:local-build? #t)]
    Return an object representing the store item @var{name}, a file or
    directory computed by @var{gexp}.  @var{options}
    is a list of additional arguments to pass to @code{gexp->derivation}.
    
    This is the declarative counterpart of @code{gexp->derivation}.
    @end deffn
    
    @deffn {Monadic Procedure} gexp->script @var{name} @var{exp}
    Return an executable script @var{name} that runs @var{exp} using
    @var{guile}, with @var{exp}'s imported modules in its search path.
    
    The example below builds a script that simply invokes the @command{ls}
    command:
    
    @example
    (use-modules (guix gexp) (gnu packages base))
    
    (gexp->script "list-files"
                  #~(execl (string-append #$coreutils "/bin/ls")
                           "ls"))
    @end example
    
    When ``running'' it through the store (@pxref{The Store Monad,
    @code{run-with-store}}), we obtain a derivation that produces an
    executable file @file{/gnu/store/@dots{}-list-files} along these lines:
    
    @example
    #!/gnu/store/@dots{}-guile-2.0.11/bin/guile -ds
    !#
    (execl (string-append "/gnu/store/@dots{}-coreutils-8.22"/bin/ls")
           "ls")
    @end example
    @end deffn
    
    @deffn {Scheme Procedure} program-file @var{name} @var{exp} @
              [#:guile #f]
    Return an object representing the executable store item @var{name} that
    runs @var{gexp}.  @var{guile} is the Guile package used to execute that
    script.
    
    This is the declarative counterpart of @code{gexp->script}.
    @end deffn
    
    @deffn {Monadic Procedure} gexp->file @var{name} @var{exp} @
                [#:set-load-path? #t]
    Return a derivation that builds a file @var{name} containing @var{exp}.
    When @var{set-load-path?} is true, emit code in the resulting file to
    set @code{%load-path} and @code{%load-compiled-path} to honor
    @var{exp}'s imported modules.
    
    The resulting file holds references to all the dependencies of @var{exp}
    or a subset thereof.
    @end deffn
    
    @deffn {Scheme Procedure} scheme-file @var{name} @var{exp}
    Return an object representing the Scheme file @var{name} that contains
    @var{exp}.
    
    This is the declarative counterpart of @code{gexp->file}.
    @end deffn
    
    @deffn {Monadic Procedure} text-file* @var{name} @var{text} @dots{}
    Return as a monadic value a derivation that builds a text file
    containing all of @var{text}.  @var{text} may list, in addition to
    strings, objects of any type that can be used in a gexp: packages,
    derivations, local file objects, etc.  The resulting store file holds
    references to all these.
    
    This variant should be preferred over @code{text-file} anytime the file
    to create will reference items from the store.  This is typically the
    case when building a configuration file that embeds store file names,
    like this:
    
    @example
    (define (profile.sh)
      ;; Return the name of a shell script in the store that
      ;; initializes the 'PATH' environment variable.
      (text-file* "profile.sh"
                  "export PATH=" coreutils "/bin:"
                  grep "/bin:" sed "/bin\n"))
    @end example
    
    In this example, the resulting @file{/gnu/store/@dots{}-profile.sh} file
    will reference @var{coreutils}, @var{grep}, and @var{sed}, thereby
    preventing them from being garbage-collected during its lifetime.
    @end deffn
    
    @deffn {Scheme Procedure} mixed-text-file @var{name} @var{text} @dots{}
    Return an object representing store file @var{name} containing
    @var{text}.  @var{text} is a sequence of strings and file-like objects,
    as in:
    
    @example
    (mixed-text-file "profile"
                     "export PATH=" coreutils "/bin:" grep "/bin")
    @end example
    
    This is the declarative counterpart of @code{text-file*}.
    @end deffn
    
    Of course, in addition to gexps embedded in ``host'' code, there are
    also modules containing build tools.  To make it clear that they are
    meant to be used in the build stratum, these modules are kept in the
    @code{(guix build @dots{})} name space.
    
    @cindex lowering, of high-level objects in gexps
    Internally, high-level objects are @dfn{lowered}, using their compiler,
    to either derivations or store items.  For instance, lowering a package
    yields a derivation, and lowering a @code{plain-file} yields a store
    item.  This is achieved using the @code{lower-object} monadic procedure.
    
    @deffn {Monadic Procedure} lower-object @var{obj} [@var{system}] @
               [#:target #f]
    Return as a value in @var{%store-monad} the derivation or store item
    corresponding to @var{obj} for @var{system}, cross-compiling for
    @var{target} if @var{target} is true.  @var{obj} must be an object that
    has an associated gexp compiler, such as a @code{<package>}.
    @end deffn
    
    
    @c *********************************************************************
    @node Utilities
    @chapter Utilities
    
    This section describes Guix command-line utilities.  Some of them are
    primarily targeted at developers and users who write new package
    definitions, while others are more generally useful.  They complement
    the Scheme programming interface of Guix in a convenient way.
    
    @menu
    * Invoking guix build::         Building packages from the command line.
    * Invoking guix edit::          Editing package definitions.
    * Invoking guix download::      Downloading a file and printing its hash.
    * Invoking guix hash::          Computing the cryptographic hash of a file.
    * Invoking guix import::        Importing package definitions.
    * Invoking guix refresh::       Updating package definitions.
    * Invoking guix lint::          Finding errors in package definitions.
    * Invoking guix size::          Profiling disk usage.
    * Invoking guix graph::         Visualizing the graph of packages.
    * Invoking guix environment::   Setting up development environments.
    * Invoking guix publish::       Sharing substitutes.
    * Invoking guix challenge::     Challenging substitute servers.
    * Invoking guix container::     Process isolation.
    @end menu
    
    @node Invoking guix build
    @section Invoking @command{guix build}
    
    The @command{guix build} command builds packages or derivations and
    their dependencies, and prints the resulting store paths.  Note that it
    does not modify the user's profile---this is the job of the
    @command{guix package} command (@pxref{Invoking guix package}).  Thus,
    it is mainly useful for distribution developers.
    
    The general syntax is:
    
    @example
    guix build @var{options} @var{package-or-derivation}@dots{}
    @end example
    
    As an example, the following command builds the latest versions of Emacs
    and of Guile, displays their build logs, and finally displays the
    resulting directories:
    
    @example
    guix build emacs guile
    @end example
    
    Similarly, the following command builds all the available packages:
    
    @example
    guix build --quiet --keep-going \
      `guix package -A | cut -f1,2 --output-delimiter=@@`
    @end example
    
    @var{package-or-derivation} may be either the name of a package found in
    the software distribution such as @code{coreutils} or
    @code{coreutils-8.20}, or a derivation such as
    @file{/gnu/store/@dots{}-coreutils-8.19.drv}.  In the former case, a
    package with the corresponding name (and optionally version) is searched
    for among the GNU distribution modules (@pxref{Package Modules}).
    
    Alternatively, the @code{--expression} option may be used to specify a
    Scheme expression that evaluates to a package; this is useful when
    disambiguating among several same-named packages or package variants is
    needed.
    
    There may be zero or more @var{options}.  The available options are
    described in the subsections below.
    
    @menu
    * Common Build Options::        Build options for most commands.
    * Package Transformation Options::    Creating variants of packages.
    * Additional Build Options::    Options specific to 'guix build'.
    @end menu
    
    @node Common Build Options
    @subsection Common Build Options
    
    A number of options that control the build process are common to
    @command{guix build} and other commands that can spawn builds, such as
    @command{guix package} or @command{guix archive}.  These are the
    following:
    
    @table @code
    
    @item --load-path=@var{directory}
    @itemx -L @var{directory}
    Add @var{directory} to the front of the package module search path
    (@pxref{Package Modules}).
    
    This allows users to define their own packages and make them visible to
    the command-line tools.
    
    @item --keep-failed
    @itemx -K
    Keep the build tree of failed builds.  Thus, if a build fails, its build
    tree is kept under @file{/tmp}, in a directory whose name is shown at
    the end of the build log.  This is useful when debugging build issues.
    
    @item --keep-going
    @itemx -k
    Keep going when some of the derivations fail to build; return only once
    all the builds have either completed or failed.
    
    The default behavior is to stop as soon as one of the specified
    derivations has failed.
    
    @item --dry-run
    @itemx -n
    Do not build the derivations.
    
    @item --fallback
    When substituting a pre-built binary fails, fall back to building
    packages locally.
    
    @item --substitute-urls=@var{urls}
    @anchor{client-substitute-urls}
    Consider @var{urls} the whitespace-separated list of substitute source
    URLs, overriding the default list of URLs of @command{guix-daemon}
    (@pxref{daemon-substitute-urls,, @command{guix-daemon} URLs}).
    
    This means that substitutes may be downloaded from @var{urls}, provided
    they are signed by a key authorized by the system administrator
    (@pxref{Substitutes}).
    
    When @var{urls} is the empty string, substitutes are effectively
    disabled.
    
    @item --no-substitutes
    Do not use substitutes for build products.  That is, always build things
    locally instead of allowing downloads of pre-built binaries
    (@pxref{Substitutes}).
    
    @item --no-grafts
    Do not ``graft'' packages.  In practice, this means that package updates
    available as grafts are not applied.  @xref{Security Updates}, for more
    information on grafts.
    
    @item --rounds=@var{n}
    Build each derivation @var{n} times in a row, and raise an error if
    consecutive build results are not bit-for-bit identical.
    
    This is a useful way to detect non-deterministic builds processes.
    Non-deterministic build processes are a problem because they make it
    practically impossible for users to @emph{verify} whether third-party
    binaries are genuine.  @xref{Invoking guix challenge}, for more.
    
    Note that, currently, the differing build results are not kept around,
    so you will have to manually investigate in case of an error---e.g., by
    stashing one of the build results with @code{guix archive --export}
    (@pxref{Invoking guix archive}), then rebuilding, and finally comparing
    the two results.
    
    @item --no-build-hook
    Do not attempt to offload builds @i{via} the ``build hook'' of the daemon
    (@pxref{Daemon Offload Setup}).  That is, always build things locally
    instead of offloading builds to remote machines.
    
    @item --max-silent-time=@var{seconds}
    When the build or substitution process remains silent for more than
    @var{seconds}, terminate it and report a build failure.
    
    @item --timeout=@var{seconds}
    Likewise, when the build or substitution process lasts for more than
    @var{seconds}, terminate it and report a build failure.
    
    By default there is no timeout.  This behavior can be restored with
    @code{--timeout=0}.
    
    @item --verbosity=@var{level}
    Use the given verbosity level.  @var{level} must be an integer between 0
    and 5; higher means more verbose output.  Setting a level of 4 or more
    may be helpful when debugging setup issues with the build daemon.
    
    @item --cores=@var{n}
    @itemx -c @var{n}
    Allow the use of up to @var{n} CPU cores for the build.  The special
    value @code{0} means to use as many CPU cores as available.
    
    @item --max-jobs=@var{n}
    @itemx -M @var{n}
    Allow at most @var{n} build jobs in parallel.  @xref{Invoking
    guix-daemon, @code{--max-jobs}}, for details about this option and the
    equivalent @command{guix-daemon} option.
    
    @end table
    
    Behind the scenes, @command{guix build} is essentially an interface to
    the @code{package-derivation} procedure of the @code{(guix packages)}
    module, and to the @code{build-derivations} procedure of the @code{(guix
    derivations)} module.
    
    In addition to options explicitly passed on the command line,
    @command{guix build} and other @command{guix} commands that support
    building honor the @code{GUIX_BUILD_OPTIONS} environment variable.
    
    @defvr {Environment Variable} GUIX_BUILD_OPTIONS
    Users can define this variable to a list of command line options that
    will automatically be used by @command{guix build} and other
    @command{guix} commands that can perform builds, as in the example
    below:
    
    @example
    $ export GUIX_BUILD_OPTIONS="--no-substitutes -c 2 -L /foo/bar"
    @end example
    
    These options are parsed independently, and the result is appended to
    the parsed command-line options.
    @end defvr
    
    
    @node Package Transformation Options
    @subsection Package Transformation Options
    
    @cindex package variants
    Another set of command-line options supported by @command{guix build}
    and also @command{guix package} are @dfn{package transformation
    options}.  These are options that make it possible to define @dfn{package
    variants}---for instance, packages built from different source code.
    This is a convenient way to create customized packages on the fly
    without having to type in the definitions of package variants
    (@pxref{Defining Packages}).
    
    @table @code
    
    @item --with-source=@var{source}
    Use @var{source} as the source of the corresponding package.
    @var{source} must be a file name or a URL, as for @command{guix
    download} (@pxref{Invoking guix download}).
    
    The ``corresponding package'' is taken to be the one specified on the
    command line the name of which matches the base of @var{source}---e.g.,
    if @var{source} is @code{/src/guile-2.0.10.tar.gz}, the corresponding
    package is @code{guile}.  Likewise, the version string is inferred from
    @var{source}; in the previous example, it is @code{2.0.10}.
    
    This option allows users to try out versions of packages other than the
    one provided by the distribution.  The example below downloads
    @file{ed-1.7.tar.gz} from a GNU mirror and uses that as the source for
    the @code{ed} package:
    
    @example
    guix build ed --with-source=mirror://gnu/ed/ed-1.7.tar.gz
    @end example
    
    As a developer, @code{--with-source} makes it easy to test release
    candidates:
    
    @example
    guix build guile --with-source=../guile-2.0.9.219-e1bb7.tar.xz
    @end example
    
    @dots{} or to build from a checkout in a pristine environment:
    
    @example
    $ git clone git://git.sv.gnu.org/guix.git
    $ guix build guix --with-source=./guix
    @end example
    
    @item --with-input=@var{package}=@var{replacement}
    Replace dependency on @var{package} by a dependency on
    @var{replacement}.  @var{package} must be a package name, and
    @var{replacement} must be a package specification such as @code{guile}
    or @code{guile@@1.8}.
    
    For instance, the following command builds Guix, but replaces its
    dependency on the current stable version of Guile with a dependency on
    the development version of Guile, @code{guile-next}:
    
    @example
    guix build --with-input=guile=guile-next guix
    @end example
    
    This is a recursive, deep replacement.  So in this example, both
    @code{guix} and its dependency @code{guile-json} (which also depends on
    @code{guile}) get rebuilt against @code{guile-next}.
    
    However, implicit inputs are left unchanged.
    @end table
    
    @node Additional Build Options
    @subsection Additional Build Options
    
    The command-line options presented below are specific to @command{guix
    build}.
    
    @table @code
    
    @item --quiet
    @itemx -q
    Build quietly, without displaying the build log.  Upon completion, the
    build log is kept in @file{/var} (or similar) and can always be
    retrieved using the @option{--log-file} option.
    
    @item --file=@var{file}
    @itemx -f @var{file}
    
    Build the package or derivation that the code within @var{file}
    evaluates to.
    
    As an example, @var{file} might contain a package definition like this
    (@pxref{Defining Packages}):
    
    @example
    @verbatiminclude package-hello.scm
    @end example
    
    @item --expression=@var{expr}
    @itemx -e @var{expr}
    Build the package or derivation @var{expr} evaluates to.
    
    For example, @var{expr} may be @code{(@@ (gnu packages guile)
    guile-1.8)}, which unambiguously designates this specific variant of
    version 1.8 of Guile.
    
    Alternatively, @var{expr} may be a G-expression, in which case it is used
    as a build program passed to @code{gexp->derivation}
    (@pxref{G-Expressions}).
    
    Lastly, @var{expr} may refer to a zero-argument monadic procedure
    (@pxref{The Store Monad}).  The procedure must return a derivation as a
    monadic value, which is then passed through @code{run-with-store}.
    
    @item --source
    @itemx -S
    Build the source derivations of the packages, rather than the packages
    themselves.
    
    For instance, @code{guix build -S gcc} returns something like
    @file{/gnu/store/@dots{}-gcc-4.7.2.tar.bz2}, which is the GCC
    source tarball.
    
    The returned source tarball is the result of applying any patches and
    code snippets specified in the package @code{origin} (@pxref{Defining
    Packages}).
    
    @item --sources
    Fetch and return the source of @var{package-or-derivation} and all their
    dependencies, recursively.  This is a handy way to obtain a local copy
    of all the source code needed to build @var{packages}, allowing you to
    eventually build them even without network access.  It is an extension
    of the @code{--source} option and can accept one of the following
    optional argument values:
    
    @table @code
    @item package
    This value causes the @code{--sources} option to behave in the same way
    as the @code{--source} option.
    
    @item all
    Build the source derivations of all packages, including any source that
    might be listed as @code{inputs}.  This is the default value.
    
    @example
    $ guix build --sources tzdata
    The following derivations will be built:
       /gnu/store/@dots{}-tzdata2015b.tar.gz.drv
       /gnu/store/@dots{}-tzcode2015b.tar.gz.drv
    @end example
    
    @item transitive
    Build the source derivations of all packages, as well of all transitive
    inputs to the packages.  This can be used e.g. to
    prefetch package source for later offline building.
    
    @example
    $ guix build --sources=transitive tzdata
    The following derivations will be built:
       /gnu/store/@dots{}-tzcode2015b.tar.gz.drv
       /gnu/store/@dots{}-findutils-4.4.2.tar.xz.drv
       /gnu/store/@dots{}-grep-2.21.tar.xz.drv
       /gnu/store/@dots{}-coreutils-8.23.tar.xz.drv
       /gnu/store/@dots{}-make-4.1.tar.xz.drv
       /gnu/store/@dots{}-bash-4.3.tar.xz.drv
    @dots{}
    @end example
    
    @end table
    
    @item --system=@var{system}
    @itemx -s @var{system}
    Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of
    the system type of the build host.
    
    An example use of this is on Linux-based systems, which can emulate
    different personalities.  For instance, passing
    @code{--system=i686-linux} on an @code{x86_64-linux} system allows users
    to build packages in a complete 32-bit environment.
    
    @item --target=@var{triplet}
    @cindex cross-compilation
    Cross-build for @var{triplet}, which must be a valid GNU triplet, such
    as @code{"mips64el-linux-gnu"} (@pxref{Configuration Names, GNU
    configuration triplets,, configure, GNU Configure and Build System}).
    
    @anchor{build-check}
    @item --check
    @cindex determinism, checking
    @cindex reproducibility, checking
    Rebuild @var{package-or-derivation}, which are already available in the
    store, and raise an error if the build results are not bit-for-bit
    identical.
    
    This mechanism allows you to check whether previously installed
    substitutes are genuine (@pxref{Substitutes}), or whether the build result
    of a package is deterministic.  @xref{Invoking guix challenge}, for more
    background information and tools.
    
    When used in conjunction with @option{--keep-failed}, the differing
    output is kept in the store, under @file{/gnu/store/@dots{}-check}.
    This makes it easy to look for differences between the two results.
    
    @item --derivations
    @itemx -d
    Return the derivation paths, not the output paths, of the given
    packages.
    
    @item --root=@var{file}
    @itemx -r @var{file}
    Make @var{file} a symlink to the result, and register it as a garbage
    collector root.
    
    @item --log-file
    Return the build log file names or URLs for the given
    @var{package-or-derivation}, or raise an error if build logs are
    missing.
    
    This works regardless of how packages or derivations are specified.  For
    instance, the following invocations are equivalent:
    
    @example
    guix build --log-file `guix build -d guile`
    guix build --log-file `guix build guile`
    guix build --log-file guile
    guix build --log-file -e '(@@ (gnu packages guile) guile-2.0)'
    @end example
    
    If a log is unavailable locally, and unless @code{--no-substitutes} is
    passed, the command looks for a corresponding log on one of the
    substitute servers (as specified with @code{--substitute-urls}.)
    
    So for instance, imagine you want to see the build log of GDB on MIPS,
    but you are actually on an @code{x86_64} machine:
    
    @example
    $ guix build --log-file gdb -s mips64el-linux
    https://hydra.gnu.org/log/@dots{}-gdb-7.10
    @end example
    
    You can freely access a huge library of build logs!
    @end table
    
    
    @node Invoking guix edit
    @section Invoking @command{guix edit}
    
    @cindex package definition, editing
    So many packages, so many source files!  The @command{guix edit} command
    facilitates the life of packagers by pointing their editor at the source
    file containing the definition of the specified packages.  For instance:
    
    @example
    guix edit gcc@@4.9 vim
    @end example
    
    @noindent
    launches the program specified in the @code{VISUAL} or in the
    @code{EDITOR} environment variable to edit the recipe of GCC@tie{}4.9.3
    and that of Vim.
    
    If you are using Emacs, note that the Emacs user interface provides the
    @kbd{M-x guix-edit} command and a similar functionality in the ``package
    info'' and ``package list'' buffers created by the @kbd{M-x
    guix-search-by-name} and similar commands (@pxref{Emacs Commands}).
    
    
    @node Invoking guix download
    @section Invoking @command{guix download}
    
    When writing a package definition, developers typically need to download
    a source tarball, compute its SHA256 hash, and write that
    hash in the package definition (@pxref{Defining Packages}).  The
    @command{guix download} tool helps with this task: it downloads a file
    from the given URI, adds it to the store, and prints both its file name
    in the store and its SHA256 hash.
    
    The fact that the downloaded file is added to the store saves bandwidth:
    when the developer eventually tries to build the newly defined package
    with @command{guix build}, the source tarball will not have to be
    downloaded again because it is already in the store.  It is also a
    convenient way to temporarily stash files, which may be deleted
    eventually (@pxref{Invoking guix gc}).
    
    The @command{guix download} command supports the same URIs as used in
    package definitions.  In particular, it supports @code{mirror://} URIs.
    @code{https} URIs (HTTP over TLS) are supported @emph{provided} the
    Guile bindings for GnuTLS are available in the user's environment; when
    they are not available, an error is raised.  @xref{Guile Preparations,
    how to install the GnuTLS bindings for Guile,, gnutls-guile,
    GnuTLS-Guile}, for more information.
    
    The following option is available:
    
    @table @code
    @item --format=@var{fmt}
    @itemx -f @var{fmt}
    Write the hash in the format specified by @var{fmt}.  For more
    information on the valid values for @var{fmt}, @pxref{Invoking guix hash}.
    @end table
    
    @node Invoking guix hash
    @section Invoking @command{guix hash}
    
    The @command{guix hash} command computes the SHA256 hash of a file.
    It is primarily a convenience tool for anyone contributing to the
    distribution: it computes the cryptographic hash of a file, which can be
    used in the definition of a package (@pxref{Defining Packages}).
    
    The general syntax is:
    
    @example
    guix hash @var{option} @var{file}
    @end example
    
    @command{guix hash} has the following option:
    
    @table @code
    
    @item --format=@var{fmt}
    @itemx -f @var{fmt}
    Write the hash in the format specified by @var{fmt}.
    
    Supported formats: @code{nix-base32}, @code{base32}, @code{base16}
    (@code{hex} and @code{hexadecimal} can be used as well).
    
    If the @option{--format} option is not specified, @command{guix hash}
    will output the hash in @code{nix-base32}.  This representation is used
    in the definitions of packages.
    
    @item --recursive
    @itemx -r
    Compute the hash on @var{file} recursively.
    
    In this case, the hash is computed on an archive containing @var{file},
    including its children if it is a directory.  Some of the metadata of
    @var{file} is part of the archive; for instance, when @var{file} is a
    regular file, the hash is different depending on whether @var{file} is
    executable or not.  Metadata such as time stamps has no impact on the
    hash (@pxref{Invoking guix archive}).
    @c FIXME: Replace xref above with xref to an ``Archive'' section when
    @c it exists.
    
    @end table
    
    @node Invoking guix import
    @section Invoking @command{guix import}
    
    @cindex importing packages
    @cindex package import
    @cindex package conversion
    The @command{guix import} command is useful for people who would like to
    add a package to the distribution with as little work as
    possible---a legitimate demand.  The command knows of a few
    repositories from which it can ``import'' package metadata.  The result
    is a package definition, or a template thereof, in the format we know
    (@pxref{Defining Packages}).
    
    The general syntax is:
    
    @example
    guix import @var{importer} @var{options}@dots{}
    @end example
    
    @var{importer} specifies the source from which to import package
    metadata, and @var{options} specifies a package identifier and other
    options specific to @var{importer}.  Currently, the available
    ``importers'' are:
    
    @table @code
    @item gnu
    Import metadata for the given GNU package.  This provides a template
    for the latest version of that GNU package, including the hash of its
    source tarball, and its canonical synopsis and description.
    
    Additional information such as the package dependencies and its
    license needs to be figured out manually.
    
    For example, the following command returns a package definition for
    GNU@tie{}Hello:
    
    @example
    guix import gnu hello
    @end example
    
    Specific command-line options are:
    
    @table @code
    @item --key-download=@var{policy}
    As for @code{guix refresh}, specify the policy to handle missing OpenPGP
    keys when verifying the package signature.  @xref{Invoking guix
    refresh, @code{--key-download}}.
    @end table
    
    @item pypi
    @cindex pypi
    Import metadata from the @uref{https://pypi.python.org/, Python Package
    Index}@footnote{This functionality requires Guile-JSON to be installed.
    @xref{Requirements}.}.  Information is taken from the JSON-formatted
    description available at @code{pypi.python.org} and usually includes all
    the relevant information, including package dependencies.  For maximum
    efficiency, it is recommended to install the @command{unzip} utility, so
    that the importer can unzip Python wheels and gather data from them.
    
    The command below imports metadata for the @code{itsdangerous} Python
    package:
    
    @example
    guix import pypi itsdangerous
    @end example
    
    @item gem
    @cindex gem
    Import metadata from @uref{https://rubygems.org/,
    RubyGems}@footnote{This functionality requires Guile-JSON to be
    installed.  @xref{Requirements}.}.  Information is taken from the
    JSON-formatted description available at @code{rubygems.org} and includes
    most relevant information, including runtime dependencies.  There are
    some caveats, however.  The metadata doesn't distinguish between
    synopses and descriptions, so the same string is used for both fields.
    Additionally, the details of non-Ruby dependencies required to build
    native extensions is unavailable and left as an exercise to the
    packager.
    
    The command below imports metadata for the @code{rails} Ruby package:
    
    @example
    guix import gem rails
    @end example
    
    @item cpan
    @cindex CPAN
    Import metadata from @uref{https://www.metacpan.org/, MetaCPAN}@footnote{This
    functionality requires Guile-JSON to be installed.
    @xref{Requirements}.}.
    Information is taken from the JSON-formatted metadata provided through
    @uref{https://api.metacpan.org/, MetaCPAN's API} and includes most
    relevant information, such as module dependencies.  License information
    should be checked closely.  If Perl is available in the store, then the
    @code{corelist} utility will be used to filter core modules out of the
    list of dependencies.
    
    The command command below imports metadata for the @code{Acme::Boolean}
    Perl module:
    
    @example
    guix import cpan Acme::Boolean
    @end example
    
    @item cran
    @cindex CRAN
    @cindex Bioconductor
    Import metadata from @uref{http://cran.r-project.org/, CRAN}, the
    central repository for the @uref{http://r-project.org, GNU@tie{}R
    statistical and graphical environment}.
    
    Information is extracted from the @code{DESCRIPTION} file of the package.
    
    The command command below imports metadata for the @code{Cairo}
    R package:
    
    @example
    guix import cran Cairo
    @end example
    
    When @code{--archive=bioconductor} is added, metadata is imported from
    @uref{http://www.bioconductor.org/, Bioconductor}, a repository of R
    packages for for the analysis and comprehension of high-throughput
    genomic data in bioinformatics.
    
    Information is extracted from the @code{DESCRIPTION} file of a package
    published on the web interface of the Bioconductor SVN repository.
    
    The command below imports metadata for the @code{GenomicRanges}
    R package:
    
    @example
    guix import cran --archive=bioconductor GenomicRanges
    @end example
    
    @item nix
    Import metadata from a local copy of the source of the
    @uref{http://nixos.org/nixpkgs/, Nixpkgs distribution}@footnote{This
    relies on the @command{nix-instantiate} command of
    @uref{http://nixos.org/nix/, Nix}.}.  Package definitions in Nixpkgs are
    typically written in a mixture of Nix-language and Bash code.  This
    command only imports the high-level package structure that is written in
    the Nix language.  It normally includes all the basic fields of a
    package definition.
    
    When importing a GNU package, the synopsis and descriptions are replaced
    by their canonical upstream variant.
    
    Usually, you will first need to do:
    
    @example
    export NIX_REMOTE=daemon
    @end example
    
    @noindent
    so that @command{nix-instantiate} does not try to open the Nix database.
    
    As an example, the command below imports the package definition of
    LibreOffice (more precisely, it imports the definition of the package
    bound to the @code{libreoffice} top-level attribute):
    
    @example
    guix import nix ~/path/to/nixpkgs libreoffice
    @end example
    
    @item hackage
    @cindex hackage
    Import metadata from the Haskell community's central package archive
    @uref{https://hackage.haskell.org/, Hackage}.  Information is taken from
    Cabal files and includes all the relevant information, including package
    dependencies.
    
    Specific command-line options are:
    
    @table @code
    @item --stdin
    @itemx -s
    Read a Cabal file from standard input.
    @item --no-test-dependencies
    @itemx -t
    Do not include dependencies required only by the test suites.
    @item --cabal-environment=@var{alist}
    @itemx -e @var{alist}
    @var{alist} is a Scheme alist defining the environment in which the
    Cabal conditionals are evaluated.  The accepted keys are: @code{os},
    @code{arch}, @code{impl} and a string representing the name of a flag.
    The value associated with a flag has to be either the symbol
    @code{true} or @code{false}.  The value associated with other keys
    has to conform to the Cabal file format definition.  The default value
    associated with the keys @code{os}, @code{arch} and @code{impl} is
    @samp{linux}, @samp{x86_64} and @samp{ghc}, respectively.
    @end table
    
    The command below imports metadata for the latest version of the
    @code{HTTP} Haskell package without including test dependencies and
    specifying the value of the flag @samp{network-uri} as @code{false}:
    
    @example
    guix import hackage -t -e "'((\"network-uri\" . false))" HTTP
    @end example
    
    A specific package version may optionally be specified by following the
    package name by an at-sign and a version number as in the following example:
    
    @example
    guix import hackage mtl@@2.1.3.1
    @end example
    
    @item elpa
    @cindex elpa
    Import metadata from an Emacs Lisp Package Archive (ELPA) package
    repository (@pxref{Packages,,, emacs, The GNU Emacs Manual}).
    
    Specific command-line options are:
    
    @table @code
    @item --archive=@var{repo}
    @itemx -a @var{repo}
    @var{repo} identifies the archive repository from which to retrieve the
    information.  Currently the supported repositories and their identifiers
    are:
    @itemize -
    @item
    @uref{http://elpa.gnu.org/packages, GNU}, selected by the @code{gnu}
    identifier.  This is the default.
    
    @item
    @uref{http://stable.melpa.org/packages, MELPA-Stable}, selected by the
    @code{melpa-stable} identifier.
    
    @item
    @uref{http://melpa.org/packages, MELPA}, selected by the @code{melpa}
    identifier.
    @end itemize
    @end table
    @end table
    
    The structure of the @command{guix import} code is modular.  It would be
    useful to have more importers for other package formats, and your help
    is welcome here (@pxref{Contributing}).
    
    @node Invoking guix refresh
    @section Invoking @command{guix refresh}
    
    The primary audience of the @command{guix refresh} command is developers
    of the GNU software distribution.  By default, it reports any packages
    provided by the distribution that are outdated compared to the latest
    upstream version, like this:
    
    @example
    $ guix refresh
    gnu/packages/gettext.scm:29:13: gettext would be upgraded from 0.18.1.1 to 0.18.2.1
    gnu/packages/glib.scm:77:12: glib would be upgraded from 2.34.3 to 2.37.0
    @end example
    
    It does so by browsing the FTP directory of each package and determining
    the highest version number of the source tarballs therein.  The command
    knows how to update specific types of packages: GNU packages, ELPA
    packages, etc.---see the documentation for @option{--type} below.  The
    are many packages, though, for which it lacks a method to determine
    whether a new upstream release is available.  However, the mechanism is
    extensible, so feel free to get in touch with us to add a new method!
    
    When passed @code{--update}, it modifies distribution source files to
    update the version numbers and source tarball hashes of those package
    recipes (@pxref{Defining Packages}).  This is achieved by downloading
    each package's latest source tarball and its associated OpenPGP
    signature, authenticating the downloaded tarball against its signature
    using @command{gpg}, and finally computing its hash.  When the public
    key used to sign the tarball is missing from the user's keyring, an
    attempt is made to automatically retrieve it from a public key server;
    when this is successful, the key is added to the user's keyring; otherwise,
    @command{guix refresh} reports an error.
    
    The following options are supported:
    
    @table @code
    
    @item --expression=@var{expr}
    @itemx -e @var{expr}
    Consider the package @var{expr} evaluates to.
    
    This is useful to precisely refer to a package, as in this example:
    
    @example
    guix refresh -l -e '(@@@@ (gnu packages commencement) glibc-final)'
    @end example
    
    This command lists the dependents of the ``final'' libc (essentially all
    the packages.)
    
    @item --update
    @itemx -u
    Update distribution source files (package recipes) in place.  This is
    usually run from a checkout of the Guix source tree (@pxref{Running
    Guix Before It Is Installed}):
    
    @example
    $ ./pre-inst-env guix refresh -s non-core
    @end example
    
    @xref{Defining Packages}, for more information on package definitions.
    
    @item --select=[@var{subset}]
    @itemx -s @var{subset}
    Select all the packages in @var{subset}, one of @code{core} or
    @code{non-core}.
    
    The @code{core} subset refers to all the packages at the core of the
    distribution---i.e., packages that are used to build ``everything
    else''.  This includes GCC, libc, Binutils, Bash, etc.  Usually,
    changing one of these packages in the distribution entails a rebuild of
    all the others.  Thus, such updates are an inconvenience to users in
    terms of build time or bandwidth used to achieve the upgrade.
    
    The @code{non-core} subset refers to the remaining packages.  It is
    typically useful in cases where an update of the core packages would be
    inconvenient.
    
    @item --type=@var{updater}
    @itemx -t @var{updater}
    Select only packages handled by @var{updater} (may be a comma-separated
    list of updaters).  Currently, @var{updater} may be one of:
    
    @table @code
    @item gnu
    the updater for GNU packages;
    @item gnome
    the updater for GNOME packages;
    @item xorg
    the updater for X.org packages;
    @item elpa
    the updater for @uref{http://elpa.gnu.org/, ELPA} packages;
    @item cran
    the updater for @uref{http://cran.r-project.org/, CRAN} packages;
    @item bioconductor
    the updater for @uref{http://www.bioconductor.org/, Bioconductor} R packages;
    @item pypi
    the updater for @uref{https://pypi.python.org, PyPI} packages.
    @item gem
    the updater for @uref{https://rubygems.org, RubyGems} packages.
    @item github
    the updater for @uref{https://github.com, GitHub} packages.
    @item hackage
    the updater for @uref{https://hackage.haskell.org, Hackage} packages.
    @end table
    
    For instance, the following command only checks for updates of Emacs
    packages hosted at @code{elpa.gnu.org} and for updates of CRAN packages:
    
    @example
    $ guix refresh --type=elpa,cran
    gnu/packages/statistics.scm:819:13: r-testthat would be upgraded from 0.10.0 to 0.11.0
    gnu/packages/emacs.scm:856:13: emacs-auctex would be upgraded from 11.88.6 to 11.88.9
    @end example
    
    @end table
    
    In addition, @command{guix refresh} can be passed one or more package
    names, as in this example:
    
    @example
    $ ./pre-inst-env guix refresh -u emacs idutils gcc-4.8.4
    @end example
    
    @noindent
    The command above specifically updates the @code{emacs} and
    @code{idutils} packages.  The @code{--select} option would have no
    effect in this case.
    
    When considering whether to upgrade a package, it is sometimes
    convenient to know which packages would be affected by the upgrade and
    should be checked for compatibility.  For this the following option may
    be used when passing @command{guix refresh} one or more package names:
    
    @table @code
    
    @item --list-updaters
    @itemx -L
    List available updaters and exit (see @option{--type} above.)
    
    @item --list-dependent
    @itemx -l
    List top-level dependent packages that would need to be rebuilt as a
    result of upgrading one or more packages.
    
    @end table
    
    Be aware that the @code{--list-dependent} option only
    @emph{approximates} the rebuilds that would be required as a result of
    an upgrade.  More rebuilds might be required under some circumstances.
    
    @example
    $ guix refresh --list-dependent flex
    Building the following 120 packages would ensure 213 dependent packages are rebuilt:
    hop-2.4.0 geiser-0.4 notmuch-0.18 mu-0.9.9.5 cflow-1.4 idutils-4.6 @dots{}
    @end example
    
    The command above lists a set of packages that could be built to check
    for compatibility with an upgraded @code{flex} package.
    
    The following options can be used to customize GnuPG operation:
    
    @table @code
    
    @item --gpg=@var{command}
    Use @var{command} as the GnuPG 2.x command.  @var{command} is searched
    for in @code{$PATH}.
    
    @item --key-download=@var{policy}
    Handle missing OpenPGP keys according to @var{policy}, which may be one
    of:
    
    @table @code
    @item always
    Always download missing OpenPGP keys from the key server, and add them
    to the user's GnuPG keyring.
    
    @item never
    Never try to download missing OpenPGP keys.  Instead just bail out.
    
    @item interactive
    When a package signed with an unknown OpenPGP key is encountered, ask
    the user whether to download it or not.  This is the default behavior.
    @end table
    
    @item --key-server=@var{host}
    Use @var{host} as the OpenPGP key server when importing a public key.
    
    @end table
    
    The @code{github} updater uses the
    @uref{https://developer.github.com/v3/, GitHub API} to query for new
    releases.  When used repeatedly e.g. when refreshing all packages,
    GitHub will eventually refuse to answer any further API requests.  By
    default 60 API requests per hour are allowed, and a full refresh on all
    GitHub packages in Guix requires more than this.  Authentication with
    GitHub through the use of an API token alleviates these limits.  To use
    an API token, set the environment variable @code{GUIX_GITHUB_TOKEN} to a
    token procured from @uref{https://github.com/settings/tokens} or
    otherwise.
    
    
    @node Invoking guix lint
    @section Invoking @command{guix lint}
    The @command{guix lint} command is meant to help package developers avoid
    common errors and use a consistent style.  It runs a number of checks on
    a given set of packages in order to find common mistakes in their
    definitions.  Available @dfn{checkers} include (see
    @code{--list-checkers} for a complete list):
    
    @table @code
    @item synopsis
    @itemx description
    Validate certain typographical and stylistic rules about package
    descriptions and synopses.
    
    @item inputs-should-be-native
    Identify inputs that should most likely be native inputs.
    
    @item source
    @itemx home-page
    @itemx source-file-name
    Probe @code{home-page} and @code{source} URLs and report those that are
    invalid.  Check that the source file name is meaningful, e.g. is not
    just a version number or ``git-checkout'', without a declared
    @code{file-name} (@pxref{origin Reference}).
    
    @item cve
    @cindex security vulnerabilities
    @cindex CVE, Common Vulnerabilities and Exposures
    Report known vulnerabilities found in the Common Vulnerabilities and
    Exposures (CVE) databases of the current and past year
    @uref{https://nvd.nist.gov/download.cfm#CVE_FEED, published by the US
    NIST}.
    
    To view information about a particular vulnerability, visit pages such as:
    
    @itemize
    @item
    @indicateurl{https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-YYYY-ABCD}
    @item
    @indicateurl{https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-YYYY-ABCD}
    @end itemize
    
    @noindent
    where @code{CVE-YYYY-ABCD} is the CVE identifier---e.g.,
    @code{CVE-2015-7554}.
    
    Package developers can specify in package recipes the
    @uref{https://nvd.nist.gov/cpe.cfm,Common Platform Enumeration (CPE)}
    name and version of the package when they differ from the name that Guix
    uses, as in this example:
    
    @example
    (package
      (name "grub")
      ;; @dots{}
      ;; CPE calls this package "grub2".
      (properties '((cpe-name . "grub2"))))
    @end example
    
    @item formatting
    Warn about obvious source code formatting issues: trailing white space,
    use of tabulations, etc.
    @end table
    
    The general syntax is:
    
    @example
    guix lint @var{options} @var{package}@dots{}
    @end example
    
    If no package is given on the command line, then all packages are checked.
    The @var{options} may be zero or more of the following:
    
    @table @code
    @item --list-checkers
    @itemx -l
    List and describe all the available checkers that will be run on packages
    and exit.
    
    @item --checkers
    @itemx -c
    Only enable the checkers specified in a comma-separated list using the
    names returned by @code{--list-checkers}.
    
    @end table
    
    @node Invoking guix size
    @section Invoking @command{guix size}
    
    The @command{guix size} command helps package developers profile the
    disk usage of packages.  It is easy to overlook the impact of an
    additional dependency added to a package, or the impact of using a
    single output for a package that could easily be split (@pxref{Packages
    with Multiple Outputs}).  Such are the typical issues that
    @command{guix size} can highlight.
    
    The command can be passed a package specification such as @code{gcc-4.8}
    or @code{guile:debug}, or a file name in the store.  Consider this
    example:
    
    @example
    $ guix size coreutils
    store item                               total    self
    /gnu/store/@dots{}-coreutils-8.23          70.0    13.9  19.8%
    /gnu/store/@dots{}-gmp-6.0.0a              55.3     2.5   3.6%
    /gnu/store/@dots{}-acl-2.2.52              53.7     0.5   0.7%
    /gnu/store/@dots{}-attr-2.4.46             53.2     0.3   0.5%
    /gnu/store/@dots{}-gcc-4.8.4-lib           52.9    15.7  22.4%
    /gnu/store/@dots{}-glibc-2.21              37.2    37.2  53.1%
    @end example
    
    @cindex closure
    The store items listed here constitute the @dfn{transitive closure} of
    Coreutils---i.e., Coreutils and all its dependencies, recursively---as
    would be returned by:
    
    @example
    $ guix gc -R /gnu/store/@dots{}-coreutils-8.23
    @end example
    
    Here the output shows three columns next to store items.  The first column,
    labeled ``total'', shows the size in mebibytes (MiB) of the closure of
    the store item---that is, its own size plus the size of all its
    dependencies.  The next column, labeled ``self'', shows the size of the
    item itself.  The last column shows the ratio of the size of the item
    itself to the space occupied by all the items listed here.
    
    In this example, we see that the closure of Coreutils weighs in at
    70@tie{}MiB, half of which is taken by libc.  (That libc represents a
    large fraction of the closure is not a problem @i{per se} because it is
    always available on the system anyway.)
    
    When the package passed to @command{guix size} is available in the
    store, @command{guix size} queries the daemon to determine its
    dependencies, and measures its size in the store, similar to @command{du
    -ms --apparent-size} (@pxref{du invocation,,, coreutils, GNU
    Coreutils}).
    
    When the given package is @emph{not} in the store, @command{guix size}
    reports information based on the available substitutes
    (@pxref{Substitutes}).  This makes it possible it to profile disk usage of
    store items that are not even on disk, only available remotely.
    
    You can also specify several package names:
    
    @example
    $ guix size coreutils grep sed bash
    store item                               total    self
    /gnu/store/@dots{}-coreutils-8.24          77.8    13.8  13.4%
    /gnu/store/@dots{}-grep-2.22               73.1     0.8   0.8%
    /gnu/store/@dots{}-bash-4.3.42             72.3     4.7   4.6%
    /gnu/store/@dots{}-readline-6.3            67.6     1.2   1.2%
    @dots{}
    total: 102.3 MiB
    @end example
    
    @noindent
    In this example we see that the combination of the four packages takes
    102.3@tie{}MiB in total, which is much less than the sum of each closure
    since they have a lot of dependencies in common.
    
    The available options are:
    
    @table @option
    
    @item --substitute-urls=@var{urls}
    Use substitute information from @var{urls}.
    @xref{client-substitute-urls, the same option for @code{guix build}}.
    
    @item --map-file=@var{file}
    Write a graphical map of disk usage in PNG format to @var{file}.
    
    For the example above, the map looks like this:
    
    @image{images/coreutils-size-map,5in,, map of Coreutils disk usage
    produced by @command{guix size}}
    
    This option requires that
    @uref{http://wingolog.org/software/guile-charting/, Guile-Charting} be
    installed and visible in Guile's module search path.  When that is not
    the case, @command{guix size} fails as it tries to load it.
    
    @item --system=@var{system}
    @itemx -s @var{system}
    Consider packages for @var{system}---e.g., @code{x86_64-linux}.
    
    @end table
    
    @node Invoking guix graph
    @section Invoking @command{guix graph}
    
    @cindex DAG
    Packages and their dependencies form a @dfn{graph}, specifically a
    directed acyclic graph (DAG).  It can quickly become difficult to have a
    mental model of the package DAG, so the @command{guix graph} command
    provides a visual representation of the DAG.  @command{guix graph}
    emits a DAG representation in the input format of
    @uref{http://www.graphviz.org/, Graphviz}, so its output can be passed
    directly to the @command{dot} command of Graphviz.  The general
    syntax is:
    
    @example
    guix graph @var{options} @var{package}@dots{}
    @end example
    
    For example, the following command generates a PDF file representing the
    package DAG for the GNU@tie{}Core Utilities, showing its build-time
    dependencies:
    
    @example
    guix graph coreutils | dot -Tpdf > dag.pdf
    @end example
    
    The output looks like this:
    
    @image{images/coreutils-graph,2in,,Dependency graph of the GNU Coreutils}
    
    Nice little graph, no?
    
    But there is more than one graph!  The one above is concise: it is the
    graph of package objects, omitting implicit inputs such as GCC, libc,
    grep, etc.  It is often useful to have such a concise graph, but
    sometimes one may want to see more details.  @command{guix graph} supports
    several types of graphs, allowing you to choose the level of detail:
    
    @table @code
    @item package
    This is the default type used in the example above.  It shows the DAG of
    package objects, excluding implicit dependencies.  It is concise, but
    filters out many details.
    
    @item bag-emerged
    This is the package DAG, @emph{including} implicit inputs.
    
    For instance, the following command:
    
    @example
    guix graph --type=bag-emerged coreutils | dot -Tpdf > dag.pdf
    @end example
    
    ... yields this bigger graph:
    
    @image{images/coreutils-bag-graph,,5in,Detailed dependency graph of the GNU Coreutils}
    
    At the bottom of the graph, we see all the implicit inputs of
    @var{gnu-build-system} (@pxref{Build Systems, @code{gnu-build-system}}).
    
    Now, note that the dependencies of these implicit inputs---that is, the
    @dfn{bootstrap dependencies} (@pxref{Bootstrapping})---are not shown
    here, for conciseness.
    
    @item bag
    Similar to @code{bag-emerged}, but this time including all the bootstrap
    dependencies.
    
    @item bag-with-origins
    Similar to @code{bag}, but also showing origins and their dependencies.
    
    @item derivations
    This is the most detailed representation: It shows the DAG of
    derivations (@pxref{Derivations}) and plain store items.  Compared to
    the above representation, many additional nodes are visible, including
    build scripts, patches, Guile modules, etc.
    
    For this type of graph, it is also possible to pass a @file{.drv} file
    name instead of a package name, as in:
    
    @example
    guix graph -t derivation `guix system build -d my-config.scm`
    @end example
    @end table
    
    All the types above correspond to @emph{build-time dependencies}.  The
    following graph type represents the @emph{run-time dependencies}:
    
    @table @code
    @item references
    This is the graph of @dfn{references} of a package output, as returned
    by @command{guix gc --references} (@pxref{Invoking guix gc}).
    
    If the given package output is not available in the store, @command{guix
    graph} attempts to obtain dependency information from substitutes.
    
    Here you can also pass a store file name instead of a package name.  For
    example, the command below produces the reference graph of your profile
    (which can be big!):
    
    @example
    guix graph -t references `readlink -f ~/.guix-profile`
    @end example
    @end table
    
    The available options are the following:
    
    @table @option
    @item --type=@var{type}
    @itemx -t @var{type}
    Produce a graph output of @var{type}, where @var{type} must be one of
    the values listed above.
    
    @item --list-types
    List the supported graph types.
    
    @item --expression=@var{expr}
    @itemx -e @var{expr}
    Consider the package @var{expr} evaluates to.
    
    This is useful to precisely refer to a package, as in this example:
    
    @example
    guix graph -e '(@@@@ (gnu packages commencement) gnu-make-final)'
    @end example
    @end table
    
    
    @node Invoking guix environment
    @section Invoking @command{guix environment}
    
    @cindex reproducible build environments
    @cindex development environments
    The purpose of @command{guix environment} is to assist hackers in
    creating reproducible development environments without polluting their
    package profile.  The @command{guix environment} tool takes one or more
    packages, builds all of their inputs, and creates a shell
    environment to use them.
    
    The general syntax is:
    
    @example
    guix environment @var{options} @var{package}@dots{}
    @end example
    
    The following example spawns a new shell set up for the development of
    GNU@tie{}Guile:
    
    @example
    guix environment guile
    @end example
    
    If the needed dependencies are not built yet, @command{guix environment}
    automatically builds them.  The environment of the new shell is an augmented
    version of the environment that @command{guix environment} was run in.
    It contains the necessary search paths for building the given package
    added to the existing environment variables.  To create a ``pure''
    environment, in which the original environment variables have been unset,
    use the @code{--pure} option@footnote{Users sometimes wrongfully augment
    environment variables such as @code{PATH} in their @file{~/.bashrc}
    file.  As a consequence, when @code{guix environment} launches it, Bash
    may read @file{~/.bashrc}, thereby introducing ``impurities'' in these
    environment variables.  It is an error to define such environment
    variables in @file{.bashrc}; instead, they should be defined in
    @file{.bash_profile}, which is sourced only by log-in shells.
    @xref{Bash Startup Files,,, bash, The GNU Bash Reference Manual}, for
    details on Bash start-up files.}.
    
    @vindex GUIX_ENVIRONMENT
    @command{guix environment} defines the @code{GUIX_ENVIRONMENT}
    variable in the shell it spawns.  This allows users to, say, define a
    specific prompt for development environments in their @file{.bashrc}
    (@pxref{Bash Startup Files,,, bash, The GNU Bash Reference Manual}):
    
    @example
    if [ -n "$GUIX_ENVIRONMENT" ]
    then
        export PS1="\u@@\h \w [dev]\$ "
    fi
    @end example
    
    Additionally, more than one package may be specified, in which case the
    union of the inputs for the given packages are used.  For example, the
    command below spawns a shell where all of the dependencies of both Guile
    and Emacs are available:
    
    @example
    guix environment guile emacs
    @end example
    
    Sometimes an interactive shell session is not desired.  An arbitrary
    command may be invoked by placing the @code{--} token to separate the
    command from the rest of the arguments:
    
    @example
    guix environment guile -- make -j4
    @end example
    
    In other situations, it is more convenient to specify the list of
    packages needed in the environment.  For example, the following command
    runs @command{python} from an environment containing Python@tie{}2.7 and
    NumPy:
    
    @example
    guix environment --ad-hoc python2-numpy python-2.7 -- python
    @end example
    
    Furthermore, one might want the dependencies of a package and also some
    additional packages that are not build-time or runtime dependencies, but
    are useful when developing nonetheless.  Because of this, the
    @code{--ad-hoc} flag is positional.  Packages appearing before
    @code{--ad-hoc} are interpreted as packages whose dependencies will be
    added to the environment.  Packages appearing after are interpreted as
    packages that will be added to the environment directly.  For example,
    the following command creates a Guix development environment that
    additionally includes Git and strace:
    
    @example
    guix environment guix --ad-hoc git strace
    @end example
    
    Sometimes it is desirable to isolate the environment as much as
    possible, for maximal purity and reproducibility.  In particular, when
    using Guix on a host distro that is not GuixSD, it is desirable to
    prevent access to @file{/usr/bin} and other system-wide resources from
    the development environment.  For example, the following command spawns
    a Guile REPL in a ``container'' where only the store and the current
    working directory are mounted:
    
    @example
    guix environment --ad-hoc --container guile -- guile
    @end example
    
    @quotation Note
    The @code{--container} option requires Linux-libre 3.19 or newer.
    @end quotation
    
    The available options are summarized below.
    
    @table @code
    @item --expression=@var{expr}
    @itemx -e @var{expr}
    Create an environment for the package or list of packages that
    @var{expr} evaluates to.
    
    For example, running:
    
    @example
    guix environment -e '(@@ (gnu packages maths) petsc-openmpi)'
    @end example
    
    starts a shell with the environment for this specific variant of the
    PETSc package.
    
    Running:
    
    @example
    guix environment --ad-hoc -e '(@@ (gnu) %base-packages)'
    @end example
    
    starts a shell with all the GuixSD base packages available.
    
    The above commands only the use default output of the given packages.
    To select other outputs, two element tuples can be specified:
    
    @example
    guix environment --ad-hoc -e '(list (@ (gnu packages bash) bash) "include")'
    @end example
    
    @item --load=@var{file}
    @itemx -l @var{file}
    Create an environment for the package or list of packages that the code
    within @var{file} evaluates to.
    
    As an example, @var{file} might contain a definition like this
    (@pxref{Defining Packages}):
    
    @example
    @verbatiminclude environment-gdb.scm
    @end example
    
    @item --ad-hoc
    Include all specified packages in the resulting environment, as if an
    @i{ad hoc} package were defined with them as inputs.  This option is
    useful for quickly creating an environment without having to write a
    package expression to contain the desired inputs.
    
    For instance, the command:
    
    @example
    guix environment --ad-hoc guile guile-sdl -- guile
    @end example
    
    runs @command{guile} in an environment where Guile and Guile-SDL are
    available.
    
    Note that this example implicitly asks for the default output of
    @code{guile} and @code{guile-sdl}, but it is possible to ask for a
    specific output---e.g., @code{glib:bin} asks for the @code{bin} output
    of @code{glib} (@pxref{Packages with Multiple Outputs}).
    
    This option may be composed with the default behavior of @command{guix
    environment}.  Packages appearing before @code{--ad-hoc} are interpreted
    as packages whose dependencies will be added to the environment, the
    default behavior.  Packages appearing after are interpreted as packages
    that will be added to the environment directly.
    
    @item --pure
    Unset existing environment variables when building the new environment.
    This has the effect of creating an environment in which search paths
    only contain package inputs.
    
    @item --search-paths
    Display the environment variable definitions that make up the
    environment.
    
    @item --system=@var{system}
    @itemx -s @var{system}
    Attempt to build for @var{system}---e.g., @code{i686-linux}.
    
    @item --container
    @itemx -C
    @cindex container
    Run @var{command} within an isolated container.  The current working
    directory outside the container is mapped inside the container.
    Additionally, a dummy home directory is created that matches the current
    user's home directory, and @file{/etc/passwd} is configured accordingly.
    The spawned process runs as the current user outside the container, but
    has root privileges in the context of the container.
    
    @item --network
    @itemx -N
    For containers, share the network namespace with the host system.
    Containers created without this flag only have access to the loopback
    device.
    
    @item --expose=@var{source}[=@var{target}]
    For containers, expose the file system @var{source} from the host system
    as the read-only file system @var{target} within the container.  If
    @var{target} is not specified, @var{source} is used as the target mount
    point in the container.
    
    The example below spawns a Guile REPL in a container in which the user's
    home directory is accessible read-only via the @file{/exchange}
    directory:
    
    @example
    guix environment --container --expose=$HOME=/exchange guile -- guile
    @end example
    
    @item --share=@var{source}[=@var{target}]
    For containers, share the file system @var{source} from the host system
    as the writable file system @var{target} within the container.  If
    @var{target} is not specified, @var{source} is used as the target mount
    point in the container.
    
    The example below spawns a Guile REPL in a container in which the user's
    home directory is accessible for both reading and writing via the
    @file{/exchange} directory:
    
    @example
    guix environment --container --share=$HOME=/exchange guile -- guile
    @end example
    @end table
    
    It also supports all of the common build options that @command{guix
    build} supports (@pxref{Common Build Options}).
    
    @node Invoking guix publish
    @section Invoking @command{guix publish}
    
    The purpose of @command{guix publish} is to enable users to easily share
    their store with others, who can then use it as a substitute server
    (@pxref{Substitutes}).
    
    When @command{guix publish} runs, it spawns an HTTP server which allows
    anyone with network access to obtain substitutes from it.  This means
    that any machine running Guix can also act as if it were a build farm,
    since the HTTP interface is compatible with Hydra, the software behind
    the @code{hydra.gnu.org} build farm.
    
    For security, each substitute is signed, allowing recipients to check
    their authenticity and integrity (@pxref{Substitutes}).  Because
    @command{guix publish} uses the signing key of the system, which is only
    readable by the system administrator, it must be started as root; the
    @code{--user} option makes it drop root privileges early on.
    
    The signing key pair must be generated before @command{guix publish} is
    launched, using @command{guix archive --generate-key} (@pxref{Invoking
    guix archive}).
    
    The general syntax is:
    
    @example
    guix publish @var{options}@dots{}
    @end example
    
    Running @command{guix publish} without any additional arguments will
    spawn an HTTP server on port 8080:
    
    @example
    guix publish
    @end example
    
    Once a publishing server has been authorized (@pxref{Invoking guix
    archive}), the daemon may download substitutes from it:
    
    @example
    guix-daemon --substitute-urls=http://example.org:8080
    @end example
    
    The following options are available:
    
    @table @code
    @item --port=@var{port}
    @itemx -p @var{port}
    Listen for HTTP requests on @var{port}.
    
    @item --listen=@var{host}
    Listen on the network interface for @var{host}.  The default is to
    accept connections from any interface.
    
    @item --user=@var{user}
    @itemx -u @var{user}
    Change privileges to @var{user} as soon as possible---i.e., once the
    server socket is open and the signing key has been read.
    
    @item --ttl=@var{ttl}
    Produce @code{Cache-Control} HTTP headers that advertise a time-to-live
    (TTL) of @var{ttl}.  @var{ttl} must denote a duration: @code{5d} means 5
    days, @code{1m} means 1 month, and so on.
    
    This allows the user's Guix to keep substitute information in cache for
    @var{ttl}.  However, note that @code{guix publish} does not itself
    guarantee that the store items it provides will indeed remain available
    for as long as @var{ttl}.
    
    @item --repl[=@var{port}]
    @itemx -r [@var{port}]
    Spawn a Guile REPL server (@pxref{REPL Servers,,, guile, GNU Guile
    Reference Manual}) on @var{port} (37146 by default).  This is used
    primarily for debugging a running @command{guix publish} server.
    @end table
    
    Enabling @command{guix publish} on a GuixSD system is a one-liner: just
    add a call to @code{guix-publish-service} in the @code{services} field
    of the @code{operating-system} declaration (@pxref{guix-publish-service,
    @code{guix-publish-service}}).
    
    
    @node Invoking guix challenge
    @section Invoking @command{guix challenge}
    
    @cindex reproducible builds
    @cindex verifiable builds
    
    Do the binaries provided by this server really correspond to the source
    code it claims to build?  Is a package build process deterministic?
    These are the questions the @command{guix challenge} command attempts to
    answer.
    
    The former is obviously an important question: Before using a substitute
    server (@pxref{Substitutes}), one had better @emph{verify} that it
    provides the right binaries, and thus @emph{challenge} it.  The latter
    is what enables the former: If package builds are deterministic, then
    independent builds of the package should yield the exact same result,
    bit for bit; if a server provides a binary different from the one
    obtained locally, it may be either corrupt or malicious.
    
    We know that the hash that shows up in @file{/gnu/store} file names is
    the hash of all the inputs of the process that built the file or
    directory---compilers, libraries, build scripts,
    etc. (@pxref{Introduction}).  Assuming deterministic build processes,
    one store file name should map to exactly one build output.
    @command{guix challenge} checks whether there is, indeed, a single
    mapping by comparing the build outputs of several independent builds of
    any given store item.
    
    The command output looks like this:
    
    @smallexample
    $ guix challenge --substitute-urls="https://hydra.gnu.org https://guix.example.org"
    updating list of substitutes from 'https://hydra.gnu.org'... 100.0%
    updating list of substitutes from 'https://guix.example.org'... 100.0%
    /gnu/store/@dots{}-openssl-1.0.2d contents differ:
      local hash: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q
      https://hydra.gnu.org/nar/@dots{}-openssl-1.0.2d: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q
      https://guix.example.org/nar/@dots{}-openssl-1.0.2d: 1zy4fmaaqcnjrzzajkdn3f5gmjk754b43qkq47llbyak9z0qjyim
    /gnu/store/@dots{}-git-2.5.0 contents differ:
      local hash: 00p3bmryhjxrhpn2gxs2fy0a15lnip05l97205pgbk5ra395hyha
      https://hydra.gnu.org/nar/@dots{}-git-2.5.0: 069nb85bv4d4a6slrwjdy8v1cn4cwspm3kdbmyb81d6zckj3nq9f
      https://guix.example.org/nar/@dots{}-git-2.5.0: 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73
    /gnu/store/@dots{}-pius-2.1.1 contents differ:
      local hash: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
      https://hydra.gnu.org/nar/@dots{}-pius-2.1.1: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
      https://guix.example.org/nar/@dots{}-pius-2.1.1: 1cy25x1a4fzq5rk0pmvc8xhwyffnqz95h2bpvqsz2mpvlbccy0gs
    @end smallexample
    
    @noindent
    In this example, @command{guix challenge} first scans the store to
    determine the set of locally-built derivations---as opposed to store
    items that were downloaded from a substitute server---and then queries
    all the substitute servers.  It then reports those store items for which
    the servers obtained a result different from the local build.
    
    @cindex non-determinism, in package builds
    As an example, @code{guix.example.org} always gets a different answer.
    Conversely, @code{hydra.gnu.org} agrees with local builds, except in the
    case of Git.  This might indicate that the build process of Git is
    non-deterministic, meaning that its output varies as a function of
    various things that Guix does not fully control, in spite of building
    packages in isolated environments (@pxref{Features}).  Most common
    sources of non-determinism include the addition of timestamps in build
    results, the inclusion of random numbers, and directory listings sorted
    by inode number.  See @uref{https://reproducible-builds.org/docs/}, for
    more information.
    
    To find out what is wrong with this Git binary, we can do something along
    these lines (@pxref{Invoking guix archive}):
    
    @example
    $ wget -q -O - https://hydra.gnu.org/nar/@dots{}-git-2.5.0 \
       | guix archive -x /tmp/git
    $ diff -ur --no-dereference /gnu/store/@dots{}-git.2.5.0 /tmp/git
    @end example
    
    This command shows the difference between the files resulting from the
    local build, and the files resulting from the build on
    @code{hydra.gnu.org} (@pxref{Overview, Comparing and Merging Files,,
    diffutils, Comparing and Merging Files}).  The @command{diff} command
    works great for text files.  When binary files differ, a better option
    is @uref{https://diffoscope.org/, Diffoscope}, a tool that helps
    visualize differences for all kinds of files.
    
    Once you have done that work, you can tell whether the differences are due
    to a non-deterministic build process or to a malicious server.  We try
    hard to remove sources of non-determinism in packages to make it easier
    to verify substitutes, but of course, this is a process that
    involves not just Guix, but a large part of the free software community.
    In the meantime, @command{guix challenge} is one tool to help address
    the problem.
    
    If you are writing packages for Guix, you are encouraged to check
    whether @code{hydra.gnu.org} and other substitute servers obtain the
    same build result as you did with:
    
    @example
    $ guix challenge @var{package}
    @end example
    
    @noindent
    where @var{package} is a package specification such as
    @code{guile@@2.0} or @code{glibc:debug}.
    
    The general syntax is:
    
    @example
    guix challenge @var{options} [@var{packages}@dots{}]
    @end example
    
    When a difference is found between the hash of a locally-built item and
    that of a server-provided substitute, or among substitutes provided by
    different servers, the command displays it as in the example above and
    its exit code is 2 (other non-zero exit codes denote other kinds of
    errors.)
    
    The one option that matters is:
    
    @table @code
    
    @item --substitute-urls=@var{urls}
    Consider @var{urls} the whitespace-separated list of substitute source
    URLs to compare to.
    
    @end table
    
    
    @node Invoking guix container
    @section Invoking @command{guix container}
    @cindex container
    
    @quotation Note
    As of version @value{VERSION}, this tool is experimental.  The interface
    is subject to radical change in the future.
    @end quotation
    
    The purpose of @command{guix container} is to manipulate processes
    running within an isolated environment, commonly known as a
    ``container'', typically created by the @command{guix environment}
    (@pxref{Invoking guix environment}) and @command{guix system container}
    (@pxref{Invoking guix system}) commands.
    
    The general syntax is:
    
    @example
    guix container @var{action} @var{options}@dots{}
    @end example
    
    @var{action} specifies the operation to perform with a container, and
    @var{options} specifies the context-specific arguments for the action.
    
    The following actions are available:
    
    @table @code
    @item exec
    Execute a command within the context of a running container.
    
    The syntax is:
    
    @example
    guix container exec @var{pid} @var{program} @var{arguments}@dots{}
    @end example
    
    @var{pid} specifies the process ID of the running container.
    @var{program} specifies an executable file name within the root file
    system of the container.  @var{arguments} are the additional options that
    will be passed to @var{program}.
    
    The following command launches an interactive login shell inside a
    GuixSD container, started by @command{guix system container}, and whose
    process ID is 9001:
    
    @example
    guix container exec 9001 /run/current-system/profile/bin/bash --login
    @end example
    
    Note that the @var{pid} cannot be the parent process of a container.  It
    must be PID 1 of the container or one of its child processes.
    
    @end table
    
    @c *********************************************************************
    @node GNU Distribution
    @chapter GNU Distribution
    
    @cindex Guix System Distribution
    @cindex GuixSD
    Guix comes with a distribution of the GNU system consisting entirely of
    free software@footnote{The term ``free'' here refers to the
    @url{http://www.gnu.org/philosophy/free-sw.html,freedom provided to
    users of that software}.}.  The
    distribution can be installed on its own (@pxref{System Installation}),
    but it is also possible to install Guix as a package manager on top of
    an installed GNU/Linux system (@pxref{Installation}).  To distinguish
    between the two, we refer to the standalone distribution as the Guix
    System Distribution, or GuixSD.
    
    The distribution provides core GNU packages such as GNU libc, GCC, and
    Binutils, as well as many GNU and non-GNU applications.  The complete
    list of available packages can be browsed
    @url{http://www.gnu.org/software/guix/packages,on-line} or by
    running @command{guix package} (@pxref{Invoking guix package}):
    
    @example
    guix package --list-available
    @end example
    
    Our goal is to provide a practical 100% free software distribution of
    Linux-based and other variants of GNU, with a focus on the promotion and
    tight integration of GNU components, and an emphasis on programs and
    tools that help users exert that freedom.
    
    Packages are currently available on the following platforms:
    
    @table @code
    
    @item x86_64-linux
    Intel/AMD @code{x86_64} architecture, Linux-Libre kernel;
    
    @item i686-linux
    Intel 32-bit architecture (IA32), Linux-Libre kernel;
    
    @item armhf-linux
    ARMv7-A architecture with hard float, Thumb-2 and NEON,
    using the EABI hard-float application binary interface (ABI),
    and Linux-Libre kernel.
    
    @item mips64el-linux
    little-endian 64-bit MIPS processors, specifically the Loongson series,
    n32 ABI, and Linux-Libre kernel.
    
    @end table
    
    GuixSD itself is currently only available on @code{i686} and @code{x86_64}.
    
    @noindent
    For information on porting to other architectures or kernels,
    @pxref{Porting}.
    
    @menu
    * System Installation::         Installing the whole operating system.
    * System Configuration::        Configuring the operating system.
    * Installing Debugging Files::  Feeding the debugger.
    * Security Updates::            Deploying security fixes quickly.
    * Package Modules::             Packages from the programmer's viewpoint.
    * Packaging Guidelines::        Growing the distribution.
    * Bootstrapping::               GNU/Linux built from scratch.
    * Porting::                     Targeting another platform or kernel.
    @end menu
    
    Building this distribution is a cooperative effort, and you are invited
    to join!  @xref{Contributing}, for information about how you can help.
    
    @node System Installation
    @section System Installation
    
    @cindex Guix System Distribution
    This section explains how to install the Guix System Distribution
    on a machine.  The Guix package manager can
    also be installed on top of a running GNU/Linux system,
    @pxref{Installation}.
    
    @ifinfo
    @quotation Note
    @c This paragraph is for people reading this from tty2 of the
    @c installation image.
    You are reading this documentation with an Info reader.  For details on
    how to use it, hit the @key{RET} key (``return'' or ``enter'') on the
    link that follows: @pxref{Top, Info reader,, info-stnd, Stand-alone GNU
    Info}.  Hit @kbd{l} afterwards to come back here.
    
    Alternately, run @command{info info} in another tty to keep the manual
    available.
    @end quotation
    @end ifinfo
    
    @menu
    * Limitations::                 What you can expect.
    * Hardware Considerations::     Supported hardware.
    * USB Stick Installation::      Preparing the installation medium.
    * Preparing for Installation::  Networking, partitioning, etc.
    * Proceeding with the Installation::  The real thing.
    * Building the Installation Image::  How this comes to be.
    @end menu
    
    @node Limitations
    @subsection Limitations
    
    As of version @value{VERSION}, the Guix System Distribution (GuixSD) is
    not production-ready.  It may contain bugs and lack important
    features.  Thus, if you are looking for a stable production system that
    respects your freedom as a computer user, a good solution at this point
    is to consider @url{http://www.gnu.org/distros/free-distros.html, one of
    the more established GNU/Linux distributions}.  We hope you can soon switch
    to the GuixSD without fear, of course.  In the meantime, you can
    also keep using your distribution and try out the package manager on top
    of it (@pxref{Installation}).
    
    Before you proceed with the installation, be aware of the following
    noteworthy limitations applicable to version @value{VERSION}:
    
    @itemize
    @item
    The installation process does not include a graphical user interface and
    requires familiarity with GNU/Linux (see the following subsections to
    get a feel of what that means.)
    
    @item
    Support for the Logical Volume Manager (LVM) is missing.
    
    @item
    Few system services are currently supported out-of-the-box
    (@pxref{Services}).
    
    @item
    More than 3,200 packages are available, but you may
    occasionally find that a useful package is missing.
    
    @item
    GNOME, Xfce, and Enlightenment are available (@pxref{Desktop Services}),
    as well as a number of X11 window managers.  However, some graphical
    applications may be missing, as well as KDE.
    @end itemize
    
    You have been warned!  But more than a disclaimer, this is an invitation
    to report issues (and success stories!), and to join us in improving it.
    @xref{Contributing}, for more info.
    
    
    @node Hardware Considerations
    @subsection Hardware Considerations
    
    @cindex hardware support on GuixSD
    GNU@tie{}GuixSD focuses on respecting the user's computing freedom.  It
    builds around the kernel Linux-libre, which means that only hardware for
    which free software drivers and firmware exist is supported.  Nowadays,
    a wide range of off-the-shelf hardware is supported on
    GNU/Linux-libre---from keyboards to graphics cards to scanners and
    Ethernet controllers.  Unfortunately, there are still areas where
    hardware vendors deny users control over their own computing, and such
    hardware is not supported on GuixSD.
    
    @cindex WiFi, hardware support
    One of the main areas where free drivers or firmware are lacking is WiFi
    devices.  WiFi devices known to work include those using Atheros chips
    (AR9271 and AR7010), which corresponds to the @code{ath9k} Linux-libre
    driver, and for which free firmware exists and is available
    out-of-the-box on GuixSD, as part of @var{%base-firmware}
    (@pxref{operating-system Reference, @code{firmware}}).
    
    @cindex RYF, Respects Your Freedom
    The @uref{https://www.fsf.org/, Free Software Foundation} runs
    @uref{https://www.fsf.org/ryf, @dfn{Respects Your Freedom}} (RYF), a
    certification program for hardware products that respect your freedom
    and your privacy and ensure that you have control over your device.  We
    encourage you to check the list of RYF-certified devices.
    
    Another useful resource is the @uref{https://www.h-node.org/, H-Node}
    web site.  It contains a catalog of hardware devices with information
    about their support in GNU/Linux.
    
    
    @node USB Stick Installation
    @subsection USB Stick Installation
    
    An installation image for USB sticks can be downloaded from
    @indicateurl{ftp://alpha.gnu.org/gnu/guix/guixsd-usb-install-@value{VERSION}.@var{system}.xz},
    where @var{system} is one of:
    
    @table @code
    @item x86_64-linux
    for a GNU/Linux system on Intel/AMD-compatible 64-bit CPUs;
    
    @item i686-linux
    for a 32-bit GNU/Linux system on Intel-compatible CPUs.
    @end table
    
    This image contains a single partition with the tools necessary for an
    installation.  It is meant to be copied @emph{as is} to a large-enough
    USB stick.
    
    To copy the image to a USB stick, follow these steps:
    
    @enumerate
    @item
    Decompress the image using the @command{xz} command:
    
    @example
    xz -d guixsd-usb-install-@value{VERSION}.@var{system}.xz
    @end example
    
    @item
    Insert a USB stick of 1@tie{}GiB or more into your machine, and determine
    its device name.  Assuming that the USB stick is known as @file{/dev/sdX},
    copy the image with:
    
    @example
    dd if=guixsd-usb-install-@value{VERSION}.x86_64 of=/dev/sdX
    @end example
    
    Access to @file{/dev/sdX} usually requires root privileges.
    @end enumerate
    
    Once this is done, you should be able to reboot the system and boot from
    the USB stick.  The latter usually requires you to get in the BIOS' boot
    menu, where you can choose to boot from the USB stick.
    
    @node Preparing for Installation
    @subsection Preparing for Installation
    
    Once you have successfully booted the image on the USB stick, you should
    end up with a root prompt.  Several console TTYs are configured and can
    be used to run commands as root.  TTY2 shows this documentation,
    browsable using the Info reader commands (@pxref{Top,,, info-stnd,
    Stand-alone GNU Info}).  The installation system runs the GPM mouse
    daemon, which allows you to select text with the left mouse button and
    to paste it with the middle button.
    
    @quotation Note
    Installation requires access to the Internet so that any missing
    dependencies of your system configuration can be downloaded.  See the
    ``Networking'' section below.
    @end quotation
    
    The installation system includes many common tools needed for this task.
    But it is also a full-blown GuixSD system, which means that you can
    install additional packages, should you need it, using @command{guix
    package} (@pxref{Invoking guix package}).
    
    @subsubsection Keyboard Layout
    
    @cindex keyboard layout
    The installation image uses the US qwerty keyboard layout.  If you want
    to change it, you can use the @command{loadkeys} command.  For example,
    the following command selects the Dvorak keyboard layout:
    
    @example
    loadkeys dvorak
    @end example
    
    See the files under @file{/run/current-system/profile/share/keymaps} for
    a list of available keyboard layouts.  Run @command{man loadkeys} for
    more information.
    
    @subsubsection Networking
    
    Run the following command see what your network interfaces are called:
    
    @example
    ifconfig -a
    @end example
    
    @noindent
    @dots{} or, using the GNU/Linux-specific @command{ip} command:
    
    @example
    ip a
    @end example
    
    @c http://cgit.freedesktop.org/systemd/systemd/tree/src/udev/udev-builtin-net_id.c#n20
    Wired interfaces have a name starting with @samp{e}; for example, the
    interface corresponding to the first on-board Ethernet controller is
    called @samp{eno1}.  Wireless interfaces have a name starting with
    @samp{w}, like @samp{w1p2s0}.
    
    @table @asis
    @item Wired connection
    To configure a wired network run the following command, substituting
    @var{interface} with the name of the wired interface you want to use.
    
    @example
    ifconfig @var{interface} up
    @end example
    
    @item Wireless connection
    To configure wireless networking, you can create a configuration file
    for the @command{wpa_supplicant} configuration tool (its location is not
    important) using one of the available text editors such as
    @command{zile}:
    
    @example
    zile wpa_supplicant.conf
    @end example
    
    As an example, the following stanza can go to this file and will work
    for many wireless networks, provided you give the actual SSID and
    passphrase for the network you are connecting to:
    
    @example
    network=@{
      ssid="@var{my-ssid}"
      key_mgmt=WPA-PSK
      psk="the network's secret passphrase"
    @}
    @end example
    
    Start the wireless service and run it in the background with the
    following command (substitute @var{interface} with the name of the
    network interface you want to use):
    
    @example
    wpa_supplicant -c wpa_supplicant.conf -i @var{interface} -B
    @end example
    
    Run @command{man wpa_supplicant} for more information.
    @end table
    
    At this point, you need to acquire an IP address.  On a network where IP
    addresses are automatically assigned @i{via} DHCP, you can run:
    
    @example
    dhclient -v @var{interface}
    @end example
    
    Try to ping a server to see if networking is up and running:
    
    @example
    ping -c 3 gnu.org
    @end example
    
    Setting up network access is almost always a requirement because the
    image does not contain all the software and tools that may be needed.
    
    @subsubsection Disk Partitioning
    
    Unless this has already been done, the next step is to partition, and
    then format the target partition(s).
    
    The installation image includes several partitioning tools, including
    Parted (@pxref{Overview,,, parted, GNU Parted User Manual}),
    @command{fdisk}, and @command{cfdisk}.  Run it and set up your disk with
    the partition layout you want:
    
    @example
    cfdisk
    @end example
    
    Once you are done partitioning the target hard disk drive, you have to
    create a file system on the relevant partition(s)@footnote{Currently
    GuixSD pretty much assumes an ext4 file system.  In particular, code
    that reads partition UUIDs and labels only works with ext4.  This will
    be fixed in the future.}.
    
    Preferably, assign partitions a label so that you can easily and
    reliably refer to them in @code{file-system} declarations (@pxref{File
    Systems}).  This is typically done using the @code{-L} option of
    @command{mkfs.ext4} and related commands.  So, assuming the target root
    partition lives at @file{/dev/sda1}, a file system with the label
    @code{my-root} can be created with:
    
    @example
    mkfs.ext4 -L my-root /dev/sda1
    @end example
    
    @c FIXME: Uncomment this once GRUB fully supports encrypted roots.
    @c A typical command sequence may be:
    @c
    @c @example
    @c # fdisk /dev/sdX
    @c @dots{} Create partitions etc.@dots{}
    @c # cryptsetup luksFormat /dev/sdX1
    @c # cryptsetup open --type luks /dev/sdX1 my-partition
    @c # mkfs.ext4 -L my-root /dev/mapper/my-partition
    @c @end example
    
    In addition to e2fsprogs, the suite of tools to manipulate
    ext2/ext3/ext4 file systems, the installation image includes
    Cryptsetup/LUKS for disk encryption.
    
    Once that is done, mount the target root partition under @file{/mnt}
    with a command like (again, assuming @file{/dev/sda1} is the root
    partition):
    
    @example
    mount /dev/sda1 /mnt
    @end example
    
    Finally, if you plan to use one or more swap partitions (@pxref{Memory
    Concepts, swap space,, libc, The GNU C Library Reference Manual}), make
    sure to initialize them with @command{mkswap}.  Assuming you have one
    swap partition on @file{/dev/sda2}, you would run:
    
    @example
    mkswap /dev/sda2
    @end example
    
    @node Proceeding with the Installation
    @subsection Proceeding with the Installation
    
    With the target partitions ready and the target root mounted on
    @file{/mnt}, we're ready to go.  First, run:
    
    @example
    herd start cow-store /mnt
    @end example
    
    This makes @file{/gnu/store} copy-on-write, such that packages added to it
    during the installation phase are written to the target disk on @file{/mnt}
    rather than kept in memory.  This is necessary because the first phase of
    the @command{guix system init} command (see below) entails downloads or
    builds to @file{/gnu/store} which, initially, is an in-memory file system.
    
    Next, you have to edit a file and
    provide the declaration of the operating system to be installed.  To
    that end, the installation system comes with two text editors: GNU nano
    (@pxref{Top,,, nano, GNU nano Manual}), and GNU Zile, an Emacs clone.
    We strongly recommend storing that file on the target root file system, say,
    as @file{/mnt/etc/config.scm}.  Failing to do that, you will have lost your
    configuration file once you have rebooted into the newly-installed system.
    
    @xref{Using the Configuration System}, for an overview of the
    configuration file.  The example configurations discussed in that
    section are available under @file{/etc/configuration} in the
    installation image.  Thus, to get started with a system configuration
    providing a graphical display server (a ``desktop'' system), you can run
    something along these lines:
    
    @example
    # mkdir /mnt/etc
    # cp /etc/configuration/desktop.scm /mnt/etc/config.scm
    # zile /mnt/etc/config.scm
    @end example
    
    You should pay attention to what your configuration file contains, and
    in particular:
    
    @itemize
    @item
    Make sure the @code{grub-configuration} form refers to the device you
    want to install GRUB on.
    
    @item
    Be sure that your partition labels match the value of their respective
    @code{device} fields in your @code{file-system} configuration, assuming
    your @code{file-system} configuration sets the value of @code{title} to
    @code{'label}.
    @end itemize
    
    Once you are done preparing the configuration file, the new system must
    be initialized (remember that the target root file system is mounted
    under @file{/mnt}):
    
    @example
    guix system init /mnt/etc/config.scm /mnt
    @end example
    
    @noindent
    This copies all the necessary files and installs GRUB on
    @file{/dev/sdX}, unless you pass the @option{--no-grub} option.  For
    more information, @pxref{Invoking guix system}.  This command may trigger
    downloads or builds of missing packages, which can take some time.
    
    Once that command has completed---and hopefully succeeded!---you can run
    @command{reboot} and boot into the new system.  The @code{root} password
    in the new system is initially empty; other users' passwords need to be
    initialized by running the @command{passwd} command as @code{root},
    unless your configuration specifies otherwise
    (@pxref{user-account-password, user account passwords}).
    
    Join us on @code{#guix} on the Freenode IRC network or on
    @file{guix-devel@@gnu.org} to share your experience---good or not so
    good.
    
    @node Building the Installation Image
    @subsection Building the Installation Image
    
    The installation image described above was built using the @command{guix
    system} command, specifically:
    
    @c FIXME: 1G is too much; see <http://bugs.gnu.org/23077>.
    @example
    guix system disk-image --image-size=1G gnu/system/install.scm
    @end example
    
    Have a look at @file{gnu/system/install.scm} in the source tree,
    and see also @ref{Invoking guix system} for more information
    about the installation image.
    
    @node System Configuration
    @section System Configuration
    
    @cindex system configuration
    The Guix System Distribution supports a consistent whole-system configuration
    mechanism.  By that we mean that all aspects of the global system
    configuration---such as the available system services, timezone and
    locale settings, user accounts---are declared in a single place.  Such
    a @dfn{system configuration} can be @dfn{instantiated}---i.e., effected.
    
    One of the advantages of putting all the system configuration under the
    control of Guix is that it supports transactional system upgrades, and
    makes it possible to roll back to a previous system instantiation,
    should something go wrong with the new one (@pxref{Features}).  Another
    advantage is that it makes it easy to replicate the exact same configuration
    across different machines, or at different points in time, without
    having to resort to additional administration tools layered on top of
    the own tools of the system.
    @c Yes, we're talking of Puppet, Chef, & co. here.  ↑
    
    This section describes this mechanism.  First we focus on the system
    administrator's viewpoint---explaining how the system is configured and
    instantiated.  Then we show how this mechanism can be extended, for
    instance to support new system services.
    
    @menu
    * Using the Configuration System::  Customizing your GNU system.
    * operating-system Reference::  Detail of operating-system declarations.
    * File Systems::                Configuring file system mounts.
    * Mapped Devices::              Block device extra processing.
    * User Accounts::               Specifying user accounts.
    * Locales::                     Language and cultural convention settings.
    * Services::                    Specifying system services.
    * Setuid Programs::             Programs running with root privileges.
    * X.509 Certificates::          Authenticating HTTPS servers.
    * Name Service Switch::         Configuring libc's name service switch.
    * Initial RAM Disk::            Linux-Libre bootstrapping.
    * GRUB Configuration::          Configuring the boot loader.
    * Invoking guix system::        Instantiating a system configuration.
    * Running GuixSD in a VM::      How to run GuixSD in a virtual machine.
    * Defining Services::           Adding new service definitions.
    @end menu
    
    @node Using the Configuration System
    @subsection Using the Configuration System
    
    The operating system is configured by providing an
    @code{operating-system} declaration in a file that can then be passed to
    the @command{guix system} command (@pxref{Invoking guix system}).  A
    simple setup, with the default system services, the default Linux-Libre
    kernel, initial RAM disk, and boot loader looks like this:
    
    @findex operating-system
    @lisp
    @include os-config-bare-bones.texi
    @end lisp
    
    This example should be self-describing.  Some of the fields defined
    above, such as @code{host-name} and @code{bootloader}, are mandatory.
    Others, such as @code{packages} and @code{services}, can be omitted, in
    which case they get a default value.
    
    Below we discuss the effect of some of the most important fields
    (@pxref{operating-system Reference}, for details about all the available
    fields), and how to @dfn{instantiate} the operating system using
    @command{guix system}.
    
    @unnumberedsubsubsec Globally-Visible Packages
    
    @vindex %base-packages
    The @code{packages} field lists packages that will be globally visible
    on the system, for all user accounts---i.e., in every user's @code{PATH}
    environment variable---in addition to the per-user profiles
    (@pxref{Invoking guix package}).  The @var{%base-packages} variable
    provides all the tools one would expect for basic user and administrator
    tasks---including the GNU Core Utilities, the GNU Networking Utilities,
    the GNU Zile lightweight text editor, @command{find}, @command{grep},
    etc.  The example above adds tcpdump to those, taken from the @code{(gnu
    packages admin)} module (@pxref{Package Modules}).
    
    @findex specification->package
    Referring to packages by variable name, like @var{tcpdump} above, has
    the advantage of being unambiguous; it also allows typos and such to be
    diagnosed right away as ``unbound variables''.  The downside is that one
    needs to know which module defines which package, and to augment the
    @code{use-package-modules} line accordingly.  To avoid that, one can use
    the @code{specification->package} procedure of the @code{(gnu packages)}
    module, which returns the best package for a given name or name and
    version:
    
    @lisp
    (use-modules (gnu packages))
    
    (operating-system
      ;; ...
      (packages (append (map specification->package
                             '("tcpdump" "htop" "gnupg@@2.0"))
                        %base-packages)))
    @end lisp
    
    @unnumberedsubsubsec System Services
    
    @vindex %base-services
    The @code{services} field lists @dfn{system services} to be made
    available when the system starts (@pxref{Services}).
    The @code{operating-system} declaration above specifies that, in
    addition to the basic services, we want the @command{lshd} secure shell
    daemon listening on port 2222 (@pxref{Networking Services,
    @code{lsh-service}}).  Under the hood,
    @code{lsh-service} arranges so that @code{lshd} is started with the
    right command-line options, possibly with supporting configuration files
    generated as needed (@pxref{Defining Services}).
    
    @cindex customization, of services
    @findex modify-services
    Occasionally, instead of using the base services as is, you will want to
    customize them.  To do this, use @code{modify-services} (@pxref{Service
    Reference, @code{modify-services}}) to modify the list.
    
    For example, suppose you want to modify @code{guix-daemon} and Mingetty
    (the console log-in) in the @var{%base-services} list (@pxref{Base
    Services, @code{%base-services}}).  To do that, you can write the
    following in your operating system declaration:
    
    @lisp
    (define %my-services
      ;; My very own list of services.
      (modify-services %base-services
        (guix-service-type config =>
                           (guix-configuration
                            (inherit config)
                            (use-substitutes? #f)
                            (extra-options '("--gc-keep-derivations"))))
        (mingetty-service-type config =>
                               (mingetty-configuration
                                (inherit config)
                                (motd (plain-file "motd" "Howdy!"))))))
    
    (operating-system
      ;; @dots{}
      (services %my-services))
    @end lisp
    
    This changes the configuration---i.e., the service parameters---of the
    @code{guix-service-type} instance, and that of all the
    @code{mingetty-service-type} instances in the @var{%base-services} list.
    Observe how this is accomplished: first, we arrange for the original
    configuration to be bound to the identifier @code{config} in the
    @var{body}, and then we write the @var{body} so that it evaluates to the
    desired configuration.  In particular, notice how we use @code{inherit}
    to create a new configuration which has the same values as the old
    configuration, but with a few modifications.
    
    The configuration for a typical ``desktop'' usage, with the X11 display
    server, GNOME and Xfce (users can choose which of these desktop
    environments to use at the log-in screen by pressing @kbd{F1}), network
    management, power management, and more, would look like this:
    
    @lisp
    @include os-config-desktop.texi
    @end lisp
    
    A graphical environment with a choice of lightweight window managers
    instead of full-blown desktop environments would look like this:
    
    @lisp
    @include os-config-lightweight-desktop.texi
    @end lisp
    
    @xref{Desktop Services}, for the exact list of services provided by
    @var{%desktop-services}.  @xref{X.509 Certificates}, for background
    information about the @code{nss-certs} package that is used here.
    
    Again, @var{%desktop-services} is just a list of service objects.  If
    you want to remove services from there, you can do so using the
    procedures for list filtering (@pxref{SRFI-1 Filtering and
    Partitioning,,, guile, GNU Guile Reference Manual}).  For instance, the
    following expression returns a list that contains all the services in
    @var{%desktop-services} minus the Avahi service:
    
    @example
    (remove (lambda (service)
              (eq? (service-kind service) avahi-service-type))
            %desktop-services)
    @end example
    
    @unnumberedsubsubsec Instantiating the System
    
    Assuming the @code{operating-system} declaration
    is stored in the @file{my-system-config.scm}
    file, the @command{guix system reconfigure my-system-config.scm} command
    instantiates that configuration, and makes it the default GRUB boot
    entry (@pxref{Invoking guix system}).
    
    The normal way to change the system configuration is by updating this
    file and re-running @command{guix system reconfigure}.  One should never
    have to touch files in @command{/etc} or to run commands that modify the
    system state such as @command{useradd} or @command{grub-install}.  In
    fact, you must avoid that since that would not only void your warranty
    but also prevent you from rolling back to previous versions of your
    system, should you ever need to.
    
    @cindex roll-back, of the operating system
    Speaking of roll-back, each time you run @command{guix system
    reconfigure}, a new @dfn{generation} of the system is created---without
    modifying or deleting previous generations.  Old system generations get
    an entry in the GRUB boot menu, allowing you to boot them in case
    something went wrong with the latest generation.  Reassuring, no?  The
    @command{guix system list-generations} command lists the system
    generations available on disk.
    
    @unnumberedsubsubsec The Programming Interface
    
    At the Scheme level, the bulk of an @code{operating-system} declaration
    is instantiated with the following monadic procedure (@pxref{The Store
    Monad}):
    
    @deffn {Monadic Procedure} operating-system-derivation os
    Return a derivation that builds @var{os}, an @code{operating-system}
    object (@pxref{Derivations}).
    
    The output of the derivation is a single directory that refers to all
    the packages, configuration files, and other supporting files needed to
    instantiate @var{os}.
    @end deffn
    
    This procedure is provided by the @code{(gnu system)} module.  Along
    with @code{(gnu services)} (@pxref{Services}), this module contains the
    guts of GuixSD.  Make sure to visit it!
    
    
    @node operating-system Reference
    @subsection @code{operating-system} Reference
    
    This section summarizes all the options available in
    @code{operating-system} declarations (@pxref{Using the Configuration
    System}).
    
    @deftp {Data Type} operating-system
    This is the data type representing an operating system configuration.
    By that, we mean all the global system configuration, not per-user
    configuration (@pxref{Using the Configuration System}).
    
    @table @asis
    @item @code{kernel} (default: @var{linux-libre})
    The package object of the operating system kernel to use@footnote{Currently
    only the Linux-libre kernel is supported.  In the future, it will be
    possible to use the GNU@tie{}Hurd.}.
    
    @item @code{kernel-arguments} (default: @code{'()})
    List of strings or gexps representing additional arguments to pass on
    the command-line of the kernel---e.g., @code{("console=ttyS0")}.
    
    @item @code{bootloader}
    The system bootloader configuration object.  @xref{GRUB Configuration}.
    
    @item @code{initrd} (default: @code{base-initrd})
    A two-argument monadic procedure that returns an initial RAM disk for
    the Linux kernel.  @xref{Initial RAM Disk}.
    
    @item @code{firmware} (default: @var{%base-firmware})
    @cindex firmware
    List of firmware packages loadable by the operating system kernel.
    
    The default includes firmware needed for Atheros-based WiFi devices
    (Linux-libre module @code{ath9k}).  @xref{Hardware Considerations}, for
    more info on supported hardware.
    
    @item @code{host-name}
    The host name.
    
    @item @code{hosts-file}
    @cindex hosts file
    A file-like object (@pxref{G-Expressions, file-like objects}) for use as
    @file{/etc/hosts} (@pxref{Host Names,,, libc, The GNU C Library
    Reference Manual}).  The default is a file with entries for
    @code{localhost} and @var{host-name}.
    
    @item @code{mapped-devices} (default: @code{'()})
    A list of mapped devices.  @xref{Mapped Devices}.
    
    @item @code{file-systems}
    A list of file systems.  @xref{File Systems}.
    
    @item @code{swap-devices} (default: @code{'()})
    @cindex swap devices
    A list of strings identifying devices to be used for ``swap space''
    (@pxref{Memory Concepts,,, libc, The GNU C Library Reference Manual}).
    For example, @code{'("/dev/sda3")}.
    
    @item @code{users} (default: @code{%base-user-accounts})
    @itemx @code{groups} (default: @var{%base-groups})
    List of user accounts and groups.  @xref{User Accounts}.
    
    @item @code{skeletons} (default: @code{(default-skeletons)})
    A list target file name/file-like object tuples (@pxref{G-Expressions,
    file-like objects}).  These are the skeleton files that will be added to
    the home directory of newly-created user accounts.
    
    For instance, a valid value may look like this:
    
    @example
    `((".bashrc" ,(plain-file "bashrc" "echo Hello\n"))
      (".guile" ,(plain-file "guile"
                             "(use-modules (ice-9 readline))
                              (activate-readline)")))
    @end example
    
    @item @code{issue} (default: @var{%default-issue})
    A string denoting the contents of the @file{/etc/issue} file, which is
    displayed when users log in on a text console.
    
    @item @code{packages} (default: @var{%base-packages})
    The set of packages installed in the global profile, which is accessible
    at @file{/run/current-system/profile}.
    
    The default set includes core utilities and it is good practice to
    install non-core utilities in user profiles (@pxref{Invoking guix
    package}).
    
    @item @code{timezone}
    A timezone identifying string---e.g., @code{"Europe/Paris"}.
    
    You can run the @command{tzselect} command to find out which timezone
    string corresponds to your region.  Choosing an invalid timezone name
    causes @command{guix system} to fail.
    
    @item @code{locale} (default: @code{"en_US.utf8"})
    The name of the default locale (@pxref{Locale Names,,, libc, The GNU C
    Library Reference Manual}).  @xref{Locales}, for more information.
    
    @item @code{locale-definitions} (default: @var{%default-locale-definitions})
    The list of locale definitions to be compiled and that may be used at
    run time.  @xref{Locales}.
    
    @item @code{locale-libcs} (default: @code{(list @var{glibc})})
    The list of GNU@tie{}libc packages whose locale data and tools are used
    to build the locale definitions.  @xref{Locales}, for compatibility
    considerations that justify this option.
    
    @item @code{name-service-switch} (default: @var{%default-nss})
    Configuration of the libc name service switch (NSS)---a
    @code{<name-service-switch>} object.  @xref{Name Service Switch}, for
    details.
    
    @item @code{services} (default: @var{%base-services})
    A list of service objects denoting system services.  @xref{Services}.
    
    @item @code{pam-services} (default: @code{(base-pam-services)})
    @cindex PAM
    @cindex pluggable authentication modules
    Linux @dfn{pluggable authentication module} (PAM) services.
    @c FIXME: Add xref to PAM services section.
    
    @item @code{setuid-programs} (default: @var{%setuid-programs})
    List of string-valued G-expressions denoting setuid programs.
    @xref{Setuid Programs}.
    
    @item @code{sudoers-file} (default: @var{%sudoers-specification})
    @cindex sudoers file
    The contents of the @file{/etc/sudoers} file as a file-like object
    (@pxref{G-Expressions, @code{local-file} and @code{plain-file}}).
    
    This file specifies which users can use the @command{sudo} command, what
    they are allowed to do, and what privileges they may gain.  The default
    is that only @code{root} and members of the @code{wheel} group may use
    @code{sudo}.
    
    @end table
    @end deftp
    
    @node File Systems
    @subsection File Systems
    
    The list of file systems to be mounted is specified in the
    @code{file-systems} field of the operating system declaration
    (@pxref{Using the Configuration System}).  Each file system is declared
    using the @code{file-system} form, like this:
    
    @example
    (file-system
      (mount-point "/home")
      (device "/dev/sda3")
      (type "ext4"))
    @end example
    
    As usual, some of the fields are mandatory---those shown in the example
    above---while others can be omitted.  These are described below.
    
    @deftp {Data Type} file-system
    Objects of this type represent file systems to be mounted.  They
    contain the following members:
    
    @table @asis
    @item @code{type}
    This is a string specifying the type of the file system---e.g.,
    @code{"ext4"}.
    
    @item @code{mount-point}
    This designates the place where the file system is to be mounted.
    
    @item @code{device}
    This names the ``source'' of the file system.  By default it is the name
    of a node under @file{/dev}, but its meaning depends on the @code{title}
    field described below.
    
    @item @code{title} (default: @code{'device})
    This is a symbol that specifies how the @code{device} field is to be
    interpreted.
    
    When it is the symbol @code{device}, then the @code{device} field is
    interpreted as a file name; when it is @code{label}, then @code{device}
    is interpreted as a partition label name; when it is @code{uuid},
    @code{device} is interpreted as a partition unique identifier (UUID).
    
    UUIDs may be converted from their string representation (as shown by the
    @command{tune2fs -l} command) using the @code{uuid} form@footnote{The
    @code{uuid} form expects 16-byte UUIDs as defined in
    @uref{https://tools.ietf.org/html/rfc4122, RFC@tie{}4122}.  This is the
    form of UUID used by the ext2 family of file systems and others, but it
    is different from ``UUIDs'' found in FAT file systems, for instance.},
    like this:
    
    @example
    (file-system
      (mount-point "/home")
      (type "ext4")
      (title 'uuid)
      (device (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
    @end example
    
    The @code{label} and @code{uuid} options offer a way to refer to disk
    partitions without having to hard-code their actual device
    name@footnote{Note that, while it is tempting to use
    @file{/dev/disk/by-uuid} and similar device names to achieve the same
    result, this is not recommended: These special device nodes are created
    by the udev daemon and may be unavailable at the time the device is
    mounted.}.
    
    However, when the source of a file system is a mapped device (@pxref{Mapped
    Devices}), its @code{device} field @emph{must} refer to the mapped
    device name---e.g., @file{/dev/mapper/root-partition}---and consequently
    @code{title} must be set to @code{'device}.  This is required so that
    the system knows that mounting the file system depends on having the
    corresponding device mapping established.
    
    @item @code{flags} (default: @code{'()})
    This is a list of symbols denoting mount flags.  Recognized flags
    include @code{read-only}, @code{bind-mount}, @code{no-dev} (disallow
    access to special files), @code{no-suid} (ignore setuid and setgid
    bits), and @code{no-exec} (disallow program execution.)
    
    @item @code{options} (default: @code{#f})
    This is either @code{#f}, or a string denoting mount options.
    
    @item @code{mount?} (default: @code{#t})
    This value indicates whether to automatically mount the file system when
    the system is brought up.  When set to @code{#f}, the file system gets
    an entry in @file{/etc/fstab} (read by the @command{mount} command) but
    is not automatically mounted.
    
    @item @code{needed-for-boot?} (default: @code{#f})
    This Boolean value indicates whether the file system is needed when
    booting.  If that is true, then the file system is mounted when the
    initial RAM disk (initrd) is loaded.  This is always the case, for
    instance, for the root file system.
    
    @item @code{check?} (default: @code{#t})
    This Boolean indicates whether the file system needs to be checked for
    errors before being mounted.
    
    @item @code{create-mount-point?} (default: @code{#f})
    When true, the mount point is created if it does not exist yet.
    
    @item @code{dependencies} (default: @code{'()})
    This is a list of @code{<file-system>} objects representing file systems
    that must be mounted before (and unmounted after) this one.
    
    As an example, consider a hierarchy of mounts: @file{/sys/fs/cgroup} is
    a dependency of @file{/sys/fs/cgroup/cpu} and
    @file{/sys/fs/cgroup/memory}.
    
    @end table
    @end deftp
    
    The @code{(gnu system file-systems)} exports the following useful
    variables.
    
    @defvr {Scheme Variable} %base-file-systems
    These are essential file systems that are required on normal systems,
    such as @var{%pseudo-terminal-file-system} and @var{%immutable-store} (see
    below.)  Operating system declarations should always contain at least
    these.
    @end defvr
    
    @defvr {Scheme Variable} %pseudo-terminal-file-system
    This is the file system to be mounted as @file{/dev/pts}.  It supports
    @dfn{pseudo-terminals} created @i{via} @code{openpty} and similar
    functions (@pxref{Pseudo-Terminals,,, libc, The GNU C Library Reference
    Manual}).  Pseudo-terminals are used by terminal emulators such as
    @command{xterm}.
    @end defvr
    
    @defvr {Scheme Variable} %shared-memory-file-system
    This file system is mounted as @file{/dev/shm} and is used to support
    memory sharing across processes (@pxref{Memory-mapped I/O,
    @code{shm_open},, libc, The GNU C Library Reference Manual}).
    @end defvr
    
    @defvr {Scheme Variable} %immutable-store
    This file system performs a read-only ``bind mount'' of
    @file{/gnu/store}, making it read-only for all the users including
    @code{root}.  This prevents against accidental modification by software
    running as @code{root} or by system administrators.
    
    The daemon itself is still able to write to the store: it remounts it
    read-write in its own ``name space.''
    @end defvr
    
    @defvr {Scheme Variable} %binary-format-file-system
    The @code{binfmt_misc} file system, which allows handling of arbitrary
    executable file types to be delegated to user space.  This requires the
    @code{binfmt.ko} kernel module to be loaded.
    @end defvr
    
    @defvr {Scheme Variable} %fuse-control-file-system
    The @code{fusectl} file system, which allows unprivileged users to mount
    and unmount user-space FUSE file systems.  This requires the
    @code{fuse.ko} kernel module to be loaded.
    @end defvr
    
    @node Mapped Devices
    @subsection Mapped Devices
    
    @cindex device mapping
    @cindex mapped devices
    The Linux kernel has a notion of @dfn{device mapping}: a block device,
    such as a hard disk partition, can be @dfn{mapped} into another device,
    with additional processing over the data that flows through
    it@footnote{Note that the GNU@tie{}Hurd makes no difference between the
    concept of a ``mapped device'' and that of a file system: both boil down
    to @emph{translating} input/output operations made on a file to
    operations on its backing store.  Thus, the Hurd implements mapped
    devices, like file systems, using the generic @dfn{translator} mechanism
    (@pxref{Translators,,, hurd, The GNU Hurd Reference Manual}).}.  A
    typical example is encryption device mapping: all writes to the mapped
    device are encrypted, and all reads are deciphered, transparently.
    
    Mapped devices are declared using the @code{mapped-device} form:
    
    @example
    (mapped-device
      (source "/dev/sda3")
      (target "home")
      (type luks-device-mapping))
    @end example
    
    Or, better yet, like this:
    
    @example
    (mapped-device
      (source (uuid "cb67fc72-0d54-4c88-9d4b-b225f30b0f44"))
      (target "home")
      (type luks-device-mapping))
    @end example
    
    @cindex disk encryption
    @cindex LUKS
    This example specifies a mapping from @file{/dev/sda3} to
    @file{/dev/mapper/home} using LUKS---the
    @url{http://code.google.com/p/cryptsetup,Linux Unified Key Setup}, a
    standard mechanism for disk encryption.  In the second example, the UUID
    (unique identifier) is the LUKS UUID returned for the device by a
    command like:
    
    @example
    cryptsetup luksUUID /dev/sdx9
    @end example
    
    The @file{/dev/mapper/home}
    device can then be used as the @code{device} of a @code{file-system}
    declaration (@pxref{File Systems}).  The @code{mapped-device} form is
    detailed below.
    
    @deftp {Data Type} mapped-device
    Objects of this type represent device mappings that will be made when
    the system boots up.
    
    @table @code
    @item source
    This string specifies the name of the block device to be mapped, such as
    @code{"/dev/sda3"}.
    
    @item target
    This string specifies the name of the mapping to be established.  For
    example, specifying @code{"my-partition"} will lead to the creation of
    the @code{"/dev/mapper/my-partition"} device.
    
    @item type
    This must be a @code{mapped-device-kind} object, which specifies how
    @var{source} is mapped to @var{target}.
    @end table
    @end deftp
    
    @defvr {Scheme Variable} luks-device-mapping
    This defines LUKS block device encryption using the @command{cryptsetup}
    command from the package with the same name.  It relies on the
    @code{dm-crypt} Linux kernel module.
    @end defvr
    
    @node User Accounts
    @subsection User Accounts
    
    User accounts and groups are entirely managed through the
    @code{operating-system} declaration.  They are specified with the
    @code{user-account} and @code{user-group} forms:
    
    @example
    (user-account
      (name "alice")
      (group "users")
      (supplementary-groups '("wheel"   ;allow use of sudo, etc.
                              "audio"   ;sound card
                              "video"   ;video devices such as webcams
                              "cdrom")) ;the good ol' CD-ROM
      (comment "Bob's sister")
      (home-directory "/home/alice"))
    @end example
    
    When booting or upon completion of @command{guix system reconfigure},
    the system ensures that only the user accounts and groups specified in
    the @code{operating-system} declaration exist, and with the specified
    properties.  Thus, account or group creations or modifications made by
    directly invoking commands such as @command{useradd} are lost upon
    reconfiguration or reboot.  This ensures that the system remains exactly
    as declared.
    
    @deftp {Data Type} user-account
    Objects of this type represent user accounts.  The following members may
    be specified:
    
    @table @asis
    @item @code{name}
    The name of the user account.
    
    @item @code{group}
    This is the name (a string) or identifier (a number) of the user group
    this account belongs to.
    
    @item @code{supplementary-groups} (default: @code{'()})
    Optionally, this can be defined as a list of group names that this
    account belongs to.
    
    @item @code{uid} (default: @code{#f})
    This is the user ID for this account (a number), or @code{#f}.  In the
    latter case, a number is automatically chosen by the system when the
    account is created.
    
    @item @code{comment} (default: @code{""})
    A comment about the account, such as the account owner's full name.
    
    @item @code{home-directory}
    This is the name of the home directory for the account.
    
    @item @code{shell} (default: Bash)
    This is a G-expression denoting the file name of a program to be used as
    the shell (@pxref{G-Expressions}).
    
    @item @code{system?} (default: @code{#f})
    This Boolean value indicates whether the account is a ``system''
    account.  System accounts are sometimes treated specially; for instance,
    graphical login managers do not list them.
    
    @anchor{user-account-password}
    @item @code{password} (default: @code{#f})
    You would normally leave this field to @code{#f}, initialize user
    passwords as @code{root} with the @command{passwd} command, and then let
    users change it with @command{passwd}.  Passwords set with
    @command{passwd} are of course preserved across reboot and
    reconfiguration.
    
    If you @emph{do} want to have a preset password for an account, then
    this field must contain the encrypted password, as a string.
    @xref{crypt,,, libc, The GNU C Library Reference Manual}, for more information
    on password encryption, and @ref{Encryption,,, guile, GNU Guile Reference
    Manual}, for information on Guile's @code{crypt} procedure.
    
    @end table
    @end deftp
    
    User group declarations are even simpler:
    
    @example
    (user-group (name "students"))
    @end example
    
    @deftp {Data Type} user-group
    This type is for, well, user groups.  There are just a few fields:
    
    @table @asis
    @item @code{name}
    The name of the group.
    
    @item @code{id} (default: @code{#f})
    The group identifier (a number).  If @code{#f}, a new number is
    automatically allocated when the group is created.
    
    @item @code{system?} (default: @code{#f})
    This Boolean value indicates whether the group is a ``system'' group.
    System groups have low numerical IDs.
    
    @item @code{password} (default: @code{#f})
    What, user groups can have a password?  Well, apparently yes.  Unless
    @code{#f}, this field specifies the password of the group.
    
    @end table
    @end deftp
    
    For convenience, a variable lists all the basic user groups one may
    expect:
    
    @defvr {Scheme Variable} %base-groups
    This is the list of basic user groups that users and/or packages expect
    to be present on the system.  This includes groups such as ``root'',
    ``wheel'', and ``users'', as well as groups used to control access to
    specific devices such as ``audio'', ``disk'', and ``cdrom''.
    @end defvr
    
    @defvr {Scheme Variable} %base-user-accounts
    This is the list of basic system accounts that programs may expect to
    find on a GNU/Linux system, such as the ``nobody'' account.
    
    Note that the ``root'' account is not included here.  It is a
    special-case and is automatically added whether or not it is specified.
    @end defvr
    
    @node Locales
    @subsection Locales
    
    @cindex locale
    A @dfn{locale} defines cultural conventions for a particular language
    and region of the world (@pxref{Locales,,, libc, The GNU C Library
    Reference Manual}).  Each locale has a name that typically has the form
    @code{@var{language}_@var{territory}.@var{codeset}}---e.g.,
    @code{fr_LU.utf8} designates the locale for the French language, with
    cultural conventions from Luxembourg, and using the UTF-8 encoding.
    
    @cindex locale definition
    Usually, you will want to specify the default locale for the machine
    using the @code{locale} field of the @code{operating-system} declaration
    (@pxref{operating-system Reference, @code{locale}}).
    
    The selected locale is automatically added to the @dfn{locale
    definitions} known to the system if needed, with its codeset inferred
    from its name---e.g., @code{bo_CN.utf8} will be assumed to use the
    @code{UTF-8} codeset.  Additional locale definitions can be specified in
    the @code{locale-definitions} slot of @code{operating-system}---this is
    useful, for instance, if the codeset could not be inferred from the
    locale name.  The default set of locale definitions includes some widely
    used locales, but not all the available locales, in order to save space.
    
    For instance, to add the North Frisian locale for Germany, the value of
    that field may be:
    
    @example
    (cons (locale-definition
            (name "fy_DE.utf8") (source "fy_DE"))
          %default-locale-definitions)
    @end example
    
    Likewise, to save space, one might want @code{locale-definitions} to
    list only the locales that are actually used, as in:
    
    @example
    (list (locale-definition
            (name "ja_JP.eucjp") (source "ja_JP")
            (charset "EUC-JP")))
    @end example
    
    @vindex LOCPATH
    The compiled locale definitions are available at
    @file{/run/current-system/locale/X.Y}, where @code{X.Y} is the libc
    version, which is the default location where the GNU@tie{}libc provided
    by Guix looks for locale data.  This can be overridden using the
    @code{LOCPATH} environment variable (@pxref{locales-and-locpath,
    @code{LOCPATH} and locale packages}).
    
    The @code{locale-definition} form is provided by the @code{(gnu system
    locale)} module.  Details are given below.
    
    @deftp {Data Type} locale-definition
    This is the data type of a locale definition.
    
    @table @asis
    
    @item @code{name}
    The name of the locale.  @xref{Locale Names,,, libc, The GNU C Library
    Reference Manual}, for more information on locale names.
    
    @item @code{source}
    The name of the source for that locale.  This is typically the
    @code{@var{language}_@var{territory}} part of the locale name.
    
    @item @code{charset} (default: @code{"UTF-8"})
    The ``character set'' or ``code set'' for that locale,
    @uref{http://www.iana.org/assignments/character-sets, as defined by
    IANA}.
    
    @end table
    @end deftp
    
    @defvr {Scheme Variable} %default-locale-definitions
    A list of commonly used UTF-8 locales, used as the default
    value of the @code{locale-definitions} field of @code{operating-system}
    declarations.
    
    @cindex locale name
    @cindex normalized codeset in locale names
    These locale definitions use the @dfn{normalized codeset} for the part
    that follows the dot in the name (@pxref{Using gettextized software,
    normalized codeset,, libc, The GNU C Library Reference Manual}).  So for
    instance it has @code{uk_UA.utf8} but @emph{not}, say,
    @code{uk_UA.UTF-8}.
    @end defvr
    
    @subsubsection Locale Data Compatibility Considerations
    
    @cindex incompatibility, of locale data
    @code{operating-system} declarations provide a @code{locale-libcs} field
    to specify the GNU@tie{}libc packages that are used to compile locale
    declarations (@pxref{operating-system Reference}).  ``Why would I
    care?'', you may ask.  Well, it turns out that the binary format of
    locale data is occasionally incompatible from one libc version to
    another.
    
    @c See <https://sourceware.org/ml/libc-alpha/2015-09/msg00575.html>
    @c and <https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00737.html>.
    For instance, a program linked against libc version 2.21 is unable to
    read locale data produced with libc 2.22; worse, that program
    @emph{aborts} instead of simply ignoring the incompatible locale
    data@footnote{Versions 2.23 and later of GNU@tie{}libc will simply skip
    the incompatible locale data, which is already an improvement.}.
    Similarly, a program linked against libc 2.22 can read most, but not
    all, the locale data from libc 2.21 (specifically, @code{LC_COLLATE}
    data is incompatible); thus calls to @code{setlocale} may fail, but
    programs will not abort.
    
    The ``problem'' in GuixSD is that users have a lot of freedom: They can
    choose whether and when to upgrade software in their profiles, and might
    be using a libc version different from the one the system administrator
    used to build the system-wide locale data.
    
    Fortunately, unprivileged users can also install their own locale data
    and define @var{GUIX_LOCPATH} accordingly (@pxref{locales-and-locpath,
    @code{GUIX_LOCPATH} and locale packages}).
    
    Still, it is best if the system-wide locale data at
    @file{/run/current-system/locale} is built for all the libc versions
    actually in use on the system, so that all the programs can access
    it---this is especially crucial on a multi-user system.  To do that, the
    administrator can specify several libc packages in the
    @code{locale-libcs} field of @code{operating-system}:
    
    @example
    (use-package-modules base)
    
    (operating-system
      ;; @dots{}
      (locale-libcs (list glibc-2.21 (canonical-package glibc))))
    @end example
    
    This example would lead to a system containing locale definitions for
    both libc 2.21 and the current version of libc in
    @file{/run/current-system/locale}.
    
    
    @node Services
    @subsection Services
    
    @cindex system services
    An important part of preparing an @code{operating-system} declaration is
    listing @dfn{system services} and their configuration (@pxref{Using the
    Configuration System}).  System services are typically daemons launched
    when the system boots, or other actions needed at that time---e.g.,
    configuring network access.
    
    Services are managed by the GNU@tie{}Shepherd (@pxref{Introduction,,,
    shepherd, The GNU Shepherd Manual}).  On a running system, the
    @command{herd} command allows you to list the available services, show
    their status, start and stop them, or do other specific operations
    (@pxref{Jump Start,,, shepherd, The GNU Shepherd Manual}).  For example:
    
    @example
    # herd status
    @end example
    
    The above command, run as @code{root}, lists the currently defined
    services.  The @command{herd doc} command shows a synopsis of the given
    service:
    
    @example
    # herd doc nscd
    Run libc's name service cache daemon (nscd).
    @end example
    
    The @command{start}, @command{stop}, and @command{restart} sub-commands
    have the effect you would expect.  For instance, the commands below stop
    the nscd service and restart the Xorg display server:
    
    @example
    # herd stop nscd
    Service nscd has been stopped.
    # herd restart xorg-server
    Service xorg-server has been stopped.
    Service xorg-server has been started.
    @end example
    
    The following sections document the available services, starting with
    the core services, that may be used in an @code{operating-system}
    declaration.
    
    @menu
    * Base Services::               Essential system services.
    * Scheduled Job Execution::     The mcron service.
    * Networking Services::         Network setup, SSH daemon, etc.
    * X Window::                    Graphical display.
    * Desktop Services::            D-Bus and desktop services.
    * Database Services::           SQL databases.
    * Mail Services::               IMAP, POP3, SMTP, and all that.
    * Web Services::                Web servers.
    * Various Services::            Other services.
    @end menu
    
    @node Base Services
    @subsubsection Base Services
    
    The @code{(gnu services base)} module provides definitions for the basic
    services that one expects from the system.  The services exported by
    this module are listed below.
    
    @defvr {Scheme Variable} %base-services
    This variable contains a list of basic services (@pxref{Service Types
    and Services}, for more information on service objects) one would
    expect from the system: a login service (mingetty) on each tty, syslogd,
    the libc name service cache daemon (nscd), the udev device manager, and
    more.
    
    This is the default value of the @code{services} field of
    @code{operating-system} declarations.  Usually, when customizing a
    system, you will want to append services to @var{%base-services}, like
    this:
    
    @example
    (cons* (avahi-service) (lsh-service) %base-services)
    @end example
    @end defvr
    
    @deffn {Scheme Procedure} host-name-service @var{name}
    Return a service that sets the host name to @var{name}.
    @end deffn
    
    @deffn {Scheme Procedure} mingetty-service @var{config}
    Return a service to run mingetty according to @var{config}, a
    @code{<mingetty-configuration>} object, which specifies the tty to run, among
    other things.
    @end deffn
    
    @deftp {Data Type} mingetty-configuration
    This is the data type representing the configuration of Mingetty, which
    implements console log-in.
    
    @table @asis
    
    @item @code{tty}
    The name of the console this Mingetty runs on---e.g., @code{"tty1"}.
    
    @item @code{motd}
    A file-like object containing the ``message of the day''.
    
    @item @code{auto-login} (default: @code{#f})
    When true, this field must be a string denoting the user name under
    which the system automatically logs in.  When it is @code{#f}, a
    user name and password must be entered to log in.
    
    @item @code{login-program} (default: @code{#f})
    This must be either @code{#f}, in which case the default log-in program
    is used (@command{login} from the Shadow tool suite), or a gexp denoting
    the name of the log-in program.
    
    @item @code{login-pause?} (default: @code{#f})
    When set to @code{#t} in conjunction with @var{auto-login}, the user
    will have to press a key before the log-in shell is launched.
    
    @item @code{mingetty} (default: @var{mingetty})
    The Mingetty package to use.
    
    @end table
    @end deftp
    
    @cindex name service cache daemon
    @cindex nscd
    @deffn {Scheme Procedure} nscd-service [@var{config}] [#:glibc glibc] @
                    [#:name-services '()]
    Return a service that runs the libc name service cache daemon (nscd) with the
    given @var{config}---an @code{<nscd-configuration>} object.  @xref{Name
    Service Switch}, for an example.
    @end deffn
    
    @defvr {Scheme Variable} %nscd-default-configuration
    This is the default @code{<nscd-configuration>} value (see below) used
    by @code{nscd-service}.  It uses the caches defined by
    @var{%nscd-default-caches}; see below.
    @end defvr
    
    @deftp {Data Type} nscd-configuration
    This is the data type representing the name service cache daemon (nscd)
    configuration.
    
    @table @asis
    
    @item @code{name-services} (default: @code{'()})
    List of packages denoting @dfn{name services} that must be visible to
    the nscd---e.g., @code{(list @var{nss-mdns})}.
    
    @item @code{glibc} (default: @var{glibc})
    Package object denoting the GNU C Library providing the @command{nscd}
    command.
    
    @item @code{log-file} (default: @code{"/var/log/nscd.log"})
    Name of the nscd log file.  This is where debugging output goes when
    @code{debug-level} is strictly positive.
    
    @item @code{debug-level} (default: @code{0})
    Integer denoting the debugging levels.  Higher numbers mean that more
    debugging output is logged.
    
    @item @code{caches} (default: @var{%nscd-default-caches})
    List of @code{<nscd-cache>} objects denoting things to be cached; see
    below.
    
    @end table
    @end deftp
    
    @deftp {Data Type} nscd-cache
    Data type representing a cache database of nscd and its parameters.
    
    @table @asis
    
    @item @code{database}
    This is a symbol representing the name of the database to be cached.
    Valid values are @code{passwd}, @code{group}, @code{hosts}, and
    @code{services}, which designate the corresponding NSS database
    (@pxref{NSS Basics,,, libc, The GNU C Library Reference Manual}).
    
    @item @code{positive-time-to-live}
    @itemx @code{negative-time-to-live} (default: @code{20})
    A number representing the number of seconds during which a positive or
    negative lookup result remains in cache.
    
    @item @code{check-files?} (default: @code{#t})
    Whether to check for updates of the files corresponding to
    @var{database}.
    
    For instance, when @var{database} is @code{hosts}, setting this flag
    instructs nscd to check for updates in @file{/etc/hosts} and to take
    them into account.
    
    @item @code{persistent?} (default: @code{#t})
    Whether the cache should be stored persistently on disk.
    
    @item @code{shared?} (default: @code{#t})
    Whether the cache should be shared among users.
    
    @item @code{max-database-size} (default: 32@tie{}MiB)
    Maximum size in bytes of the database cache.
    
    @c XXX: 'suggested-size' and 'auto-propagate?' seem to be expert
    @c settings, so leave them out.
    
    @end table
    @end deftp
    
    @defvr {Scheme Variable} %nscd-default-caches
    List of @code{<nscd-cache>} objects used by default by
    @code{nscd-configuration} (see above).
    
    It enables persistent and aggressive caching of service and host name
    lookups.  The latter provides better host name lookup performance,
    resilience in the face of unreliable name servers, and also better
    privacy---often the result of host name lookups is in local cache, so
    external name servers do not even need to be queried.
    @end defvr
    
    
    @deffn {Scheme Procedure} syslog-service @
                 [#:config-file @var{%default-syslog.conf}]
    Return a service that runs @command{syslogd}.  If the configuration file
    name @var{config-file} is not specified, use some reasonable default
    settings.
    
    @xref{syslogd invocation,,, inetutils, GNU Inetutils}, for more
    information on the configuration file syntax.
    @end deffn
    
    @anchor{guix-configuration-type}
    @deftp {Data Type} guix-configuration
    This data type represents the configuration of the Guix build daemon.
    @xref{Invoking guix-daemon}, for more information.
    
    @table @asis
    @item @code{guix} (default: @var{guix})
    The Guix package to use.
    
    @item @code{build-group} (default: @code{"guixbuild"})
    Name of the group for build user accounts.
    
    @item @code{build-accounts} (default: @code{10})
    Number of build user accounts to create.
    
    @item @code{authorize-key?} (default: @code{#t})
    Whether to authorize the substitute key for @code{hydra.gnu.org}
    (@pxref{Substitutes}).
    
    @item @code{use-substitutes?} (default: @code{#t})
    Whether to use substitutes.
    
    @item @code{substitute-urls} (default: @var{%default-substitute-urls})
    The list of URLs where to look for substitutes by default.
    
    @item @code{extra-options} (default: @code{'()})
    List of extra command-line options for @command{guix-daemon}.
    
    @item @code{lsof} (default: @var{lsof})
    @itemx @code{lsh} (default: @var{lsh})
    The lsof and lsh packages to use.
    
    @end table
    @end deftp
    
    @deffn {Scheme Procedure} guix-service @var{config}
    Return a service that runs the Guix build daemon according to
    @var{config}.
    @end deffn
    
    @deffn {Scheme Procedure} udev-service [#:udev udev]
    Run @var{udev}, which populates the @file{/dev} directory dynamically.
    @end deffn
    
    @deffn {Scheme Procedure} urandom-seed-service @var{#f}
    Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
    when rebooting.
    @end deffn
    
    @defvr {Scheme Variable} %random-seed-file
    This is the name of the file where some random bytes are saved by
    @var{urandom-seed-service} to seed @file{/dev/urandom} when rebooting.
    It defaults to @file{/var/lib/random-seed}.
    @end defvr
    
    @deffn {Scheme Procedure} console-keymap-service @var{files} ...
    @cindex keyboard layout
    Return a service to load console keymaps from @var{files} using
    @command{loadkeys} command.  Most likely, you want to load some default
    keymap, which can be done like this:
    
    @example
    (console-keymap-service "dvorak")
    @end example
    
    Or, for example, for a Swedish keyboard, you may need to combine
    the following keymaps:
    @example
    (console-keymap-service "se-lat6" "se-fi-lat6")
    @end example
    
    Also you can specify a full file name (or file names) of your keymap(s).
    See @code{man loadkeys} for details.
    
    @end deffn
    
    @deffn {Scheme Procedure} gpm-service [#:gpm @var{gpm}] @
              [#:options]
    Run @var{gpm}, the general-purpose mouse daemon, with the given
    command-line @var{options}.  GPM allows users to use the mouse in the console,
    notably to select, copy, and paste text.  The default value of @var{options}
    uses the @code{ps2} protocol, which works for both USB and PS/2 mice.
    
    This service is not part of @var{%base-services}.
    @end deffn
    
    @anchor{guix-publish-service}
    @deffn {Scheme Procedure} guix-publish-service [#:guix @var{guix}] @
               [#:port 80] [#:host "localhost"]
    Return a service that runs @command{guix publish} listening on @var{host}
    and @var{port} (@pxref{Invoking guix publish}).
    
    This assumes that @file{/etc/guix} already contains a signing key pair as
    created by @command{guix archive --generate-key} (@pxref{Invoking guix
    archive}).  If that is not the case, the service will fail to start.
    @end deffn
    
    @anchor{rngd-service}
    @deffn {Scheme Procedure} rngd-service [#:rng-tools @var{rng-tools}] @
                [#:device "/dev/hwrng"]
    Return a service that runs the @command{rngd} program from @var{rng-tools}
    to add @var{device} to the kernel's entropy pool.  The service will fail if
    @var{device} does not exist.
    @end deffn
    
    @node Scheduled Job Execution
    @subsubsection Scheduled Job Execution
    
    @cindex cron
    @cindex scheduling jobs
    The @code{(gnu services mcron)} module provides an interface to
    GNU@tie{}mcron, a daemon to run jobs at scheduled times (@pxref{Top,,,
    mcron, GNU@tie{}mcron}).  GNU@tie{}mcron is similar to the traditional
    Unix @command{cron} daemon; the main difference is that it is
    implemented in Guile Scheme, which provides a lot of flexibility when
    specifying the scheduling of jobs and their actions.
    
    The example below defines an operating system that runs the
    @command{updatedb} (@pxref{Invoking updatedb,,, find, Finding Files})
    and the @command{guix gc} commands (@pxref{Invoking guix gc}) daily, as
    well as the @command{mkid} command on behalf of an unprivileged user
    (@pxref{mkid invocation,,, idutils, ID Database Utilities}).  It uses
    gexps to introduce job definitions that are passed to mcron
    (@pxref{G-Expressions}).
    
    @lisp
    (use-modules (guix) (gnu) (gnu services mcron))
    (use-package-modules base idutils)
    
    (define updatedb-job
      ;; Run 'updatedb' at 3AM every day.  Here we write the
      ;; job's action as a Scheme procedure.
      #~(job '(next-hour '(3))
             (lambda ()
               (execl (string-append #$findutils "/bin/updatedb")
                      "updatedb"
                      "--prunepaths=/tmp /var/tmp /gnu/store"))))
    
    (define garbage-collector-job
      ;; Collect garbage 5 minutes after midnight every day.
      ;; The job's action is a shell command.
      #~(job "5 0 * * *"            ;Vixie cron syntax
             "guix gc -F 1G"))
    
    (define idutils-jobs
      ;; Update the index database as user "charlie" at 12:15PM
      ;; and 19:15PM.  This runs from the user's home directory.
      #~(job '(next-minute-from (next-hour '(12 19)) '(15))
             (string-append #$idutils "/bin/mkid src")
             #:user "charlie"))
    
    (operating-system
      ;; @dots{}
      (services (cons (mcron-service (list garbage-collector-job
                                           updatedb-job
                                           idutils-job))
                      %base-services)))
    @end lisp
    
    @xref{Guile Syntax, mcron job specifications,, mcron, GNU@tie{}mcron},
    for more information on mcron job specifications.  Below is the
    reference of the mcron service.
    
    @deffn {Scheme Procedure} mcron-service @var{jobs} [#:mcron @var{mcron2}]
    Return an mcron service running @var{mcron} that schedules @var{jobs}, a
    list of gexps denoting mcron job specifications.
    
    This is a shorthand for:
    @example
      (service mcron-service-type
               (mcron-configuration (mcron mcron) (jobs jobs)))
    @end example
    @end deffn
    
    @defvr {Scheme Variable} mcron-service-type
    This is the type of the @code{mcron} service, whose value is an
    @code{mcron-configuration} object.
    
    This service type can be the target of a service extension that provides
    it additional job specifications (@pxref{Service Composition}).  In
    other words, it is possible to define services that provide addition
    mcron jobs to run.
    @end defvr
    
    @deftp {Data Type} mcron-configuration
    Data type representing the configuration of mcron.
    
    @table @asis
    @item @code{mcron} (default: @var{mcron2})
    The mcron package to use.
    
    @item @code{jobs}
    This is a list of gexps (@pxref{G-Expressions}), where each gexp
    corresponds to an mcron job specification (@pxref{Syntax, mcron job
    specifications,, mcron, GNU@tie{}mcron}).
    @end table
    @end deftp
    
    
    @node Networking Services
    @subsubsection Networking Services
    
    The @code{(gnu services networking)} module provides services to configure
    the network interface.
    
    @cindex DHCP, networking service
    @deffn {Scheme Procedure} dhcp-client-service [#:dhcp @var{isc-dhcp}]
    Return a service that runs @var{dhcp}, a Dynamic Host Configuration
    Protocol (DHCP) client, on all the non-loopback network interfaces.
    @end deffn
    
    @deffn {Scheme Procedure} static-networking-service @var{interface} @var{ip} @
           [#:gateway #f] [#:name-servers @code{'()}]
    Return a service that starts @var{interface} with address @var{ip}.  If
    @var{gateway} is true, it must be a string specifying the default network
    gateway.
    @end deffn
    
    @cindex wicd
    @cindex network management
    @deffn {Scheme Procedure} wicd-service [#:wicd @var{wicd}]
    Return a service that runs @url{https://launchpad.net/wicd,Wicd}, a network
    management daemon that aims to simplify wired and wireless networking.
    
    This service adds the @var{wicd} package to the global profile, providing
    several commands to interact with the daemon and configure networking:
    @command{wicd-client}, a graphical user interface, and the @command{wicd-cli}
    and @command{wicd-curses} user interfaces.
    @end deffn
    
    @cindex NetworkManager
    @deffn {Scheme Procedure} network-manager-service @
           [#:network-manager @var{network-manager}]
    Return a service that runs NetworkManager, a network connection manager
    attempting to keep network connectivity active when available.
    @end deffn
    
    @cindex Connman
    @deffn {Scheme Procedure} connman-service @
           [#:connman @var{connman}]
    Return a service that runs @url{https://01.org/connman,Connman}, a network
    connection manager.
    
    This service adds the @var{connman} package to the global profile, providing
    several the @command{connmanctl} command to interact with the daemon and
    configure networking."
    @end deffn
    
    @deffn {Scheme Procedure} ntp-service [#:ntp @var{ntp}] @
      [#:name-service @var{%ntp-servers}]
    Return a service that runs the daemon from @var{ntp}, the
    @uref{http://www.ntp.org, Network Time Protocol package}.  The daemon will
    keep the system clock synchronized with that of @var{servers}.
    @end deffn
    
    @defvr {Scheme Variable} %ntp-servers
    List of host names used as the default NTP servers.
    @end defvr
    
    @deffn {Scheme Procedure} tor-service [@var{config-file}] [#:tor @var{tor}]
    Return a service to run the @uref{https://torproject.org, Tor} anonymous
    networking daemon.
    
    The daemon runs as the @code{tor} unprivileged user.  It is passed
    @var{config-file}, a file-like object, with an additional @code{User tor} line
    and lines for hidden services added via @code{tor-hidden-service}.  Run
    @command{man tor} for information about the configuration file.
    @end deffn
    
    @cindex hidden service
    @deffn {Scheme Procedure} tor-hidden-service @var{name} @var{mapping}
    Define a new Tor @dfn{hidden service} called @var{name} and implementing
    @var{mapping}.  @var{mapping} is a list of port/host tuples, such as:
    
    @example
     '((22 "127.0.0.1:22")
       (80 "127.0.0.1:8080"))
    @end example
    
    In this example, port 22 of the hidden service is mapped to local port 22, and
    port 80 is mapped to local port 8080.
    
    This creates a @file{/var/lib/tor/hidden-services/@var{name}} directory, where
    the @file{hostname} file contains the @code{.onion} host name for the hidden
    service.
    
    See @uref{https://www.torproject.org/docs/tor-hidden-service.html.en, the Tor
    project's documentation} for more information.
    @end deffn
    
    @deffn {Scheme Procedure} bitlbee-service [#:bitlbee bitlbee] @
             [#:interface "127.0.0.1"] [#:port 6667] @
             [#:extra-settings ""]
    Return a service that runs @url{http://bitlbee.org,BitlBee}, a daemon that
    acts as a gateway between IRC and chat networks.
    
    The daemon will listen to the interface corresponding to the IP address
    specified in @var{interface}, on @var{port}.  @code{127.0.0.1} means that only
    local clients can connect, whereas @code{0.0.0.0} means that connections can
    come from any networking interface.
    
    In addition, @var{extra-settings} specifies a string to append to the
    configuration file.
    @end deffn
    
    Furthermore, @code{(gnu services ssh)} provides the following service.
    
    @deffn {Scheme Procedure} lsh-service [#:host-key "/etc/lsh/host-key"] @
           [#:daemonic? #t] [#:interfaces '()] [#:port-number 22] @
           [#:allow-empty-passwords? #f] [#:root-login? #f] @
           [#:syslog-output? #t] [#:x11-forwarding? #t] @
           [#:tcp/ip-forwarding? #t] [#:password-authentication? #t] @
           [#:public-key-authentication? #t] [#:initialize? #t]
    Run the @command{lshd} program from @var{lsh} to listen on port @var{port-number}.
    @var{host-key} must designate a file containing the host key, and readable
    only by root.
    
    When @var{daemonic?} is true, @command{lshd} will detach from the
    controlling terminal and log its output to syslogd, unless one sets
    @var{syslog-output?} to false.  Obviously, it also makes lsh-service
    depend on existence of syslogd service.  When @var{pid-file?} is true,
    @command{lshd} writes its PID to the file called @var{pid-file}.
    
    When @var{initialize?} is true, automatically create the seed and host key
    upon service activation if they do not exist yet.  This may take long and
    require interaction.
    
    When @var{initialize?} is false, it is up to the user to initialize the
    randomness generator (@pxref{lsh-make-seed,,, lsh, LSH Manual}), and to create
    a key pair with the private key stored in file @var{host-key} (@pxref{lshd
    basics,,, lsh, LSH Manual}).
    
    When @var{interfaces} is empty, lshd listens for connections on all the
    network interfaces; otherwise, @var{interfaces} must be a list of host names
    or addresses.
    
    @var{allow-empty-passwords?} specifies whether to accept log-ins with empty
    passwords, and @var{root-login?} specifies whether to accept log-ins as
    root.
    
    The other options should be self-descriptive.
    @end deffn
    
    @defvr {Scheme Variable} %facebook-host-aliases
    This variable contains a string for use in @file{/etc/hosts}
    (@pxref{Host Names,,, libc, The GNU C Library Reference Manual}).  Each
    line contains a entry that maps a known server name of the Facebook
    on-line service---e.g., @code{www.facebook.com}---to the local
    host---@code{127.0.0.1} or its IPv6 equivalent, @code{::1}.
    
    This variable is typically used in the @code{hosts-file} field of an
    @code{operating-system} declaration (@pxref{operating-system Reference,
    @file{/etc/hosts}}):
    
    @example
    (use-modules (gnu) (guix))
    
    (operating-system
      (host-name "mymachine")
      ;; ...
      (hosts-file
        ;; Create a /etc/hosts file with aliases for "localhost"
        ;; and "mymachine", as well as for Facebook servers.
        (plain-file "hosts"
                    (string-append (local-host-aliases host-name)
                                   %facebook-host-aliases))))
    @end example
    
    This mechanism can prevent programs running locally, such as Web
    browsers, from accessing Facebook.
    @end defvr
    
    The @code{(gnu services avahi)} provides the following definition.
    
    @deffn {Scheme Procedure} avahi-service [#:avahi @var{avahi}] @
              [#:host-name #f] [#:publish? #t] [#:ipv4? #t] @
              [#:ipv6? #t] [#:wide-area? #f] @
              [#:domains-to-browse '()] [#:debug? #f]
    Return a service that runs @command{avahi-daemon}, a system-wide
    mDNS/DNS-SD responder that allows for service discovery and
    "zero-configuration" host name lookups (see @uref{http://avahi.org/}), and
    extends the name service cache daemon (nscd) so that it can resolve
    @code{.local} host names using
    @uref{http://0pointer.de/lennart/projects/nss-mdns/, nss-mdns}.  Additionally,
    add the @var{avahi} package to the system profile so that commands such as
    @command{avahi-browse} are directly usable.
    
    If @var{host-name} is different from @code{#f}, use that as the host name to
    publish for this machine; otherwise, use the machine's actual host name.
    
    When @var{publish?} is true, publishing of host names and services is allowed;
    in particular, avahi-daemon will publish the machine's host name and IP
    address via mDNS on the local network.
    
    When @var{wide-area?} is true, DNS-SD over unicast DNS is enabled.
    
    Boolean values @var{ipv4?} and @var{ipv6?} determine whether to use IPv4/IPv6
    sockets.
    @end deffn
    
    
    @node X Window
    @subsubsection X Window
    
    Support for the X Window graphical display system---specifically
    Xorg---is provided by the @code{(gnu services xorg)} module.  Note that
    there is no @code{xorg-service} procedure.  Instead, the X server is
    started by the @dfn{login manager}, currently SLiM.
    
    @deffn {Scheme Procedure} slim-service [#:allow-empty-passwords? #f] @
      [#:auto-login? #f] [#:default-user ""] [#:startx] @
      [#:theme @var{%default-slim-theme}] @
      [#:theme-name @var{%default-slim-theme-name}]
    Return a service that spawns the SLiM graphical login manager, which in
    turn starts the X display server with @var{startx}, a command as returned by
    @code{xorg-start-command}.
    
    @cindex X session
    
    SLiM automatically looks for session types described by the @file{.desktop}
    files in @file{/run/current-system/profile/share/xsessions} and allows users
    to choose a session from the log-in screen using @kbd{F1}.  Packages such as
    @var{xfce}, @var{sawfish}, and @var{ratpoison} provide @file{.desktop} files;
    adding them to the system-wide set of packages automatically makes them
    available at the log-in screen.
    
    In addition, @file{~/.xsession} files are honored.  When available,
    @file{~/.xsession} must be an executable that starts a window manager
    and/or other X clients.
    
    When @var{allow-empty-passwords?} is true, allow logins with an empty
    password.  When @var{auto-login?} is true, log in automatically as
    @var{default-user}.
    
    If @var{theme} is @code{#f}, use the default log-in theme; otherwise
    @var{theme} must be a gexp denoting the name of a directory containing the
    theme to use.  In that case, @var{theme-name} specifies the name of the
    theme.
    @end deffn
    
    @defvr {Scheme Variable} %default-theme
    @defvrx {Scheme Variable} %default-theme-name
    The G-Expression denoting the default SLiM theme and its name.
    @end defvr
    
    @deffn {Scheme Procedure} xorg-start-command [#:guile] @
      [#:configuration-file #f] [#:xorg-server @var{xorg-server}]
    Return a derivation that builds a @var{guile} script to start the X server
    from @var{xorg-server}.  @var{configuration-file} is the server configuration
    file or a derivation that builds it; when omitted, the result of
    @code{xorg-configuration-file} is used.
    
    Usually the X server is started by a login manager.
    @end deffn
    
    @deffn {Scheme Procedure} xorg-configuration-file @
      [#:drivers '()] [#:resolutions '()] [#:extra-config '()]
    Return a configuration file for the Xorg server containing search paths for
    all the common drivers.
    
    @var{drivers} must be either the empty list, in which case Xorg chooses a
    graphics driver automatically, or a list of driver names that will be tried in
    this order---e.g., @code{(\"modesetting\" \"vesa\")}.
    
    Likewise, when @var{resolutions} is the empty list, Xorg chooses an
    appropriate screen resolution; otherwise, it must be a list of
    resolutions---e.g., @code{((1024 768) (640 480))}.
    
    Last, @var{extra-config} is a list of strings or objects appended to the
    @code{text-file*} argument list.  It is used to pass extra text to be added
    verbatim to the configuration file.
    @end deffn
    
    @deffn {Scheme Procedure} screen-locker-service @var{package} [@var{name}]
    Add @var{package}, a package for a screen-locker or screen-saver whose
    command is @var{program}, to the set of setuid programs and add a PAM entry
    for it.  For example:
    
    @lisp
    (screen-locker-service xlockmore "xlock")
    @end lisp
    
    makes the good ol' XlockMore usable.
    @end deffn
    
    
    @node Desktop Services
    @subsubsection Desktop Services
    
    The @code{(gnu services desktop)} module provides services that are
    usually useful in the context of a ``desktop'' setup---that is, on a
    machine running a graphical display server, possibly with graphical user
    interfaces, etc.  It also defines services that provide specific desktop
    environments like GNOME and XFCE.
    
    To simplify things, the module defines a variable containing the set of
    services that users typically expect on a machine with a graphical
    environment and networking:
    
    @defvr {Scheme Variable} %desktop-services
    This is a list of services that builds upon @var{%base-services} and
    adds or adjusts services for a typical ``desktop'' setup.
    
    In particular, it adds a graphical login manager (@pxref{X Window,
    @code{slim-service}}), screen lockers,
    a network management tool (@pxref{Networking
    Services, @code{wicd-service}}), energy and color management services,
    the @code{elogind} login and seat manager, the Polkit privilege service,
    the GeoClue location service, an NTP client (@pxref{Networking
    Services}), the Avahi daemon, and has the name service switch service
    configured to be able to use @code{nss-mdns} (@pxref{Name Service
    Switch, mDNS}).
    @end defvr
    
    The @var{%desktop-services} variable can be used as the @code{services}
    field of an @code{operating-system} declaration (@pxref{operating-system
    Reference, @code{services}}).
    
    Additionally, the @code{gnome-desktop-service} and
    @code{xfce-desktop-service} procedures can add GNOME and/or XFCE to a
    system.  To ``add GNOME'' means that system-level services like the
    backlight adjustment helpers and the power management utilities are
    added to the system, extending @code{polkit} and @code{dbus}
    appropriately, allowing GNOME to operate with elevated privileges on a
    limited number of special-purpose system interfaces.  Additionally,
    adding a service made by @code{gnome-desktop-service} adds the GNOME
    metapackage to the system profile.  Likewise, adding the XFCE service
    not only adds the @code{xfce} metapackage to the system profile, but it
    also gives the Thunar file manager the ability to open a ``root-mode''
    file management window, if the user authenticates using the
    administrator's password via the standard polkit graphical interface.
    
    @deffn {Scheme Procedure} gnome-desktop-service
    Return a service that adds the @code{gnome} package to the system
    profile, and extends polkit with the actions from
    @code{gnome-settings-daemon}.
    @end deffn
    
    @deffn {Scheme Procedure} xfce-desktop-service
    Return a service that adds the @code{xfce} package to the system profile,
    and extends polkit with the abilit for @code{thunar} to manipulate the
    file system as root from within a user session, after the user has
    authenticated with the administrator's password.
    @end deffn
    
    Because the GNOME and XFCE desktop services pull in so many packages,
    the default @code{%desktop-services} variable doesn't include either of
    them by default.  To add GNOME or XFCE, just @code{cons} them onto
    @code{%desktop-services} in the @code{services} field of your
    @code{operating-system}:
    
    @example
    (use-modules (gnu))
    (use-service-modules desktop)
    (operating-system
      ...
      ;; cons* adds items to the list given as its last argument.
      (services (cons* (gnome-desktop-service)
                       (xfce-desktop-service)
                       %desktop-services))
      ...)
    @end example
    
    These desktop environments will then be available as options in the
    graphical login window.
    
    The actual service definitions included in @code{%desktop-services} and
    provided by @code{(gnu services dbus)} and @code{(gnu services desktop)}
    are described below.
    
    @deffn {Scheme Procedure} dbus-service [#:dbus @var{dbus}] [#:services '()]
    Return a service that runs the ``system bus'', using @var{dbus}, with
    support for @var{services}.
    
    @uref{http://dbus.freedesktop.org/, D-Bus} is an inter-process communication
    facility.  Its system bus is used to allow system services to communicate
    and to be notified of system-wide events.
    
    @var{services} must be a list of packages that provide an
    @file{etc/dbus-1/system.d} directory containing additional D-Bus configuration
    and policy files.  For example, to allow avahi-daemon to use the system bus,
    @var{services} must be equal to @code{(list avahi)}.
    @end deffn
    
    @deffn {Scheme Procedure} elogind-service [#:config @var{config}]
    Return a service that runs the @code{elogind} login and
    seat management daemon.  @uref{https://github.com/andywingo/elogind,
    Elogind} exposes a D-Bus interface that can be used to know which users
    are logged in, know what kind of sessions they have open, suspend the
    system, inhibit system suspend, reboot the system, and other tasks.
    
    Elogind handles most system-level power events for a computer, for
    example suspending the system when a lid is closed, or shutting it down
    when the power button is pressed.
    
    The @var{config} keyword argument specifies the configuration for
    elogind, and should be the result of an @code{(elogind-configuration
    (@var{parameter} @var{value})...)} invocation.  Available parameters and
    their default values are:
    
    @table @code
    @item kill-user-processes?
    @code{#f}
    @item kill-only-users
    @code{()}
    @item kill-exclude-users
    @code{("root")}
    @item inhibit-delay-max-seconds
    @code{5}
    @item handle-power-key
    @code{poweroff}
    @item handle-suspend-key
    @code{suspend}
    @item handle-hibernate-key
    @code{hibernate}
    @item handle-lid-switch
    @code{suspend}
    @item handle-lid-switch-docked
    @code{ignore}
    @item power-key-ignore-inhibited?
    @code{#f}
    @item suspend-key-ignore-inhibited?
    @code{#f}
    @item hibernate-key-ignore-inhibited?
    @code{#f}
    @item lid-switch-ignore-inhibited?
    @code{#t}
    @item holdoff-timeout-seconds
    @code{30}
    @item idle-action
    @code{ignore}
    @item idle-action-seconds
    @code{(* 30 60)}
    @item runtime-directory-size-percent
    @code{10}
    @item runtime-directory-size
    @code{#f}
    @item remove-ipc?
    @code{#t}
    @item suspend-state
    @code{("mem" "standby" "freeze")}
    @item suspend-mode
    @code{()}
    @item hibernate-state
    @code{("disk")}
    @item hibernate-mode
    @code{("platform" "shutdown")}
    @item hybrid-sleep-state
    @code{("disk")}
    @item hybrid-sleep-mode
    @code{("suspend" "platform" "shutdown")}
    @end table
    @end deffn
    
    @deffn {Scheme Procedure} polkit-service @
                             [#:polkit @var{polkit}]
    Return a service that runs the
    @uref{http://www.freedesktop.org/wiki/Software/polkit/, Polkit privilege
    management service}, which allows system administrators to grant access to
    privileged operations in a structured way.  By querying the Polkit service, a
    privileged system component can know when it should grant additional
    capabilities to ordinary users.  For example, an ordinary user can be granted
    the capability to suspend the system if the user is logged in locally.
    @end deffn
    
    @deffn {Scheme Procedure} upower-service [#:upower @var{upower}] @
                             [#:watts-up-pro? #f] @
                             [#:poll-batteries? #t] @
                             [#:ignore-lid? #f] @
                             [#:use-percentage-for-policy? #f] @
                             [#:percentage-low 10] @
                             [#:percentage-critical 3] @
                             [#:percentage-action 2] @
                             [#:time-low 1200] @
                             [#:time-critical 300] @
                             [#:time-action 120] @
                             [#:critical-power-action 'hybrid-sleep]
    Return a service that runs @uref{http://upower.freedesktop.org/,
    @command{upowerd}}, a system-wide monitor for power consumption and battery
    levels, with the given configuration settings.  It implements the
    @code{org.freedesktop.UPower} D-Bus interface, and is notably used by
    GNOME.
    @end deffn
    
    @deffn {Scheme Procedure} udisks-service [#:udisks @var{udisks}]
    Return a service for @uref{http://udisks.freedesktop.org/docs/latest/,
    UDisks}, a @dfn{disk management} daemon that provides user interfaces with
    notifications and ways to mount/unmount disks.  Programs that talk to UDisks
    include the @command{udisksctl} command, part of UDisks, and GNOME Disks.
    @end deffn
    
    @deffn {Scheme Procedure} colord-service [#:colord @var{colord}]
    Return a service that runs @command{colord}, a system service with a D-Bus
    interface to manage the color profiles of input and output devices such as
    screens and scanners.  It is notably used by the GNOME Color Manager graphical
    tool.  See @uref{http://www.freedesktop.org/software/colord/, the colord web
    site} for more information.
    @end deffn
    
    @deffn {Scheme Procedure} geoclue-application name [#:allowed? #t] [#:system? #f] [#:users '()]
    Return a configuration allowing an application to access GeoClue
    location data.  @var{name} is the Desktop ID of the application, without
    the @code{.desktop} part.  If @var{allowed?} is true, the application
    will have access to location information by default.  The boolean
    @var{system?}  value indicates whether an application is a system component
    or not.  Finally @var{users} is a list of UIDs of all users for which
    this application is allowed location info access.  An empty users list
    means that all users are allowed.
    @end deffn
    
    @defvr {Scheme Variable} %standard-geoclue-applications
    The standard list of well-known GeoClue application configurations,
    granting authority to the GNOME date-and-time utility to ask for the
    current location in order to set the time zone, and allowing the
    IceCat and Epiphany web browsers to request location information.
    IceCat and Epiphany both query the user before allowing a web page to
    know the user's location.
    @end defvr
    
    @deffn {Scheme Procedure} geoclue-service [#:colord @var{colord}] @
                             [#:whitelist '()] @
                             [#:wifi-geolocation-url "https://location.services.mozilla.com/v1/geolocate?key=geoclue"] @
                             [#:submit-data? #f]
                             [#:wifi-submission-url "https://location.services.mozilla.com/v1/submit?key=geoclue"] @
                             [#:submission-nick "geoclue"] @
                             [#:applications %standard-geoclue-applications]
    Return a service that runs the GeoClue location service.  This service
    provides a D-Bus interface to allow applications to request access to a
    user's physical location, and optionally to add information to online
    location databases.  See
    @uref{https://wiki.freedesktop.org/www/Software/GeoClue/, the GeoClue
    web site} for more information.
    @end deffn
    
    @deffn {Scheme Procedure} bluetooth-service [#:bluez @var{bluez}]
    Return a service that runs the @command{bluetoothd} daemon, which manages
    all the Bluetooth devices and provides a number of D-Bus interfaces.
    
    Users need to be in the @code{lp} group to access the D-Bus service.
    @end deffn
    
    @node Database Services
    @subsubsection Database Services
    
    The @code{(gnu services databases)} module provides the following services.
    
    @deffn {Scheme Procedure} postgresql-service [#:postgresql postgresql] @
           [#:config-file] [#:data-directory ``/var/lib/postgresql/data'']
    Return a service that runs @var{postgresql}, the PostgreSQL database
    server.
    
    The PostgreSQL daemon loads its runtime configuration from
    @var{config-file} and stores the database cluster in
    @var{data-directory}.
    @end deffn
    
    @deffn {Scheme Procedure} mysql-service [#:config (mysql-configuration)]
    Return a service that runs @command{mysqld}, the MySQL or MariaDB
    database server.
    
    The optional @var{config} argument specifies the configuration for
    @command{mysqld}, which should be a @code{<mysql-configuraiton>} object.
    @end deffn
    
    @deftp {Data Type} mysql-configuration
    Data type representing the configuration of @var{mysql-service}.
    
    @table @asis
    @item @code{mysql} (default: @var{mariadb})
    Package object of the MySQL database server, can be either @var{mariadb}
    or @var{mysql}.
    
    For MySQL, a temorary root password will be displayed at activation time.
    For MariaDB, the root password is empty.
    @end table
    @end deftp
    
    @node Mail Services
    @subsubsection Mail Services
    
    The @code{(gnu services mail)} module provides Guix service definitions
    for mail services.  Currently the only implemented service is Dovecot,
    an IMAP, POP3, and LMTP server.
    
    Guix does not yet have a mail transfer agent (MTA), although for some
    lightweight purposes the @code{esmtp} relay-only MTA may suffice.  Help
    is needed to properly integrate a full MTA, such as Postfix.  Patches
    welcome!
    
    To add an IMAP/POP3 server to a GuixSD system, add a
    @code{dovecot-service} to the operating system definition:
    
    @deffn {Scheme Procedure} dovecot-service [#:config (dovecot-configuration)]
    Return a service that runs the Dovecot IMAP/POP3/LMTP mail server.
    @end deffn
    
    By default, Dovecot does not need much configuration; the default
    configuration object created by @code{(dovecot-configuration)} will
    suffice if your mail is delivered to @code{~/Maildir}.  A self-signed
    certificate will be generated for TLS-protected connections, though
    Dovecot will also listen on cleartext ports by default.  There are a
    number of options, though, which mail administrators might need to change,
    and as is the case with other services, Guix allows the system
    administrator to specify these parameters via a uniform Scheme interface.
    
    For example, to specify that mail is located at @code{maildir~/.mail},
    one would instantiate the Dovecot service like this:
    
    @example
    (dovecot-service #:config
                     (dovecot-configuration
                      (mail-location "maildir:~/.mail")))
    @end example
    
    The available configuration parameters follow.  Each parameter
    definition is preceded by its type; for example, @samp{string-list foo}
    indicates that the @code{foo} parameter should be specified as a list of
    strings.  There is also a way to specify the configuration as a string,
    if you have an old @code{dovecot.conf} file that you want to port over
    from some other system; see the end for more details.
    
    @c The following documentation was initially generated by
    @c (generate-documentation) in (gnu services mail).  Manually maintained
    @c documentation is better, so we shouldn't hesitate to edit below as
    @c needed.  However if the change you want to make to this documentation
    @c can be done in an automated way, it's probably easier to change
    @c (generate-documentation) than to make it below and have to deal with
    @c the churn as dovecot updates.
    
    Available @code{dovecot-configuration} fields are:
    
    @deftypevr {@code{dovecot-configuration} parameter} package dovecot
    The dovecot package.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} comma-separated-string-list listen
    A list of IPs or hosts where to listen for connections.  @samp{*}
    listens on all IPv4 interfaces, @samp{::} listens on all IPv6
    interfaces.  If you want to specify non-default ports or anything more
    complex, customize the address and port fields of the
    @samp{inet-listener} of the specific services you are interested in.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} protocol-configuration-list protocols
    List of protocols we want to serve.  Available protocols include
    @samp{imap}, @samp{pop3}, and @samp{lmtp}.
    
    Available @code{protocol-configuration} fields are:
    
    @deftypevr {@code{protocol-configuration} parameter} string name
    The name of the protocol.
    @end deftypevr
    
    @deftypevr {@code{protocol-configuration} parameter} string auth-socket-path
    UNIX socket path to the master authentication server to find users.
    This is used by imap (for shared users) and lda.
    It defaults to @samp{"/var/run/dovecot/auth-userdb"}.
    @end deftypevr
    
    @deftypevr {@code{protocol-configuration} parameter} space-separated-string-list mail-plugins
    Space separated list of plugins to load.
    @end deftypevr
    
    @deftypevr {@code{protocol-configuration} parameter} non-negative-integer mail-max-userip-connections
    Maximum number of IMAP connections allowed for a user from each IP
    address.  NOTE: The username is compared case-sensitively.
    Defaults to @samp{10}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} service-configuration-list services
    List of services to enable.  Available services include @samp{imap},
    @samp{imap-login}, @samp{pop3}, @samp{pop3-login}, @samp{auth}, and
    @samp{lmtp}.
    
    Available @code{service-configuration} fields are:
    
    @deftypevr {@code{service-configuration} parameter} string kind
    The service kind.  Valid values include @code{director},
    @code{imap-login}, @code{pop3-login}, @code{lmtp}, @code{imap},
    @code{pop3}, @code{auth}, @code{auth-worker}, @code{dict},
    @code{tcpwrap}, @code{quota-warning}, or anything else.
    @end deftypevr
    
    @deftypevr {@code{service-configuration} parameter} listener-configuration-list listeners
    Listeners for the service.  A listener is either a
    @code{unix-listener-configuration}, a @code{fifo-listener-configuration}, or
    an @code{inet-listener-configuration}.
    Defaults to @samp{()}.
    
    Available @code{unix-listener-configuration} fields are:
    
    @deftypevr {@code{unix-listener-configuration} parameter} file-name path
    The file name on which to listen.
    @end deftypevr
    
    @deftypevr {@code{unix-listener-configuration} parameter} string mode
    The access mode for the socket.
    Defaults to @samp{"0600"}.
    @end deftypevr
    
    @deftypevr {@code{unix-listener-configuration} parameter} string user
    The user to own the socket.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{unix-listener-configuration} parameter} string group
    The group to own the socket.
    Defaults to @samp{""}.
    @end deftypevr
    
    
    Available @code{fifo-listener-configuration} fields are:
    
    @deftypevr {@code{fifo-listener-configuration} parameter} file-name path
    The file name on which to listen.
    @end deftypevr
    
    @deftypevr {@code{fifo-listener-configuration} parameter} string mode
    The access mode for the socket.
    Defaults to @samp{"0600"}.
    @end deftypevr
    
    @deftypevr {@code{fifo-listener-configuration} parameter} string user
    The user to own the socket.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{fifo-listener-configuration} parameter} string group
    The group to own the socket.
    Defaults to @samp{""}.
    @end deftypevr
    
    
    Available @code{inet-listener-configuration} fields are:
    
    @deftypevr {@code{inet-listener-configuration} parameter} string protocol
    The protocol to listen for.
    @end deftypevr
    
    @deftypevr {@code{inet-listener-configuration} parameter} string address
    The address on which to listen, or empty for all addresses.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{inet-listener-configuration} parameter} non-negative-integer port
    The port on which to listen.
    @end deftypevr
    
    @deftypevr {@code{inet-listener-configuration} parameter} boolean ssl?
    Whether to use SSL for this service; @samp{yes}, @samp{no}, or
    @samp{required}.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{service-configuration} parameter} non-negative-integer service-count
    Number of connections to handle before starting a new process.
    Typically the only useful values are 0 (unlimited) or 1.  1 is more
    secure, but 0 is faster.  <doc/wiki/LoginProcess.txt>.
    Defaults to @samp{1}.
    @end deftypevr
    
    @deftypevr {@code{service-configuration} parameter} non-negative-integer process-min-avail
    Number of processes to always keep waiting for more connections.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{service-configuration} parameter} non-negative-integer vsz-limit
    If you set @samp{service-count 0}, you probably need to grow
    this.
    Defaults to @samp{256000000}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} dict-configuration dict
    Dict configuration, as created by the @code{dict-configuration}
    constructor.
    
    Available @code{dict-configuration} fields are:
    
    @deftypevr {@code{dict-configuration} parameter} free-form-fields entries
    A list of key-value pairs that this dict should hold.
    Defaults to @samp{()}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} passdb-configuration-list passdbs
    A list of passdb configurations, each one created by the
    @code{passdb-configuration} constructor.
    
    Available @code{passdb-configuration} fields are:
    
    @deftypevr {@code{passdb-configuration} parameter} string driver
    The driver that the passdb should use.  Valid values include
    @samp{pam}, @samp{passwd}, @samp{shadow}, @samp{bsdauth}, and
    @samp{static}.
    Defaults to @samp{"pam"}.
    @end deftypevr
    
    @deftypevr {@code{passdb-configuration} parameter} free-form-args args
    A list of key-value args to the passdb driver.
    Defaults to @samp{()}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} userdb-configuration-list userdbs
    List of userdb configurations, each one created by the
    @code{userdb-configuration} constructor.
    
    Available @code{userdb-configuration} fields are:
    
    @deftypevr {@code{userdb-configuration} parameter} string driver
    The driver that the userdb should use.  Valid values include
    @samp{passwd} and @samp{static}.
    Defaults to @samp{"passwd"}.
    @end deftypevr
    
    @deftypevr {@code{userdb-configuration} parameter} free-form-args args
    A list of key-value args to the userdb driver.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{userdb-configuration} parameter} free-form-args override-fields
    Override fields from passwd.
    Defaults to @samp{()}.
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} plugin-configuration plugin-configuration
    Plug-in configuration, created by the @code{plugin-configuration}
    constructor.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} list-of-namespace-configuration namespaces
    List of namespaces.  Each item in the list is created by the
    @code{namespace-configuration} constructor.
    
    Available @code{namespace-configuration} fields are:
    
    @deftypevr {@code{namespace-configuration} parameter} string name
    Name for this namespace.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} string type
    Namespace type: @samp{private}, @samp{shared} or @samp{public}.
    Defaults to @samp{"private"}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} string separator
    Hierarchy separator to use. You should use the same separator for
    all namespaces or some clients get confused.  @samp{/} is usually a good
    one.  The default however depends on the underlying mail storage
    format.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} string prefix
    Prefix required to access this namespace.  This needs to be
    different for all namespaces. For example @samp{Public/}.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} string location
    Physical location of the mailbox. This is in the same format as
    mail_location, which is also the default for it.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} boolean inbox?
    There can be only one INBOX, and this setting defines which
    namespace has it.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} boolean hidden?
    If namespace is hidden, it's not advertised to clients via NAMESPACE
    extension. You'll most likely also want to set @samp{list? #f}.  This is mostly
    useful when converting from another server with different namespaces
    which you want to deprecate but still keep working.  For example you can
    create hidden namespaces with prefixes @samp{~/mail/}, @samp{~%u/mail/}
    and @samp{mail/}.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} boolean list?
    Show the mailboxes under this namespace with the LIST command. This
    makes the namespace visible for clients that do not support the NAMESPACE
    extension.  The special @code{children} value lists child mailboxes, but
    hides the namespace prefix.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} boolean subscriptions?
    Namespace handles its own subscriptions.  If set to @code{#f}, the
    parent namespace handles them.  The empty prefix should always have this
    as @code{#t}).
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{namespace-configuration} parameter} mailbox-configuration-list mailboxes
    List of predefined mailboxes in this namespace.
    Defaults to @samp{()}.
    
    Available @code{mailbox-configuration} fields are:
    
    @deftypevr {@code{mailbox-configuration} parameter} string name
    Name for this mailbox.
    @end deftypevr
    
    @deftypevr {@code{mailbox-configuration} parameter} string auto
    @samp{create} will automatically create this mailbox.
    @samp{subscribe} will both create and subscribe to the mailbox.
    Defaults to @samp{"no"}.
    @end deftypevr
    
    @deftypevr {@code{mailbox-configuration} parameter} space-separated-string-list special-use
    List of IMAP @code{SPECIAL-USE} attributes as specified by RFC 6154.
    Valid values are @code{\All}, @code{\Archive}, @code{\Drafts},
    @code{\Flagged}, @code{\Junk}, @code{\Sent}, and @code{\Trash}.
    Defaults to @samp{()}.
    @end deftypevr
    
    @end deftypevr
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name base-dir
    Base directory where to store runtime data.
    Defaults to @samp{"/var/run/dovecot/"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string login-greeting
    Greeting message for clients.
    Defaults to @samp{"Dovecot ready."}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list login-trusted-networks
    List of trusted network ranges.  Connections from these IPs are
    allowed to override their IP addresses and ports (for logging and for
    authentication checks).  @samp{disable-plaintext-auth} is also ignored
    for these networks.  Typically you would specify your IMAP proxy servers
    here.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list login-access-sockets
    List of login access check sockets (e.g. tcpwrap).
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean verbose-proctitle?
    Show more verbose process titles (in ps).  Currently shows user name
    and IP address.  Useful for seeing who is actually using the IMAP
    processes (e.g. shared mailboxes or if the same uid is used for multiple
    accounts).
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean shutdown-clients?
    Should all processes be killed when Dovecot master process shuts down.
    Setting this to @code{#f} means that Dovecot can be upgraded without
    forcing existing client connections to close (although that could also
    be a problem if the upgrade is e.g. due to a security fix).
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer doveadm-worker-count
    If non-zero, run mail commands via this many connections to doveadm
    server, instead of running them directly in the same process.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string doveadm-socket-path
    UNIX socket or host:port used for connecting to doveadm server.
    Defaults to @samp{"doveadm-server"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list import-environment
    List of environment variables that are preserved on Dovecot startup
    and passed down to all of its child processes.  You can also give
    key=value pairs to always set specific settings.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean disable-plaintext-auth?
    Disable LOGIN command and all other plaintext authentications unless
    SSL/TLS is used (LOGINDISABLED capability).  Note that if the remote IP
    matches the local IP (i.e. you're connecting from the same computer),
    the connection is considered secure and plaintext authentication is
    allowed.  See also ssl=required setting.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer auth-cache-size
    Authentication cache size (e.g. @samp{#e10e6}).  0 means it's disabled.
    Note that bsdauth, PAM and vpopmail require @samp{cache-key} to be set
    for caching to be used.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-cache-ttl
    Time to live for cached data.  After TTL expires the cached record
    is no longer used, *except* if the main database lookup returns internal
    failure.  We also try to handle password changes automatically: If
    user's previous authentication was successful, but this one wasn't, the
    cache isn't used.  For now this works only with plaintext
    authentication.
    Defaults to @samp{"1 hour"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-cache-negative-ttl
    TTL for negative hits (user not found, password mismatch).
    0 disables caching them completely.
    Defaults to @samp{"1 hour"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list auth-realms
    List of realms for SASL authentication mechanisms that need them.
    You can leave it empty if you don't want to support multiple realms.
    Many clients simply use the first one listed here, so keep the default
    realm first.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-default-realm
    Default realm/domain to use if none was specified.  This is used for
    both SASL realms and appending @@domain to username in plaintext
    logins.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-username-chars
    List of allowed characters in username.  If the user-given username
    contains a character not listed in here, the login automatically fails.
    This is just an extra check to make sure user can't exploit any
    potential quote escaping vulnerabilities with SQL/LDAP databases.  If
    you want to allow all characters, set this value to empty.
    Defaults to @samp{"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@@"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-username-translation
    Username character translations before it's looked up from
    databases.  The value contains series of from -> to characters.  For
    example @samp{#@@/@@} means that @samp{#} and @samp{/} characters are
    translated to @samp{@@}.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-username-format
    Username formatting before it's looked up from databases.  You can
    use the standard variables here, e.g. %Lu would lowercase the username,
    %n would drop away the domain if it was given, or @samp{%n-AT-%d} would
    change the @samp{@@} into @samp{-AT-}.  This translation is done after
    @samp{auth-username-translation} changes.
    Defaults to @samp{"%Lu"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-master-user-separator
    If you want to allow master users to log in by specifying the master
    username within the normal username string (i.e. not using SASL
    mechanism's support for it), you can specify the separator character
    here.  The format is then <username><separator><master username>.
    UW-IMAP uses @samp{*} as the separator, so that could be a good
    choice.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-anonymous-username
    Username to use for users logging in with ANONYMOUS SASL
    mechanism.
    Defaults to @samp{"anonymous"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer auth-worker-max-count
    Maximum number of dovecot-auth worker processes.  They're used to
    execute blocking passdb and userdb queries (e.g. MySQL and PAM).
    They're automatically created and destroyed as needed.
    Defaults to @samp{30}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-gssapi-hostname
    Host name to use in GSSAPI principal names.  The default is to use
    the name returned by gethostname().  Use @samp{$ALL} (with quotes) to
    allow all keytab entries.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-krb5-keytab
    Kerberos keytab to use for the GSSAPI mechanism.  Will use the
    system default (usually /etc/krb5.keytab) if not specified.  You may
    need to change the auth service to run as root to be able to read this
    file.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-use-winbind?
    Do NTLM and GSS-SPNEGO authentication using Samba's winbind daemon
    and @samp{ntlm-auth} helper.
    <doc/wiki/Authentication/Mechanisms/Winbind.txt>.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name auth-winbind-helper-path
    Path for Samba's @samp{ntlm-auth} helper binary.
    Defaults to @samp{"/usr/bin/ntlm_auth"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string auth-failure-delay
    Time to delay before replying to failed authentications.
    Defaults to @samp{"2 secs"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-ssl-require-client-cert?
    Require a valid SSL client certificate or the authentication
    fails.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-ssl-username-from-cert?
    Take the username from client's SSL certificate, using
    @code{X509_NAME_get_text_by_NID()} which returns the subject's DN's
    CommonName.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list auth-mechanisms
    List of wanted authentication mechanisms.  Supported mechanisms are:
    @samp{plain}, @samp{login}, @samp{digest-md5}, @samp{cram-md5},
    @samp{ntlm}, @samp{rpa}, @samp{apop}, @samp{anonymous}, @samp{gssapi},
    @samp{otp}, @samp{skey}, and @samp{gss-spnego}.  NOTE: See also
    @samp{disable-plaintext-auth} setting.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list director-servers
    List of IPs or hostnames to all director servers, including ourself.
    Ports can be specified as ip:port.  The default port is the same as what
    director service's @samp{inet-listener} is using.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list director-mail-servers
    List of IPs or hostnames to all backend mail servers.  Ranges are
    allowed too, like 10.0.0.10-10.0.0.30.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string director-user-expire
    How long to redirect users to a specific server after it no longer
    has any connections.
    Defaults to @samp{"15 min"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer director-doveadm-port
    TCP/IP port that accepts doveadm connections (instead of director
    connections) If you enable this, you'll also need to add
    @samp{inet-listener} for the port.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string director-username-hash
    How the username is translated before being hashed.  Useful values
    include %Ln if user can log in with or without @@domain, %Ld if mailboxes
    are shared within domain.
    Defaults to @samp{"%Lu"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string log-path
    Log file to use for error messages.  @samp{syslog} logs to syslog,
    @samp{/dev/stderr} logs to stderr.
    Defaults to @samp{"syslog"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string info-log-path
    Log file to use for informational messages.  Defaults to
    @samp{log-path}.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string debug-log-path
    Log file to use for debug messages.  Defaults to
    @samp{info-log-path}.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string syslog-facility
    Syslog facility to use if you're logging to syslog.  Usually if you
    don't want to use @samp{mail}, you'll use local0..local7.  Also other
    standard facilities are supported.
    Defaults to @samp{"mail"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-verbose?
    Log unsuccessful authentication attempts and the reasons why they
    failed.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-verbose-passwords?
    In case of password mismatches, log the attempted password.  Valid
    values are no, plain and sha1.  sha1 can be useful for detecting brute
    force password attempts vs.  user simply trying the same password over
    and over again.  You can also truncate the value to n chars by appending
    ":n" (e.g. sha1:6).
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-debug?
    Even more verbose logging for debugging purposes.  Shows for example
    SQL queries.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean auth-debug-passwords?
    In case of password mismatches, log the passwords and used scheme so
    the problem can be debugged.  Enabling this also enables
    @samp{auth-debug}.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mail-debug?
    Enable mail process debugging.  This can help you figure out why
    Dovecot isn't finding your mails.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean verbose-ssl?
    Show protocol level SSL errors.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string log-timestamp
    Prefix for each line written to log file.  % codes are in
    strftime(3) format.
    Defaults to @samp{"\"%b %d %H:%M:%S \""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list login-log-format-elements
    List of elements we want to log.  The elements which have a
    non-empty variable value are joined together to form a comma-separated
    string.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string login-log-format
    Login log format.  %s contains @samp{login-log-format-elements}
    string, %$ contains the data we want to log.
    Defaults to @samp{"%$: %s"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-log-prefix
    Log prefix for mail processes.  See doc/wiki/Variables.txt for list
    of possible variables you can use.
    Defaults to @samp{"\"%s(%u): \""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string deliver-log-format
    Format to use for logging mail deliveries.  You can use variables:
    @table @code
    @item %$
    Delivery status message (e.g. @samp{saved to INBOX})
    @item %m
    Message-ID
    @item %s
    Subject
    @item %f
    From address
    @item %p
    Physical size
    @item %w
    Virtual size.
    @end table
    Defaults to @samp{"msgid=%m: %$"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-location
    Location for users' mailboxes.  The default is empty, which means
    that Dovecot tries to find the mailboxes automatically.  This won't work
    if the user doesn't yet have any mail, so you should explicitly tell
    Dovecot the full location.
    
    If you're using mbox, giving a path to the INBOX
    file (e.g. /var/mail/%u) isn't enough.  You'll also need to tell Dovecot
    where the other mailboxes are kept.  This is called the "root mail
    directory", and it must be the first path given in the
    @samp{mail-location} setting.
    
    There are a few special variables you can use, eg.:
    
    @table @samp
    @item %u
    username
    @item %n
    user part in user@@domain, same as %u if there's no domain
    @item %d
    domain part in user@@domain, empty if there's no domain
    @item %h
    home director
    @end table
    
    See doc/wiki/Variables.txt for full list.  Some examples:
    @table @samp
    @item maildir:~/Maildir
    @item mbox:~/mail:INBOX=/var/mail/%u
    @item mbox:/var/mail/%d/%1n/%n:INDEX=/var/indexes/%d/%1n/%
    @end table
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-uid
    System user and group used to access mails.  If you use multiple,
    userdb can override these by returning uid or gid fields.  You can use
    either numbers or names.  <doc/wiki/UserIds.txt>.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-gid
    
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-privileged-group
    Group to enable temporarily for privileged operations.  Currently
    this is used only with INBOX when either its initial creation or
    dotlocking fails.  Typically this is set to "mail" to give access to
    /var/mail.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-access-groups
    Grant access to these supplementary groups for mail processes.
    Typically these are used to set up access to shared mailboxes.  Note
    that it may be dangerous to set these if users can create
    symlinks (e.g. if "mail" group is set here, ln -s /var/mail ~/mail/var
    could allow a user to delete others' mailboxes, or ln -s
    /secret/shared/box ~/mail/mybox would allow reading it).
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mail-full-filesystem-access?
    Allow full filesystem access to clients.  There's no access checks
    other than what the operating system does for the active UID/GID.  It
    works with both maildir and mboxes, allowing you to prefix mailboxes
    names with e.g. /path/ or ~user/.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mmap-disable?
    Don't use mmap() at all.  This is required if you store indexes to
    shared filesystems (NFS or clustered filesystem).
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean dotlock-use-excl?
    Rely on @samp{O_EXCL} to work when creating dotlock files.  NFS
    supports @samp{O_EXCL} since version 3, so this should be safe to use
    nowadays by default.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-fsync
    When to use fsync() or fdatasync() calls:
    @table @code
    @item optimized
    Whenever necessary to avoid losing important data
    @item always
    Useful with e.g. NFS when write()s are delayed
    @item never
    Never use it (best performance, but crashes can lose data).
    @end table
    Defaults to @samp{"optimized"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mail-nfs-storage?
    Mail storage exists in NFS.  Set this to yes to make Dovecot flush
    NFS caches whenever needed.  If you're using only a single mail server
    this isn't needed.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mail-nfs-index?
    Mail index files also exist in NFS.  Setting this to yes requires
    @samp{mmap-disable? #t} and @samp{fsync-disable? #f}.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string lock-method
    Locking method for index files.  Alternatives are fcntl, flock and
    dotlock.  Dotlocking uses some tricks which may create more disk I/O
    than other locking methods.  NFS users: flock doesn't work, remember to
    change @samp{mmap-disable}.
    Defaults to @samp{"fcntl"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name mail-temp-dir
    Directory in which LDA/LMTP temporarily stores incoming mails >128
    kB.
    Defaults to @samp{"/tmp"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer first-valid-uid
    Valid UID range for users.  This is mostly to make sure that users can't
    log in as daemons or other system users.  Note that denying root logins is
    hardcoded to dovecot binary and can't be done even if @samp{first-valid-uid}
    is set to 0.
    Defaults to @samp{500}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer last-valid-uid
    
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer first-valid-gid
    Valid GID range for users.  Users having non-valid GID as primary group ID
    aren't allowed to log in.  If user belongs to supplementary groups with
    non-valid GIDs, those groups are not set.
    Defaults to @samp{1}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer last-valid-gid
    
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer mail-max-keyword-length
    Maximum allowed length for mail keyword name.  It's only forced when
    trying to create new keywords.
    Defaults to @samp{50}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} colon-separated-file-name-list valid-chroot-dirs
    List of directories under which chrooting is allowed for mail
    processes (i.e. /var/mail will allow chrooting to /var/mail/foo/bar
    too).  This setting doesn't affect @samp{login-chroot}
    @samp{mail-chroot} or auth chroot settings.  If this setting is empty,
    "/./" in home dirs are ignored.  WARNING: Never add directories here
    which local users can modify, that may lead to root exploit.  Usually
    this should be done only if you don't allow shell access for users.
    <doc/wiki/Chrooting.txt>.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-chroot
    Default chroot directory for mail processes.  This can be overridden
    for specific users in user database by giving /./ in user's home
    directory (e.g. /home/./user chroots into /home).  Note that usually
    there is no real need to do chrooting, Dovecot doesn't allow users to
    access files outside their mail directory anyway.  If your home
    directories are prefixed with the chroot directory, append "/." to
    @samp{mail-chroot}.  <doc/wiki/Chrooting.txt>.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name auth-socket-path
    UNIX socket path to master authentication server to find users.
    This is used by imap (for shared users) and lda.
    Defaults to @samp{"/var/run/dovecot/auth-userdb"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name mail-plugin-dir
    Directory where to look up mail plugins.
    Defaults to @samp{"/usr/lib/dovecot"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list mail-plugins
    List of plugins to load for all services.  Plugins specific to IMAP,
    LDA, etc. are added to this list in their own .conf files.
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer mail-cache-min-mail-count
    The minimum number of mails in a mailbox before updates are done to
    cache file.  This allows optimizing Dovecot's behavior to do less disk
    writes at the cost of more disk reads.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mailbox-idle-check-interval
    When IDLE command is running, mailbox is checked once in a while to
    see if there are any new mails or other changes.  This setting defines
    the minimum time to wait between those checks.  Dovecot can also use
    dnotify, inotify and kqueue to find out immediately when changes
    occur.
    Defaults to @samp{"30 secs"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mail-save-crlf?
    Save mails with CR+LF instead of plain LF.  This makes sending those
    mails take less CPU, especially with sendfile() syscall with Linux and
    FreeBSD.  But it also creates a bit more disk I/O which may just make it
    slower.  Also note that if other software reads the mboxes/maildirs,
    they may handle the extra CRs wrong and cause problems.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean maildir-stat-dirs?
    By default LIST command returns all entries in maildir beginning
    with a dot.  Enabling this option makes Dovecot return only entries
    which are directories.  This is done by stat()ing each entry, so it
    causes more disk I/O.
     (For systems setting struct @samp{dirent->d_type} this check is free
    and it's done always regardless of this setting).
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean maildir-copy-with-hardlinks?
    When copying a message, do it with hard links whenever possible.
    This makes the performance much better, and it's unlikely to have any
    side effects.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean maildir-very-dirty-syncs?
    Assume Dovecot is the only MUA accessing Maildir: Scan cur/
    directory only when its mtime changes unexpectedly or when we can't find
    the mail otherwise.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list mbox-read-locks
    Which locking methods to use for locking mbox.  There are four
    available:
    
    @table @code
    @item dotlock
    Create <mailbox>.lock file.  This is the oldest and most NFS-safe
    solution.  If you want to use /var/mail/ like directory, the users will
    need write access to that directory.
    @item dotlock-try
    Same as dotlock, but if it fails because of permissions or because there
    isn't enough disk space, just skip it.
    @item fcntl
    Use this if possible.  Works with NFS too if lockd is used.
    @item flock
    May not exist in all systems.  Doesn't work with NFS.
    @item lockf
    May not exist in all systems.  Doesn't work with NFS.
    @end table
    
    You can use multiple locking methods; if you do the order they're declared
    in is important to avoid deadlocks if other MTAs/MUAs are using multiple
    locking methods as well.  Some operating systems don't allow using some of
    them simultaneously.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list mbox-write-locks
    
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mbox-lock-timeout
    Maximum time to wait for lock (all of them) before aborting.
    Defaults to @samp{"5 mins"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mbox-dotlock-change-timeout
    If dotlock exists but the mailbox isn't modified in any way,
    override the lock file after this much time.
    Defaults to @samp{"2 mins"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mbox-dirty-syncs?
    When mbox changes unexpectedly we have to fully read it to find out
    what changed.  If the mbox is large this can take a long time.  Since
    the change is usually just a newly appended mail, it'd be faster to
    simply read the new mails.  If this setting is enabled, Dovecot does
    this but still safely fallbacks to re-reading the whole mbox file
    whenever something in mbox isn't how it's expected to be.  The only real
    downside to this setting is that if some other MUA changes message
    flags, Dovecot doesn't notice it immediately.  Note that a full sync is
    done with SELECT, EXAMINE, EXPUNGE and CHECK commands.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mbox-very-dirty-syncs?
    Like @samp{mbox-dirty-syncs}, but don't do full syncs even with SELECT,
    EXAMINE, EXPUNGE or CHECK commands.  If this is set,
    @samp{mbox-dirty-syncs} is ignored.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mbox-lazy-writes?
    Delay writing mbox headers until doing a full write sync (EXPUNGE
    and CHECK commands and when closing the mailbox).  This is especially
    useful for POP3 where clients often delete all mails.  The downside is
    that our changes aren't immediately visible to other MUAs.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer mbox-min-index-size
    If mbox size is smaller than this (e.g. 100k), don't write index
    files.  If an index file already exists it's still read, just not
    updated.
    Defaults to @samp{0}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer mdbox-rotate-size
    Maximum dbox file size until it's rotated.
    Defaults to @samp{2000000}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mdbox-rotate-interval
    Maximum dbox file age until it's rotated.  Typically in days.  Day
    begins from midnight, so 1d = today, 2d = yesterday, etc.  0 = check
    disabled.
    Defaults to @samp{"1d"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean mdbox-preallocate-space?
    When creating new mdbox files, immediately preallocate their size to
    @samp{mdbox-rotate-size}.  This setting currently works only in Linux
    with some filesystems (ext4, xfs).
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-attachment-dir
    sdbox and mdbox support saving mail attachments to external files,
    which also allows single instance storage for them.  Other backends
    don't support this for now.
    
    WARNING: This feature hasn't been tested much yet.  Use at your own risk.
    
    Directory root where to store mail attachments.  Disabled, if empty.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer mail-attachment-min-size
    Attachments smaller than this aren't saved externally.  It's also
    possible to write a plugin to disable saving specific attachments
    externally.
    Defaults to @samp{128000}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-attachment-fs
    Filesystem backend to use for saving attachments:
    @table @code
    @item posix
    No SiS done by Dovecot (but this might help FS's own deduplication)
    @item sis posix
    SiS with immediate byte-by-byte comparison during saving
    @item sis-queue posix
    SiS with delayed comparison and deduplication.
    @end table
    Defaults to @samp{"sis posix"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string mail-attachment-hash
    Hash format to use in attachment filenames.  You can add any text and
    variables: @code{%@{md4@}}, @code{%@{md5@}}, @code{%@{sha1@}},
    @code{%@{sha256@}}, @code{%@{sha512@}}, @code{%@{size@}}.  Variables can be
    truncated, e.g. @code{%@{sha256:80@}} returns only first 80 bits.
    Defaults to @samp{"%@{sha1@}"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer default-process-limit
    
    Defaults to @samp{100}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer default-client-limit
    
    Defaults to @samp{1000}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer default-vsz-limit
    Default VSZ (virtual memory size) limit for service processes.
    This is mainly intended to catch and kill processes that leak memory
    before they eat up everything.
    Defaults to @samp{256000000}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string default-login-user
    Login user is internally used by login processes.  This is the most
    untrusted user in Dovecot system.  It shouldn't have access to anything
    at all.
    Defaults to @samp{"dovenull"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string default-internal-user
    Internal user is used by unprivileged processes.  It should be
    separate from login user, so that login processes can't disturb other
    processes.
    Defaults to @samp{"dovecot"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl?
    SSL/TLS support: yes, no, required.  <doc/wiki/SSL.txt>.
    Defaults to @samp{"required"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-cert
    PEM encoded X.509 SSL/TLS certificate (public key).
    Defaults to @samp{"</etc/dovecot/default.pem"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-key
    PEM encoded SSL/TLS private key.  The key is opened before
    dropping root privileges, so keep the key file unreadable by anyone but
    root.
    Defaults to @samp{"</etc/dovecot/private/default.pem"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-key-password
    If key file is password protected, give the password here.
    Alternatively give it when starting dovecot with -p parameter.  Since
    this file is often world-readable, you may want to place this setting
    instead to a different.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-ca
    PEM encoded trusted certificate authority.  Set this only if you
    intend to use @samp{ssl-verify-client-cert? #t}.  The file should
    contain the CA certificate(s) followed by the matching
    CRL(s).  (e.g. @samp{ssl-ca </etc/ssl/certs/ca.pem}).
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean ssl-require-crl?
    Require that CRL check succeeds for client certificates.
    Defaults to @samp{#t}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean ssl-verify-client-cert?
    Request client to send a certificate.  If you also want to require
    it, set @samp{auth-ssl-require-client-cert? #t} in auth section.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-cert-username-field
    Which field from certificate to use for username.  commonName and
    x500UniqueIdentifier are the usual choices.  You'll also need to set
    @samp{auth-ssl-username-from-cert? #t}.
    Defaults to @samp{"commonName"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} hours ssl-parameters-regenerate
    How often to regenerate the SSL parameters file.  Generation is
    quite CPU intensive operation.  The value is in hours, 0 disables
    regeneration entirely.
    Defaults to @samp{168}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-protocols
    SSL protocols to use.
    Defaults to @samp{"!SSLv2"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-cipher-list
    SSL ciphers to use.
    Defaults to @samp{"ALL:!LOW:!SSLv2:!EXP:!aNULL"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string ssl-crypto-device
    SSL crypto device to use, for valid values run "openssl engine".
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string postmaster-address
    Address to use when sending rejection mails.
    Default is postmaster@@<your domain>.  %d expands to recipient domain.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string hostname
    Hostname to use in various parts of sent mails (e.g. in Message-Id)
    and in LMTP replies.  Default is the system's real hostname@@domain.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean quota-full-tempfail?
    If user is over quota, return with temporary failure instead of
    bouncing the mail.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} file-name sendmail-path
    Binary to use for sending mails.
    Defaults to @samp{"/usr/sbin/sendmail"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string submission-host
    If non-empty, send mails via this SMTP host[:port] instead of
    sendmail.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string rejection-subject
    Subject: header to use for rejection mails.  You can use the same
    variables as for @samp{rejection-reason} below.
    Defaults to @samp{"Rejected: %s"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string rejection-reason
    Human readable error message for rejection mails.  You can use
    variables:
    
    @table @code
    @item %n
    CRLF
    @item %r
    reason
    @item %s
    original subject
    @item %t
    recipient
    @end table
    Defaults to @samp{"Your message to <%t> was automatically rejected:%n%r"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string recipient-delimiter
    Delimiter character between local-part and detail in email
    address.
    Defaults to @samp{"+"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string lda-original-recipient-header
    Header where the original recipient address (SMTP's RCPT TO:
    address) is taken from if not available elsewhere.  With dovecot-lda -a
    parameter overrides this.  A commonly used header for this is
    X-Original-To.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean lda-mailbox-autocreate?
    Should saving a mail to a nonexistent mailbox automatically create
    it?.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} boolean lda-mailbox-autosubscribe?
    Should automatically created mailboxes be also automatically
    subscribed?.
    Defaults to @samp{#f}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} non-negative-integer imap-max-line-length
    Maximum IMAP command line length.  Some clients generate very long
    command lines with huge mailboxes, so you may need to raise this if you
    get "Too long argument" or "IMAP command line too large" errors
    often.
    Defaults to @samp{64000}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-logout-format
    IMAP logout format string:
    @table @code
    @item %i
    total number of bytes read from client
    @item %o
    total number of bytes sent to client.
    @end table
    Defaults to @samp{"in=%i out=%o"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-capability
    Override the IMAP CAPABILITY response.  If the value begins with '+',
    add the given capabilities on top of the defaults (e.g. +XFOO XBAR).
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-idle-notify-interval
    How long to wait between "OK Still here" notifications when client
    is IDLEing.
    Defaults to @samp{"2 mins"}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-id-send
    ID field names and values to send to clients.  Using * as the value
    makes Dovecot use the default value.  The following fields have default
    values currently: name, version, os, os-version, support-url,
    support-email.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-id-log
    ID fields sent by client to log.  * means everything.
    Defaults to @samp{""}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} space-separated-string-list imap-client-workarounds
    Workarounds for various client bugs:
    
    @table @code
    @item delay-newmail
    Send EXISTS/RECENT new mail notifications only when replying to NOOP and
    CHECK commands.  Some clients ignore them otherwise, for example OSX
    Mail (<v2.1).  Outlook Express breaks more badly though, without this it
    may show user "Message no longer in server" errors.  Note that OE6
    still breaks even with this workaround if synchronization is set to
    "Headers Only".
    
    @item tb-extra-mailbox-sep
    Thunderbird gets somehow confused with LAYOUT=fs (mbox and dbox) and
    adds extra @samp{/} suffixes to mailbox names.  This option causes Dovecot to
    ignore the extra @samp{/} instead of treating it as invalid mailbox name.
    
    @item tb-lsub-flags
    Show \Noselect flags for LSUB replies with LAYOUT=fs (e.g. mbox).
    This makes Thunderbird realize they aren't selectable and show them
    greyed out, instead of only later giving "not selectable" popup error.
    @end table
    Defaults to @samp{()}.
    @end deftypevr
    
    @deftypevr {@code{dovecot-configuration} parameter} string imap-urlauth-host
    Host allowed in URLAUTH URLs sent by client.  "*" allows all.
    Defaults to @samp{""}.
    @end deftypevr
    
    
    Whew!  Lots of configuration options.  The nice thing about it though is
    that GuixSD has a complete interface to Dovecot's configuration
    language.  This allows not only a nice way to declare configurations,
    but also offers reflective capabilities as well: users can write code to
    inspect and transform configurations from within Scheme.
    
    However, it could be that you just want to get a @code{dovecot.conf} up
    and running.  In that case, you can pass an
    @code{opaque-dovecot-configuration} as the @code{#:config} paramter to
    @code{dovecot-service}.  As its name indicates, an opaque configuration
    does not have easy reflective capabilities.
    
    Available @code{opaque-dovecot-configuration} fields are:
    
    @deftypevr {@code{opaque-dovecot-configuration} parameter} package dovecot
    The dovecot package.
    @end deftypevr
    
    @deftypevr {@code{opaque-dovecot-configuration} parameter} string string
    The contents of the @code{dovecot.conf}, as a string.
    @end deftypevr
    
    For example, if your @code{dovecot.conf} is just the empty string, you
    could instantiate a dovecot service like this:
    
    @example
    (dovecot-service #:config
                     (opaque-dovecot-configuration
                      (string "")))
    @end example
    
    @node Web Services
    @subsubsection Web Services
    
    The @code{(gnu services web)} module provides the following service:
    
    @deffn {Scheme Procedure} nginx-service [#:nginx nginx] @
           [#:log-directory ``/var/log/nginx''] @
           [#:run-directory ``/var/run/nginx''] @
           [#:config-file]
    
    Return a service that runs @var{nginx}, the nginx web server.
    
    The nginx daemon loads its runtime configuration from @var{config-file}.
    Log files are written to @var{log-directory} and temporary runtime data
    files are written to @var{run-directory}.  For proper operation, these
    arguments should match what is in @var{config-file} to ensure that the
    directories are created when the service is activated.
    
    @end deffn
    
    @node Various Services
    @subsubsection Various Services
    
    The @code{(gnu services lirc)} module provides the following service.
    
    @deffn {Scheme Procedure} lirc-service [#:lirc lirc] @
           [#:device #f] [#:driver #f] [#:config-file #f] @
           [#:extra-options '()]
    Return a service that runs @url{http://www.lirc.org,LIRC}, a daemon that
    decodes infrared signals from remote controls.
    
    Optionally, @var{device}, @var{driver} and @var{config-file}
    (configuration file name) may be specified.  See @command{lircd} manual
    for details.
    
    Finally, @var{extra-options} is a list of additional command-line options
    passed to @command{lircd}.
    @end deffn
    
    @subsubsection Dictionary Services
    The @code{(gnu services dict)} module provides the following service:
    
    @deffn {Scheme Procedure} dicod-service [#:config (dicod-configuration)]
    Return a service that runs the @command{dicod} daemon, an implementation
    of DICT server (@pxref{Dicod,,, dico, GNU Dico Manual}).
    
    The optional @var{config} argument specifies the configuration for
    @command{dicod}, which should be a @code{<dicod-configuration>} object, by
    default it serves the GNU Collaborative International Dictonary of English.
    
    You can add @command{open localhost} to your @file{~/.dico} file to make
    @code{localhost} the default server for @command{dico} client
    (@pxref{Initialization File,,, dico, GNU Dico Manual}).
    @end deffn
    
    @deftp {Data Type} dicod-configuration
    Data type representing the configuration of dicod.
    
    @table @asis
    @item @code{dico} (default: @var{dico})
    Package object of the GNU Dico dictionary server.
    
    @item @code{interfaces} (default: @var{'("localhost")})
    This is the list of IP addresses and ports and possibly socket file
    names to listen to (@pxref{Server Settings, @code{listen} directive,,
    dico, GNU Dico Manual}).
    
    @item @code{databases} (default: @var{(list %dicod-database:gcide)})
    List of @code{<dicod-database>} objects denoting dictionaries to be served.
    @end table
    @end deftp
    
    @deftp {Data Type} dicod-database
    Data type representing a dictionary database.
    
    @table @asis
    @item @code{name}
    Name of the database, will be used in DICT commands.
    
    @item @code{module}
    Name of the dicod module used by this database
    (@pxref{Modules,,, dico, GNU Dico Manual}).
    
    @item @code{options}
    List of strings or gexps representing the arguments for the module handler
    (@pxref{Handlers,,, dico, GNU Dico Manual}).
    @end table
    @end deftp
    
    @defvr {Scheme Variable} %dicod-database:gcide
    A @code{<dicod-database>} object serving the GNU Collaborative International
    Dictonary of English using the @code{gcide} package.
    @end defvr
    
    @node Setuid Programs
    @subsection Setuid Programs
    
    @cindex setuid programs
    Some programs need to run with ``root'' privileges, even when they are
    launched by unprivileged users.  A notorious example is the
    @command{passwd} program, which users can run to change their
    password, and which needs to access the @file{/etc/passwd} and
    @file{/etc/shadow} files---something normally restricted to root, for
    obvious security reasons.  To address that, these executables are
    @dfn{setuid-root}, meaning that they always run with root privileges
    (@pxref{How Change Persona,,, libc, The GNU C Library Reference Manual},
    for more info about the setuid mechanism.)
    
    The store itself @emph{cannot} contain setuid programs: that would be a
    security issue since any user on the system can write derivations that
    populate the store (@pxref{The Store}).  Thus, a different mechanism is
    used: instead of changing the setuid bit directly on files that are in
    the store, we let the system administrator @emph{declare} which programs
    should be setuid root.
    
    The @code{setuid-programs} field of an @code{operating-system}
    declaration contains a list of G-expressions denoting the names of
    programs to be setuid-root (@pxref{Using the Configuration System}).
    For instance, the @command{passwd} program, which is part of the Shadow
    package, can be designated by this G-expression (@pxref{G-Expressions}):
    
    @example
    #~(string-append #$shadow "/bin/passwd")
    @end example
    
    A default set of setuid programs is defined by the
    @code{%setuid-programs} variable of the @code{(gnu system)} module.
    
    @defvr {Scheme Variable} %setuid-programs
    A list of G-expressions denoting common programs that are setuid-root.
    
    The list includes commands such as @command{passwd}, @command{ping},
    @command{su}, and @command{sudo}.
    @end defvr
    
    Under the hood, the actual setuid programs are created in the
    @file{/run/setuid-programs} directory at system activation time.  The
    files in this directory refer to the ``real'' binaries, which are in the
    store.
    
    @node X.509 Certificates
    @subsection X.509 Certificates
    
    @cindex HTTPS, certificates
    @cindex X.509 certificates
    @cindex TLS
    Web servers available over HTTPS (that is, HTTP over the transport-layer
    security mechanism, TLS) send client programs an @dfn{X.509 certificate}
    that the client can then use to @emph{authenticate} the server.  To do
    that, clients verify that the server's certificate is signed by a
    so-called @dfn{certificate authority} (CA).  But to verify the CA's
    signature, clients must have first acquired the CA's certificate.
    
    Web browsers such as GNU@tie{}IceCat include their own set of CA
    certificates, such that they are able to verify CA signatures
    out-of-the-box.
    
    However, most other programs that can talk HTTPS---@command{wget},
    @command{git}, @command{w3m}, etc.---need to be told where CA
    certificates can be found.
    
    @cindex @code{nss-certs}
    In GuixSD, this is done by adding a package that provides certificates
    to the @code{packages} field of the @code{operating-system} declaration
    (@pxref{operating-system Reference}).  GuixSD includes one such package,
    @code{nss-certs}, which is a set of CA certificates provided as part of
    Mozilla's Network Security Services.
    
    Note that it is @emph{not} part of @var{%base-packages}, so you need to
    explicitly add it.  The @file{/etc/ssl/certs} directory, which is where
    most applications and libraries look for certificates by default, points
    to the certificates installed globally.
    
    Unprivileged users, including users of Guix on a foreign distro,
    can also install their own certificate package in
    their profile.  A number of environment variables need to be defined so
    that applications and libraries know where to find them.  Namely, the
    OpenSSL library honors the @code{SSL_CERT_DIR} and @code{SSL_CERT_FILE}
    variables.  Some applications add their own environment variables; for
    instance, the Git version control system honors the certificate bundle
    pointed to by the @code{GIT_SSL_CAINFO} environment variable.  Thus, you
    would typically run something like:
    
    @example
    $ guix package -i nss-certs
    $ export SSL_CERT_DIR="$HOME/.guix-profile/etc/ssl/certs"
    $ export SSL_CERT_FILE="$HOME/.guix-profile/etc/ssl/certs/ca-certificates.crt"
    $ export GIT_SSL_CAINFO="$SSL_CERT_FILE"
    @end example
    
    @node Name Service Switch
    @subsection Name Service Switch
    
    @cindex name service switch
    @cindex NSS
    The @code{(gnu system nss)} module provides bindings to the
    configuration file of the libc @dfn{name service switch} or @dfn{NSS}
    (@pxref{NSS Configuration File,,, libc, The GNU C Library Reference
    Manual}).  In a nutshell, the NSS is a mechanism that allows libc to be
    extended with new ``name'' lookup methods for system databases, which
    includes host names, service names, user accounts, and more (@pxref{Name
    Service Switch, System Databases and Name Service Switch,, libc, The GNU
    C Library Reference Manual}).
    
    The NSS configuration specifies, for each system database, which lookup
    method is to be used, and how the various methods are chained
    together---for instance, under which circumstances NSS should try the
    next method in the list.  The NSS configuration is given in the
    @code{name-service-switch} field of @code{operating-system} declarations
    (@pxref{operating-system Reference, @code{name-service-switch}}).
    
    @cindex nss-mdns
    @cindex .local, host name lookup
    As an example, the declaration below configures the NSS to use the
    @uref{http://0pointer.de/lennart/projects/nss-mdns/, @code{nss-mdns}
    back-end}, which supports host name lookups over multicast DNS (mDNS)
    for host names ending in @code{.local}:
    
    @example
    (name-service-switch
       (hosts (list %files    ;first, check /etc/hosts
    
                    ;; If the above did not succeed, try
                    ;; with 'mdns_minimal'.
                    (name-service
                      (name "mdns_minimal")
    
                      ;; 'mdns_minimal' is authoritative for
                      ;; '.local'.  When it returns "not found",
                      ;; no need to try the next methods.
                      (reaction (lookup-specification
                                 (not-found => return))))
    
                    ;; Then fall back to DNS.
                    (name-service
                      (name "dns"))
    
                    ;; Finally, try with the "full" 'mdns'.
                    (name-service
                      (name "mdns")))))
    @end example
    
    Do not worry: the @code{%mdns-host-lookup-nss} variable (see below)
    contains this configuration, so you will not have to type it if all you
    want is to have @code{.local} host lookup working.
    
    Note that, in this case, in addition to setting the
    @code{name-service-switch} of the @code{operating-system} declaration,
    you also need to use @code{avahi-service} (@pxref{Networking Services,
    @code{avahi-service}}), or @var{%desktop-services}, which includes it
    (@pxref{Desktop Services}).  Doing this makes @code{nss-mdns} accessible
    to the name service cache daemon (@pxref{Base Services,
    @code{nscd-service}}).
    
    For convenience, the following variables provide typical NSS
    configurations.
    
    @defvr {Scheme Variable} %default-nss
    This is the default name service switch configuration, a
    @code{name-service-switch} object.
    @end defvr
    
    @defvr {Scheme Variable} %mdns-host-lookup-nss
    This is the name service switch configuration with support for host name
    lookup over multicast DNS (mDNS) for host names ending in @code{.local}.
    @end defvr
    
    The reference for name service switch configuration is given below.  It
    is a direct mapping of the configuration file format of the C library , so
    please refer to the C library manual for more information (@pxref{NSS
    Configuration File,,, libc, The GNU C Library Reference Manual}).
    Compared to the configuration file format of libc NSS, it has the advantage
    not only of adding this warm parenthetic feel that we like, but also
    static checks: you will know about syntax errors and typos as soon as you
    run @command{guix system}.
    
    @deftp {Data Type} name-service-switch
    
    This is the data type representation the configuration of libc's name
    service switch (NSS).  Each field below represents one of the supported
    system databases.
    
    @table @code
    @item aliases
    @itemx ethers
    @itemx group
    @itemx gshadow
    @itemx hosts
    @itemx initgroups
    @itemx netgroup
    @itemx networks
    @itemx password
    @itemx public-key
    @itemx rpc
    @itemx services
    @itemx shadow
    The system databases handled by the NSS.  Each of these fields must be a
    list of @code{<name-service>} objects (see below).
    @end table
    @end deftp
    
    @deftp {Data Type} name-service
    
    This is the data type representing an actual name service and the
    associated lookup action.
    
    @table @code
    @item name
    A string denoting the name service (@pxref{Services in the NSS
    configuration,,, libc, The GNU C Library Reference Manual}).
    
    Note that name services listed here must be visible to nscd.  This is
    achieved by passing the @code{#:name-services} argument to
    @code{nscd-service} the list of packages providing the needed name
    services (@pxref{Base Services, @code{nscd-service}}).
    
    @item reaction
    An action specified using the @code{lookup-specification} macro
    (@pxref{Actions in the NSS configuration,,, libc, The GNU C Library
    Reference Manual}).  For example:
    
    @example
    (lookup-specification (unavailable => continue)
                          (success => return))
    @end example
    @end table
    @end deftp
    
    @node Initial RAM Disk
    @subsection Initial RAM Disk
    
    @cindex initial RAM disk (initrd)
    @cindex initrd (initial RAM disk)
    For bootstrapping purposes, the Linux-Libre kernel is passed an
    @dfn{initial RAM disk}, or @dfn{initrd}.  An initrd contains a temporary
    root file system as well as an initialization script.  The latter is
    responsible for mounting the real root file system, and for loading any
    kernel modules that may be needed to achieve that.
    
    The @code{initrd} field of an @code{operating-system} declaration allows
    you to specify which initrd you would like to use.  The @code{(gnu
    system linux-initrd)} module provides two ways to build an initrd: the
    high-level @code{base-initrd} procedure, and the low-level
    @code{expression->initrd} procedure.
    
    The @code{base-initrd} procedure is intended to cover most common uses.
    For example, if you want to add a bunch of kernel modules to be loaded
    at boot time, you can define the @code{initrd} field of the operating
    system declaration like this:
    
    @example
    (initrd (lambda (file-systems . rest)
              ;; Create a standard initrd that has modules "foo.ko"
              ;; and "bar.ko", as well as their dependencies, in
              ;; addition to the modules available by default.
              (apply base-initrd file-systems
                     #:extra-modules '("foo" "bar")
                     rest)))
    @end example
    
    The @code{base-initrd} procedure also handles common use cases that
    involves using the system as a QEMU guest, or as a ``live'' system with
    volatile root file system.
    
    The initial RAM disk produced by @code{base-initrd} honors several
    options passed on the Linux kernel command line (that is, arguments
    passed @i{via} the @code{linux} command of GRUB, or the
    @code{-append} option) of QEMU, notably:
    
    @table @code
    @item --load=@var{boot}
    Tell the initial RAM disk to load @var{boot}, a file containing a Scheme
    program, once it has mounted the root file system.
    
    GuixSD uses this option to yield control to a boot program that runs the
    service activation programs and then spawns the GNU@tie{}Shepherd, the
    initialization system.
    
    @item --root=@var{root}
    Mount @var{root} as the root file system.  @var{root} can be a
    device name like @code{/dev/sda1}, a partition label, or a partition
    UUID.
    
    @item --system=@var{system}
    Have @file{/run/booted-system} and @file{/run/current-system} point to
    @var{system}.
    
    @item modprobe.blacklist=@var{modules}@dots{}
    @cindex module, black-listing
    @cindex black list, of kernel modules
    Instruct the initial RAM disk as well as the @command{modprobe} command
    (from the kmod package) to refuse to load @var{modules}.  @var{modules}
    must be a comma-separated list of module names---e.g.,
    @code{usbkbd,9pnet}.
    
    @item --repl
    Start a read-eval-print loop (REPL) from the initial RAM disk before it
    tries to load kernel modules and to mount the root file system.  Our
    marketing team calls it @dfn{boot-to-Guile}.  The Schemer in you will
    love it.  @xref{Using Guile Interactively,,, guile, GNU Guile Reference
    Manual}, for more information on Guile's REPL.
    
    @end table
    
    Now that you know all the features that initial RAM disks produced by
    @code{base-initrd} provide, here is how to use it and customize it
    further.
    
    @deffn {Monadic Procedure} base-initrd @var{file-systems} @
           [#:qemu-networking? #f] [#:virtio? #t] [#:volatile-root? #f] @
           [#:extra-modules '()] [#:mapped-devices '()]
    Return a monadic derivation that builds a generic initrd.  @var{file-systems} is
    a list of file systems to be mounted by the initrd, possibly in addition to
    the root file system specified on the kernel command line via @code{--root}.
    @var{mapped-devices} is a list of device mappings to realize before
    @var{file-systems} are mounted (@pxref{Mapped Devices}).
    
    When @var{qemu-networking?} is true, set up networking with the standard QEMU
    parameters.  When @var{virtio?} is true, load additional modules so that the
    initrd can be used as a QEMU guest with para-virtualized I/O drivers.
    
    When @var{volatile-root?} is true, the root file system is writable but any changes
    to it are lost.
    
    The initrd is automatically populated with all the kernel modules necessary
    for @var{file-systems} and for the given options.  However, additional kernel
    modules can be listed in @var{extra-modules}.  They will be added to the initrd, and
    loaded at boot time in the order in which they appear.
    @end deffn
    
    Needless to say, the initrds we produce and use embed a
    statically-linked Guile, and the initialization program is a Guile
    program.  That gives a lot of flexibility.  The
    @code{expression->initrd} procedure builds such an initrd, given the
    program to run in that initrd.
    
    @deffn {Monadic Procedure} expression->initrd @var{exp} @
           [#:guile %guile-static-stripped] [#:name "guile-initrd"]
    Return a derivation that builds a Linux initrd (a gzipped cpio archive)
    containing @var{guile} and that evaluates @var{exp}, a G-expression,
    upon booting.  All the derivations referenced by @var{exp} are
    automatically copied to the initrd.
    @end deffn
    
    @node GRUB Configuration
    @subsection GRUB Configuration
    
    @cindex GRUB
    @cindex boot loader
    
    The operating system uses GNU@tie{}GRUB as its boot loader
    (@pxref{Overview, overview of GRUB,, grub, GNU GRUB Manual}).  It is
    configured using a @code{grub-configuration} declaration.  This data type
    is exported by the @code{(gnu system grub)} module and described below.
    
    @deftp {Data Type} grub-configuration
    The type of a GRUB configuration declaration.
    
    @table @asis
    
    @item @code{device}
    This is a string denoting the boot device.  It must be a device name
    understood by the @command{grub-install} command, such as
    @code{/dev/sda} or @code{(hd0)} (@pxref{Invoking grub-install,,, grub,
    GNU GRUB Manual}).
    
    @item @code{menu-entries} (default: @code{()})
    A possibly empty list of @code{menu-entry} objects (see below), denoting
    entries to appear in the GRUB boot menu, in addition to the current
    system entry and the entry pointing to previous system generations.
    
    @item @code{default-entry} (default: @code{0})
    The index of the default boot menu entry.  Index 0 is for the entry of the
    current system.
    
    @item @code{timeout} (default: @code{5})
    The number of seconds to wait for keyboard input before booting.  Set to
    0 to boot immediately, and to -1 to wait indefinitely.
    
    @item @code{theme} (default: @var{%default-theme})
    The @code{grub-theme} object describing the theme to use.
    @end table
    
    @end deftp
    
    Should you want to list additional boot menu entries @i{via} the
    @code{menu-entries} field above, you will need to create them with the
    @code{menu-entry} form:
    
    @deftp {Data Type} menu-entry
    The type of an entry in the GRUB boot menu.
    
    @table @asis
    
    @item @code{label}
    The label to show in the menu---e.g., @code{"GNU"}.
    
    @item @code{linux}
    The Linux kernel to boot.
    
    @item @code{linux-arguments} (default: @code{()})
    The list of extra Linux kernel command-line arguments---e.g.,
    @code{("console=ttyS0")}.
    
    @item @code{initrd}
    A G-Expression or string denoting the file name of the initial RAM disk
    to use (@pxref{G-Expressions}).
    
    @end table
    @end deftp
    
    @c FIXME: Write documentation once it's stable.
    Themes are created using the @code{grub-theme} form, which is not
    documented yet.
    
    @defvr {Scheme Variable} %default-theme
    This is the default GRUB theme used by the operating system, with a
    fancy background image displaying the GNU and Guix logos.
    @end defvr
    
    
    @node Invoking guix system
    @subsection Invoking @code{guix system}
    
    Once you have written an operating system declaration as seen in the
    previous section, it can be @dfn{instantiated} using the @command{guix
    system} command.  The synopsis is:
    
    @example
    guix system @var{options}@dots{} @var{action} @var{file}
    @end example
    
    @var{file} must be the name of a file containing an
    @code{operating-system} declaration.  @var{action} specifies how the
    operating system is instantiated.  Currently the following values are
    supported:
    
    @table @code
    @item reconfigure
    Build the operating system described in @var{file}, activate it, and
    switch to it@footnote{This action is usable only on systems already
    running GuixSD.}.
    
    This effects all the configuration specified in @var{file}: user
    accounts, system services, global package list, setuid programs, etc.
    The command starts system services specified in @var{file} that are not
    currently running; if a service is currently running, it does not
    attempt to upgrade it since this would not be possible without stopping it
    first.
    
    It also adds a GRUB menu entry for the new OS configuration, and moves
    entries for older configurations to a submenu---unless
    @option{--no-grub} is passed.
    
    @quotation Note
    @c The paragraph below refers to the problem discussed at
    @c <http://lists.gnu.org/archive/html/guix-devel/2014-08/msg00057.html>.
    It is highly recommended to run @command{guix pull} once before you run
    @command{guix system reconfigure} for the first time (@pxref{Invoking
    guix pull}).  Failing to do that you would see an older version of Guix
    once @command{reconfigure} has completed.
    @end quotation
    
    @item build
    Build the derivation of the operating system, which includes all the
    configuration files and programs needed to boot and run the system.
    This action does not actually install anything.
    
    @item init
    Populate the given directory with all the files necessary to run the
    operating system specified in @var{file}.  This is useful for first-time
    installations of GuixSD.  For instance:
    
    @example
    guix system init my-os-config.scm /mnt
    @end example
    
    copies to @file{/mnt} all the store items required by the configuration
    specified in @file{my-os-config.scm}.  This includes configuration
    files, packages, and so on.  It also creates other essential files
    needed for the system to operate correctly---e.g., the @file{/etc},
    @file{/var}, and @file{/run} directories, and the @file{/bin/sh} file.
    
    This command also installs GRUB on the device specified in
    @file{my-os-config}, unless the @option{--no-grub} option was passed.
    
    @item vm
    @cindex virtual machine
    @cindex VM
    @anchor{guix system vm}
    Build a virtual machine that contains the operating system declared in
    @var{file}, and return a script to run that virtual machine (VM).
    Arguments given to the script are passed to QEMU.
    
    The VM shares its store with the host system.
    
    Additional file systems can be shared between the host and the VM using
    the @code{--share} and @code{--expose} command-line options: the former
    specifies a directory to be shared with write access, while the latter
    provides read-only access to the shared directory.
    
    The example below creates a VM in which the user's home directory is
    accessible read-only, and where the @file{/exchange} directory is a
    read-write mapping of @file{$HOME/tmp} on the host:
    
    @example
    guix system vm my-config.scm \
       --expose=$HOME --share=$HOME/tmp=/exchange
    @end example
    
    On GNU/Linux, the default is to boot directly to the kernel; this has
    the advantage of requiring only a very tiny root disk image since the
    store of the host can then be mounted.
    
    The @code{--full-boot} option forces a complete boot sequence, starting
    with the bootloader.  This requires more disk space since a root image
    containing at least the kernel, initrd, and bootloader data files must
    be created.  The @code{--image-size} option can be used to specify the
    size of the image.
    
    @item vm-image
    @itemx disk-image
    Return a virtual machine or disk image of the operating system declared
    in @var{file} that stands alone.  Use the @option{--image-size} option
    to specify the size of the image.
    
    When using @code{vm-image}, the returned image is in qcow2 format, which
    the QEMU emulator can efficiently use. @xref{Running GuixSD in a VM},
    for more information on how to run the image in a virtual machine.
    
    When using @code{disk-image}, a raw disk image is produced; it can be
    copied as is to a USB stick, for instance.  Assuming @code{/dev/sdc} is
    the device corresponding to a USB stick, one can copy the image to it
    using the following command:
    
    @example
    # dd if=$(guix system disk-image my-os.scm) of=/dev/sdc
    @end example
    
    @item container
    Return a script to run the operating system declared in @var{file}
    within a container.  Containers are a set of lightweight isolation
    mechanisms provided by the kernel Linux-libre.  Containers are
    substantially less resource-demanding than full virtual machines since
    the kernel, shared objects, and other resources can be shared with the
    host system; this also means they provide thinner isolation.
    
    Currently, the script must be run as root in order to support more than
    a single user and group.  The container shares its store with the host
    system.
    
    As with the @code{vm} action (@pxref{guix system vm}), additional file
    systems to be shared between the host and container can be specified
    using the @option{--share} and @option{--expose} options:
    
    @example
    guix system container my-config.scm \
       --expose=$HOME --share=$HOME/tmp=/exchange
    @end example
    
    @quotation Note
    This option requires Linux-libre 3.19 or newer.
    @end quotation
    
    @end table
    
    @var{options} can contain any of the common build options (@pxref{Common
    Build Options}).  In addition, @var{options} can contain one of the
    following:
    
    @table @option
    @item --system=@var{system}
    @itemx -s @var{system}
    Attempt to build for @var{system} instead of the host system type.
    This works as per @command{guix build} (@pxref{Invoking guix build}).
    
    @item --derivation
    @itemx -d
    Return the derivation file name of the given operating system without
    building anything.
    
    @item --image-size=@var{size}
    For the @code{vm-image} and @code{disk-image} actions, create an image
    of the given @var{size}.  @var{size} may be a number of bytes, or it may
    include a unit as a suffix (@pxref{Block size, size specifications,,
    coreutils, GNU Coreutils}).
    
    @item --on-error=@var{strategy}
    Apply @var{strategy} when an error occurs when reading @var{file}.
    @var{strategy} may be one of the following:
    
    @table @code
    @item nothing-special
    Report the error concisely and exit.  This is the default strategy.
    
    @item backtrace
    Likewise, but also display a backtrace.
    
    @item debug
    Report the error and enter Guile's debugger.  From there, you can run
    commands such as @code{,bt} to get a backtrace, @code{,locals} to
    display local variable values, and more generally inspect the state of the
    program.  @xref{Debug Commands,,, guile, GNU Guile Reference Manual}, for
    a list of available debugging commands.
    @end table
    @end table
    
    @quotation Note
    All the actions above, except @code{build} and @code{init},
    can use KVM support in the Linux-libre kernel.  Specifically, if the
    machine has hardware virtualization support, the corresponding
    KVM kernel module should be loaded, and the @file{/dev/kvm} device node
    must exist and be readable and writable by the user and by the
    build users of the daemon (@pxref{Build Environment Setup}).
    @end quotation
    
    Once you have built, configured, re-configured, and re-re-configured
    your GuixSD installation, you may find it useful to list the operating
    system generations available on disk---and that you can choose from the
    GRUB boot menu:
    
    @table @code
    
    @item list-generations
    List a summary of each generation of the operating system available on
    disk, in a human-readable way.  This is similar to the
    @option{--list-generations} option of @command{guix package}
    (@pxref{Invoking guix package}).
    
    Optionally, one can specify a pattern, with the same syntax that is used
    in @command{guix package --list-generations}, to restrict the list of
    generations displayed.  For instance, the following command displays
    generations that are up to 10 days old:
    
    @example
    $ guix system list-generations 10d
    @end example
    
    @end table
    
    The @command{guix system} command has even more to offer!  The following
    sub-commands allow you to visualize how your system services relate to
    each other:
    
    @anchor{system-extension-graph}
    @table @code
    
    @item extension-graph
    Emit in Dot/Graphviz format to standard output the @dfn{service
    extension graph} of the operating system defined in @var{file}
    (@pxref{Service Composition}, for more information on service
    extensions.)
    
    The command:
    
    @example
    $ guix system extension-graph @var{file} | dot -Tpdf > services.pdf
    @end example
    
    produces a PDF file showing the extension relations among services.
    
    @anchor{system-shepherd-graph}
    @item shepherd-graph
    Emit in Dot/Graphviz format to standard output the @dfn{dependency
    graph} of shepherd services of the operating system defined in
    @var{file}.  @xref{Shepherd Services}, for more information and for an
    example graph.
    
    @end table
    
    @node Running GuixSD in a VM
    @subsection Running GuixSD in a Virtual Machine
    
    One way to run GuixSD in a virtual machine (VM) is to build a GuixSD
    virtual machine image using @command{guix system vm-image}
    (@pxref{Invoking guix system}).  The returned image is in qcow2 format,
    which the @uref{http://qemu.org/, QEMU emulator} can efficiently use.
    
    To run the image in QEMU, copy it out of the store (@pxref{The Store})
    and give yourself permission to write to the copy.  When invoking QEMU,
    you must choose a system emulator that is suitable for your hardware
    platform.  Here is a minimal QEMU invocation that will boot the result
    of @command{guix system vm-image} on x86_64 hardware:
    
    @example
    $ qemu-system-x86_64 \
       -net user -net nic,model=virtio \
       -enable-kvm -m 256 /tmp/qemu-image
    @end example
    
    Here is what each of these options means:
    
    @table @code
    @item qemu-system-x86_64
    This specifies the hardware platform to emulate.  This should match the
    host.
    
    @item -net user
    Enable the unprivileged user-mode network stack.  The guest OS can
    access the host but not vice versa.  This is the simplest way to get the
    guest OS online.  If you do not choose a network stack, the boot will
    fail.
    
    @item -net nic,model=virtio
    You must create a network interface of a given model.  If you do not
    create a NIC, the boot will fail.  Assuming your hardware platform is
    x86_64, you can get a list of available NIC models by running
    @command{qemu-system-x86_64 -net nic,model=help}.
    
    @item -enable-kvm
    If your system has hardware virtualization extensions, enabling the
    virtual machine support (KVM) of the Linux kernel will make things run
    faster.
    
    @item -m 256
    RAM available to the guest OS, in mebibytes.  Defaults to 128@tie{}MiB,
    which may be insufficent for some operations.
    
    @item /tmp/qemu-image
    The file name of the qcow2 image.
    @end table
    
    @node Defining Services
    @subsection Defining Services
    
    The previous sections show the available services and how one can combine
    them in an @code{operating-system} declaration.  But how do we define
    them in the first place?  And what is a service anyway?
    
    @menu
    * Service Composition::         The model for composing services.
    * Service Types and Services::  Types and services.
    * Service Reference::           API reference.
    * Shepherd Services::           A particular type of service.
    @end menu
    
    @node Service Composition
    @subsubsection Service Composition
    
    @cindex services
    @cindex daemons
    Here we define a @dfn{service} as, broadly, something that extends the
    functionality of the operating system.  Often a service is a process---a
    @dfn{daemon}---started when the system boots: a secure shell server, a
    Web server, the Guix build daemon, etc.  Sometimes a service is a daemon
    whose execution can be triggered by another daemon---e.g., an FTP server
    started by @command{inetd} or a D-Bus service activated by
    @command{dbus-daemon}.  Occasionally, a service does not map to a
    daemon.  For instance, the ``account'' service collects user accounts
    and makes sure they exist when the system runs; the ``udev'' service
    collects device management rules and makes them available to the eudev
    daemon; the @file{/etc} service populates the @file{/etc} directory
    of the system.
    
    @cindex service extensions
    GuixSD services are connected by @dfn{extensions}.  For instance, the
    secure shell service @emph{extends} the Shepherd---the GuixSD
    initialization system, running as PID@tie{}1---by giving it the command
    lines to start and stop the secure shell daemon (@pxref{Networking
    Services, @code{lsh-service}}); the UPower service extends the D-Bus
    service by passing it its @file{.service} specification, and extends the
    udev service by passing it device management rules (@pxref{Desktop
    Services, @code{upower-service}}); the Guix daemon service extends the
    Shepherd by passing it the command lines to start and stop the daemon,
    and extends the account service by passing it a list of required build
    user accounts (@pxref{Base Services}).
    
    All in all, services and their ``extends'' relations form a directed
    acyclic graph (DAG).  If we represent services as boxes and extensions
    as arrows, a typical system might provide something like this:
    
    @image{images/service-graph,,5in,Typical service extension graph.}
    
    @cindex system service
    At the bottom, we see the @dfn{system service}, which produces the
    directory containing everything to run and boot the system, as returned
    by the @command{guix system build} command.  @xref{Service Reference},
    to learn about the other service types shown here.
    @xref{system-extension-graph, the @command{guix system extension-graph}
    command}, for information on how to generate this representation for a
    particular operating system definition.
    
    @cindex service types
    Technically, developers can define @dfn{service types} to express these
    relations.  There can be any number of services of a given type on the
    system---for instance, a system running two instances of the GNU secure
    shell server (lsh) has two instances of @var{lsh-service-type}, with
    different parameters.
    
    The following section describes the programming interface for service
    types and services.
    
    @node Service Types and Services
    @subsubsection Service Types and Services
    
    A @dfn{service type} is a node in the DAG described above.  Let us start
    with a simple example, the service type for the Guix build daemon
    (@pxref{Invoking guix-daemon}):
    
    @example
    (define guix-service-type
      (service-type
       (name 'guix)
       (extensions
        (list (service-extension shepherd-root-service-type guix-shepherd-service)
              (service-extension account-service-type guix-accounts)
              (service-extension activation-service-type guix-activation)))))
    @end example
    
    @noindent
    It defines two things:
    
    @enumerate
    @item
    A name, whose sole purpose is to make inspection and debugging easier.
    
    @item
    A list of @dfn{service extensions}, where each extension designates the
    target service type and a procedure that, given the parameters of the
    service, returns a list of objects to extend the service of that type.
    
    Every service type has at least one service extension.  The only
    exception is the @dfn{boot service type}, which is the ultimate service.
    @end enumerate
    
    In this example, @var{guix-service-type} extends three services:
    
    @table @var
    @item shepherd-root-service-type
    The @var{guix-shepherd-service} procedure defines how the Shepherd
    service is extended.  Namely, it returns a @code{<shepherd-service>}
    object that defines how @command{guix-daemon} is started and stopped
    (@pxref{Shepherd Services}).
    
    @item account-service-type
    This extension for this service is computed by @var{guix-accounts},
    which returns a list of @code{user-group} and @code{user-account}
    objects representing the build user accounts (@pxref{Invoking
    guix-daemon}).
    
    @item activation-service-type
    Here @var{guix-activation} is a procedure that returns a gexp, which is
    a code snippet to run at ``activation time''---e.g., when the service is
    booted.
    @end table
    
    A service of this type is instantiated like this:
    
    @example
    (service guix-service-type
             (guix-configuration
               (build-accounts 5)
               (use-substitutes? #f)))
    @end example
    
    The second argument to the @code{service} form is a value representing
    the parameters of this specific service instance.
    @xref{guix-configuration-type, @code{guix-configuration}}, for
    information about the @code{guix-configuration} data type.
    
    @var{guix-service-type} is quite simple because it extends other
    services but is not extensible itself.
    
    @c @subsubsubsection Extensible Service Types
    
    The service type for an @emph{extensible} service looks like this:
    
    @example
    (define udev-service-type
      (service-type (name 'udev)
                    (extensions
                     (list (service-extension shepherd-root-service-type
                                              udev-shepherd-service)))
    
                    (compose concatenate)       ;concatenate the list of rules
                    (extend (lambda (config rules)
                              (match config
                                (($ <udev-configuration> udev initial-rules)
                                 (udev-configuration
                                  (udev udev)   ;the udev package to use
                                  (rules (append initial-rules rules)))))))))
    @end example
    
    This is the service type for the
    @uref{https://wiki.gentoo.org/wiki/Project:Eudev, eudev device
    management daemon}.  Compared to the previous example, in addition to an
    extension of @var{shepherd-root-service-type}, we see two new fields:
    
    @table @code
    @item compose
    This is the procedure to @dfn{compose} the list of extensions to
    services of this type.
    
    Services can extend the udev service by passing it lists of rules; we
    compose those extensions simply by concatenating them.
    
    @item extend
    This procedure defines how the value of the service is @dfn{extended} with
    the composition of the extensions.
    
    Udev extensions are composed into a list of rules, but the udev service
    value is itself a @code{<udev-configuration>} record.  So here, we
    extend that record by appending the list of rules it contains to the
    list of contributed rules.
    @end table
    
    There can be only one instance of an extensible service type such as
    @var{udev-service-type}.  If there were more, the
    @code{service-extension} specifications would be ambiguous.
    
    Still here?  The next section provides a reference of the programming
    interface for services.
    
    @node Service Reference
    @subsubsection Service Reference
    
    We have seen an overview of service types (@pxref{Service Types and
    Services}).  This section provides a reference on how to manipulate
    services and service types.  This interface is provided by the
    @code{(gnu services)} module.
    
    @deffn {Scheme Procedure} service @var{type} @var{value}
    Return a new service of @var{type}, a @code{<service-type>} object (see
    below.)  @var{value} can be any object; it represents the parameters of
    this particular service instance.
    @end deffn
    
    @deffn {Scheme Procedure} service? @var{obj}
    Return true if @var{obj} is a service.
    @end deffn
    
    @deffn {Scheme Procedure} service-kind @var{service}
    Return the type of @var{service}---i.e., a @code{<service-type>} object.
    @end deffn
    
    @deffn {Scheme Procedure} service-parameters @var{service}
    Return the value associated with @var{service}.  It represents its
    parameters.
    @end deffn
    
    Here is an example of how a service is created and manipulated:
    
    @example
    (define s
      (service nginx-service-type
               (nginx-configuration
                (nginx nginx)
                (log-directory log-directory)
                (run-directory run-directory)
                (file config-file))))
    
    (service? s)
    @result{} #t
    
    (eq? (service-kind s) nginx-service-type)
    @result{} #t
    @end example
    
    The @code{modify-services} form provides a handy way to change the
    parameters of some of the services of a list such as
    @var{%base-services} (@pxref{Base Services, @code{%base-services}}).  It
    evalutes to a list of services.  Of course, you could always use
    standard list combinators such as @code{map} and @code{fold} to do that
    (@pxref{SRFI-1, List Library,, guile, GNU Guile Reference Manual});
    @code{modify-services} simply provides a more concise form for this
    common pattern.
    
    @deffn {Scheme Syntax} modify-services @var{services} @
      (@var{type} @var{variable} => @var{body}) @dots{}
    
    Modify the services listed in @var{services} according to the given
    clauses.  Each clause has the form:
    
    @example
    (@var{type} @var{variable} => @var{body})
    @end example
    
    where @var{type} is a service type---e.g.,
    @code{guix-service-type}---and @var{variable} is an identifier that is
    bound within the @var{body} to the service parameters---e.g., a
    @code{guix-configuration} instance---of the original service of that
    @var{type}.
    
    The @var{body} should evaluate to the new service parameters, which will
    be used to configure the new service.  This new service will replace the
    original in the resulting list.  Because a service's service parameters
    are created using @code{define-record-type*}, you can write a succint
    @var{body} that evaluates to the new service parameters by using the
    @code{inherit} feature that @code{define-record-type*} provides.
    
    @xref{Using the Configuration System}, for example usage.
    
    @end deffn
    
    Next comes the programming interface for service types.  This is
    something you want to know when writing new service definitions, but not
    necessarily when simply looking for ways to customize your
    @code{operating-system} declaration.
    
    @deftp {Data Type} service-type
    @cindex service type
    This is the representation of a @dfn{service type} (@pxref{Service Types
    and Services}).
    
    @table @asis
    @item @code{name}
    This is a symbol, used only to simplify inspection and debugging.
    
    @item @code{extensions}
    A non-empty list of @code{<service-extension>} objects (see below).
    
    @item @code{compose} (default: @code{#f})
    If this is @code{#f}, then the service type denotes services that cannot
    be extended---i.e., services that do not receive ``values'' from other
    services.
    
    Otherwise, it must be a one-argument procedure.  The procedure is called
    by @code{fold-services} and is passed a list of values collected from
    extensions.  It must return a value that is a valid parameter value for
    the service instance.
    
    @item @code{extend} (default: @code{#f})
    If this is @code{#f}, services of this type cannot be extended.
    
    Otherwise, it must be a two-argument procedure: @code{fold-services}
    calls it, passing it the initial value of the service as the first argument
    and the result of applying @code{compose} to the extension values as the
    second argument.
    @end table
    
    @xref{Service Types and Services}, for examples.
    @end deftp
    
    @deffn {Scheme Procedure} service-extension @var{target-type} @
                                  @var{compute}
    Return a new extension for services of type @var{target-type}.
    @var{compute} must be a one-argument procedure: @code{fold-services}
    calls it, passing it the value associated with the service that provides
    the extension; it must return a valid value for the target service.
    @end deffn
    
    @deffn {Scheme Procedure} service-extension? @var{obj}
    Return true if @var{obj} is a service extension.
    @end deffn
    
    At the core of the service abstraction lies the @code{fold-services}
    procedure, which is responsible for ``compiling'' a list of services
    down to a single directory that contains everything needed to boot and
    run the system---the directory shown by the @command{guix system build}
    command (@pxref{Invoking guix system}).  In essence, it propagates
    service extensions down the service graph, updating each node parameters
    on the way, until it reaches the root node.
    
    @deffn {Scheme Procedure} fold-services @var{services} @
                                [#:target-type @var{system-service-type}]
    Fold @var{services} by propagating their extensions down to the root of
    type @var{target-type}; return the root service adjusted accordingly.
    @end deffn
    
    Lastly, the @code{(gnu services)} module also defines several essential
    service types, some of which are listed below.
    
    @defvr {Scheme Variable} system-service-type
    This is the root of the service graph.  It produces the system directory
    as returned by the @command{guix system build} command.
    @end defvr
    
    @defvr {Scheme Variable} boot-service-type
    The type of the ``boot service'', which produces the @dfn{boot script}.
    The boot script is what the initial RAM disk runs when booting.
    @end defvr
    
    @defvr {Scheme Variable} etc-service-type
    The type of the @file{/etc} service.  This service can be extended by
    passing it name/file tuples such as:
    
    @example
    (list `("issue" ,(plain-file "issue" "Welcome!\n")))
    @end example
    
    In this example, the effect would be to add an @file{/etc/issue} file
    pointing to the given file.
    @end defvr
    
    @defvr {Scheme Variable} setuid-program-service-type
    Type for the ``setuid-program service''.  This service collects lists of
    executable file names, passed as gexps, and adds them to the set of
    setuid-root programs on the system (@pxref{Setuid Programs}).
    @end defvr
    
    @defvr {Scheme Variable} profile-service-type
    Type of the service that populates the @dfn{system profile}---i.e., the
    programs under @file{/run/current-system/profile}.  Other services can
    extend it by passing it lists of packages to add to the system profile.
    @end defvr
    
    
    @node Shepherd Services
    @subsubsection Shepherd Services
    
    @cindex PID 1
    @cindex init system
    The @code{(gnu services shepherd)} module provides a way to define
    services managed by the GNU@tie{}Shepherd, which is the GuixSD
    initialization system---the first process that is started when the
    system boots, also known as PID@tie{}1
    (@pxref{Introduction,,, shepherd, The GNU Shepherd Manual}).
    
    Services in the Shepherd can depend on each other.  For instance, the
    SSH daemon may need to be started after the syslog daemon has been
    started, which in turn can only happen once all the file systems have
    been mounted.  The simple operating system defined earlier (@pxref{Using
    the Configuration System}) results in a service graph like this:
    
    @image{images/shepherd-graph,,5in,Typical shepherd service graph.}
    
    You can actually generate such a graph for any operating system
    definition using the @command{guix system shepherd-graph} command
    (@pxref{system-shepherd-graph, @command{guix system shepherd-graph}}).
    
    The @var{%shepherd-root-service} is a service object representing
    PID@tie{}1, of type @var{shepherd-root-service-type}; it can be extended
    by passing it lists of @code{<shepherd-service>} objects.
    
    @deftp {Data Type} shepherd-service
    The data type representing a service managed by the Shepherd.
    
    @table @asis
    @item @code{provision}
    This is a list of symbols denoting what the service provides.
    
    These are the names that may be passed to @command{herd start},
    @command{herd status}, and similar commands (@pxref{Invoking herd,,,
    shepherd, The GNU Shepherd Manual}).  @xref{Slots of services, the
    @code{provides} slot,, shepherd, The GNU Shepherd Manual}, for details.
    
    @item @code{requirements} (default: @code{'()})
    List of symbols denoting the Shepherd services this one depends on.
    
    @item @code{respawn?} (default: @code{#t})
    Whether to restart the service when it stops, for instance when the
    underlying process dies.
    
    @item @code{start}
    @itemx @code{stop} (default: @code{#~(const #f)})
    The @code{start} and @code{stop} fields refer to the Shepherd's
    facilities to start and stop processes (@pxref{Service De- and
    Constructors,,, shepherd, The GNU Shepherd Manual}).  They are given as
    G-expressions that get expanded in the Shepherd configuration file
    (@pxref{G-Expressions}).
    
    @item @code{documentation}
    A documentation string, as shown when running:
    
    @example
    herd doc @var{service-name}
    @end example
    
    where @var{service-name} is one of the symbols in @var{provision}
    (@pxref{Invoking herd,,, shepherd, The GNU Shepherd Manual}).
    
    @item @code{modules} (default: @var{%default-modules})
    This is the list of modules that must be in scope when @code{start} and
    @code{stop} are evaluated.
    
    @end table
    @end deftp
    
    @defvr {Scheme Variable} shepherd-root-service-type
    The service type for the Shepherd ``root service''---i.e., PID@tie{}1.
    
    This is the service type that extensions target when they want to create
    shepherd services (@pxref{Service Types and Services}, for an example).
    Each extension must pass a list of @code{<shepherd-service>}.
    @end defvr
    
    @defvr {Scheme Variable} %shepherd-root-service
    This service represents PID@tie{}1.
    @end defvr
    
    
    @node Installing Debugging Files
    @section Installing Debugging Files
    
    @cindex debugging files
    Program binaries, as produced by the GCC compilers for instance, are
    typically written in the ELF format, with a section containing
    @dfn{debugging information}.  Debugging information is what allows the
    debugger, GDB, to map binary code to source code; it is required to
    debug a compiled program in good conditions.
    
    The problem with debugging information is that is takes up a fair amount
    of disk space.  For example, debugging information for the GNU C Library
    weighs in at more than 60 MiB.  Thus, as a user, keeping all the
    debugging info of all the installed programs is usually not an option.
    Yet, space savings should not come at the cost of an impediment to
    debugging---especially in the GNU system, which should make it easier
    for users to exert their computing freedom (@pxref{GNU Distribution}).
    
    Thankfully, the GNU Binary Utilities (Binutils) and GDB provide a
    mechanism that allows users to get the best of both worlds: debugging
    information can be stripped from the binaries and stored in separate
    files.  GDB is then able to load debugging information from those files,
    when they are available (@pxref{Separate Debug Files,,, gdb, Debugging
    with GDB}).
    
    The GNU distribution takes advantage of this by storing debugging
    information in the @code{lib/debug} sub-directory of a separate package
    output unimaginatively called @code{debug} (@pxref{Packages with
    Multiple Outputs}).  Users can choose to install the @code{debug} output
    of a package when they need it.  For instance, the following command
    installs the debugging information for the GNU C Library and for GNU
    Guile:
    
    @example
    guix package -i glibc:debug guile:debug
    @end example
    
    GDB must then be told to look for debug files in the user's profile, by
    setting the @code{debug-file-directory} variable (consider setting it
    from the @file{~/.gdbinit} file, @pxref{Startup,,, gdb, Debugging with
    GDB}):
    
    @example
    (gdb) set debug-file-directory ~/.guix-profile/lib/debug
    @end example
    
    From there on, GDB will pick up debugging information from the
    @code{.debug} files under @file{~/.guix-profile/lib/debug}.
    
    In addition, you will most likely want GDB to be able to show the source
    code being debugged.  To do that, you will have to unpack the source
    code of the package of interest (obtained with @code{guix build
    --source}, @pxref{Invoking guix build}), and to point GDB to that source
    directory using the @code{directory} command (@pxref{Source Path,
    @code{directory},, gdb, Debugging with GDB}).
    
    @c XXX: keep me up-to-date
    The @code{debug} output mechanism in Guix is implemented by the
    @code{gnu-build-system} (@pxref{Build Systems}).  Currently, it is
    opt-in---debugging information is available only for the packages
    with definitions explicitly declaring a @code{debug} output.  This may be
    changed to opt-out in the future if our build farm servers can handle
    the load.  To check whether a package has a @code{debug} output, use
    @command{guix package --list-available} (@pxref{Invoking guix package}).
    
    
    @node Security Updates
    @section Security Updates
    
    @cindex security updates
    @cindex security vulnerabilities
    Occasionally, important security vulnerabilities are discovered in software
    packages and must be patched.  Guix developers try hard to keep track of
    known vulnerabilities and to apply fixes as soon as possible in the
    @code{master} branch of Guix (we do not yet provide a ``stable'' branch
    containing only security updates.)  The @command{guix lint} tool helps
    developers find out about vulnerable versions of software packages in the
    distribution:
    
    @smallexample
    $ guix lint -c cve
    gnu/packages/base.scm:652:2: glibc-2.21: probably vulnerable to CVE-2015-1781, CVE-2015-7547
    gnu/packages/gcc.scm:334:2: gcc-4.9.3: probably vulnerable to CVE-2015-5276
    gnu/packages/image.scm:312:2: openjpeg-2.1.0: probably vulnerable to CVE-2016-1923, CVE-2016-1924
    @dots{}
    @end smallexample
    
    @xref{Invoking guix lint}, for more information.
    
    @quotation Note
    As of version @value{VERSION}, the feature described below is considered
    ``beta''.
    @end quotation
    
    Guix follows a functional
    package management discipline (@pxref{Introduction}), which implies
    that, when a package is changed, @emph{every package that depends on it}
    must be rebuilt.  This can significantly slow down the deployment of
    fixes in core packages such as libc or Bash, since basically the whole
    distribution would need to be rebuilt.  Using pre-built binaries helps
    (@pxref{Substitutes}), but deployment may still take more time than
    desired.
    
    @cindex grafts
    To address this, Guix implements @dfn{grafts}, a mechanism that allows
    for fast deployment of critical updates without the costs associated
    with a whole-distribution rebuild.  The idea is to rebuild only the
    package that needs to be patched, and then to ``graft'' it onto packages
    explicitly installed by the user and that were previously referring to
    the original package.  The cost of grafting is typically very low, and
    order of magnitudes lower than a full rebuild of the dependency chain.
    
    @cindex replacements of packages, for grafts
    For instance, suppose a security update needs to be applied to Bash.
    Guix developers will provide a package definition for the ``fixed''
    Bash, say @var{bash-fixed}, in the usual way (@pxref{Defining
    Packages}).  Then, the original package definition is augmented with a
    @code{replacement} field pointing to the package containing the bug fix:
    
    @example
    (define bash
      (package
        (name "bash")
        ;; @dots{}
        (replacement bash-fixed)))
    @end example
    
    From there on, any package depending directly or indirectly on Bash---as
    reported by @command{guix gc --requisites} (@pxref{Invoking guix
    gc})---that is installed is automatically ``rewritten'' to refer to
    @var{bash-fixed} instead of @var{bash}.  This grafting process takes
    time proportional to the size of the package, usually less than a
    minute for an ``average'' package on a recent machine.  Grafting is
    recursive: when an indirect dependency requires grafting, then grafting
    ``propagates'' up to the package that the user is installing.
    
    Currently, the graft and the package it replaces (@var{bash-fixed} and
    @var{bash} in the example above) must have the exact same @code{name}
    and @code{version} fields.  This restriction mostly comes from the fact
    that grafting works by patching files, including binary files, directly.
    Other restrictions may apply: for instance, when adding a graft to a
    package providing a shared library, the original shared library and its
    replacement must have the same @code{SONAME} and be binary-compatible.
    
    The @option{--no-grafts} command-line option allows you to forcefully
    avoid grafting (@pxref{Common Build Options, @option{--no-grafts}}).
    Thus, the command:
    
    @example
    guix build bash --no-grafts
    @end example
    
    @noindent
    returns the store file name of the original Bash, whereas:
    
    @example
    guix build bash
    @end example
    
    @noindent
    returns the store file name of the ``fixed'', replacement Bash.  This
    allows you to distinguish between the two variants of Bash.
    
    To verify which Bash your whole profile refers to, you can run
    (@pxref{Invoking guix gc}):
    
    @example
    guix gc -R `readlink -f ~/.guix-profile` | grep bash
    @end example
    
    @noindent
    @dots{} and compare the store file names that you get with those above.
    Likewise for a complete GuixSD system generation:
    
    @example
    guix gc -R `guix system build my-config.scm` | grep bash
    @end example
    
    Lastly, to check which Bash running processes are using, you can use the
    @command{lsof} command:
    
    @example
    lsof | grep /gnu/store/.*bash
    @end example
    
    
    @node Package Modules
    @section Package Modules
    
    From a programming viewpoint, the package definitions of the
    GNU distribution are provided by Guile modules in the @code{(gnu packages
    @dots{})} name space@footnote{Note that packages under the @code{(gnu
    packages @dots{})} module name space are not necessarily ``GNU
    packages''.  This module naming scheme follows the usual Guile module
    naming convention: @code{gnu} means that these modules are distributed
    as part of the GNU system, and @code{packages} identifies modules that
    define packages.}  (@pxref{Modules, Guile modules,, guile, GNU Guile
    Reference Manual}).  For instance, the @code{(gnu packages emacs)}
    module exports a variable named @code{emacs}, which is bound to a
    @code{<package>} object (@pxref{Defining Packages}).
    
    The @code{(gnu packages @dots{})} module name space is
    automatically scanned for packages by the command-line tools.  For
    instance, when running @code{guix package -i emacs}, all the @code{(gnu
    packages @dots{})} modules are scanned until one that exports a package
    object whose name is @code{emacs} is found.  This package search
    facility is implemented in the @code{(gnu packages)} module.
    
    @cindex customization, of packages
    @cindex package module search path
    Users can store package definitions in modules with different
    names---e.g., @code{(my-packages emacs)}@footnote{Note that the file
    name and module name must match.  For instance, the @code{(my-packages
    emacs)} module must be stored in a @file{my-packages/emacs.scm} file
    relative to the load path specified with @option{--load-path} or
    @code{GUIX_PACKAGE_PATH}.  @xref{Modules and the File System,,,
    guile, GNU Guile Reference Manual}, for details.}.  These package definitions
    will not be visible by default.  Users can invoke commands such as
    @command{guix package} and @command{guix build} with the
    @code{-e} option so that they know where to find the package.  Better
    yet, they can use the
    @code{-L} option of these commands to make those modules visible
    (@pxref{Invoking guix build, @code{--load-path}}), or define the
    @code{GUIX_PACKAGE_PATH} environment variable.  This environment
    variable makes it easy to extend or customize the distribution and is
    honored by all the user interfaces.
    
    @defvr {Environment Variable} GUIX_PACKAGE_PATH
    This is a colon-separated list of directories to search for additional
    package modules.  Directories listed in this variable take precedence
    over the own modules of the distribution.
    @end defvr
    
    The distribution is fully @dfn{bootstrapped} and @dfn{self-contained}:
    each package is built based solely on other packages in the
    distribution.  The root of this dependency graph is a small set of
    @dfn{bootstrap binaries}, provided by the @code{(gnu packages
    bootstrap)} module.  For more information on bootstrapping,
    @pxref{Bootstrapping}.
    
    @node Packaging Guidelines
    @section Packaging Guidelines
    
    The GNU distribution is nascent and may well lack some of your favorite
    packages.  This section describes how you can help make the distribution
    grow.  @xref{Contributing}, for additional information on how you can
    help.
    
    Free software packages are usually distributed in the form of
    @dfn{source code tarballs}---typically @file{tar.gz} files that contain
    all the source files.  Adding a package to the distribution means
    essentially two things: adding a @dfn{recipe} that describes how to
    build the package, including a list of other packages required to build
    it, and adding @dfn{package metadata} along with that recipe, such as a
    description and licensing information.
    
    In Guix all this information is embodied in @dfn{package definitions}.
    Package definitions provide a high-level view of the package.  They are
    written using the syntax of the Scheme programming language; in fact,
    for each package we define a variable bound to the package definition,
    and export that variable from a module (@pxref{Package Modules}).
    However, in-depth Scheme knowledge is @emph{not} a prerequisite for
    creating packages.  For more information on package definitions,
    @pxref{Defining Packages}.
    
    Once a package definition is in place, stored in a file in the Guix
    source tree, it can be tested using the @command{guix build} command
    (@pxref{Invoking guix build}).  For example, assuming the new package is
    called @code{gnew}, you may run this command from the Guix build tree
    (@pxref{Running Guix Before It Is Installed}):
    
    @example
    ./pre-inst-env guix build gnew --keep-failed
    @end example
    
    Using @code{--keep-failed} makes it easier to debug build failures since
    it provides access to the failed build tree.  Another useful
    command-line option when debugging is @code{--log-file}, to access the
    build log.
    
    If the package is unknown to the @command{guix} command, it may be that
    the source file contains a syntax error, or lacks a @code{define-public}
    clause to export the package variable.  To figure it out, you may load
    the module from Guile to get more information about the actual error:
    
    @example
    ./pre-inst-env guile -c '(use-modules (gnu packages gnew))'
    @end example
    
    Once your package builds correctly, please send us a patch
    (@pxref{Contributing}).  Well, if you need help, we will be happy to
    help you too.  Once the patch is committed in the Guix repository, the
    new package automatically gets built on the supported platforms by
    @url{http://hydra.gnu.org/jobset/gnu/master, our continuous integration
    system}.
    
    @cindex substituter
    Users can obtain the new package definition simply by running
    @command{guix pull} (@pxref{Invoking guix pull}).  When
    @code{hydra.gnu.org} is done building the package, installing the
    package automatically downloads binaries from there
    (@pxref{Substitutes}).  The only place where human intervention is
    needed is to review and apply the patch.
    
    
    @menu
    * Software Freedom::            What may go into the distribution.
    * Package Naming::              What's in a name?
    * Version Numbers::             When the name is not enough.
    * Synopses and Descriptions::   Helping users find the right package.
    * Python Modules::              Taming the snake.
    * Perl Modules::                Little pearls.
    * Java Packages::               Coffee break.
    * Fonts::                       Fond of fonts.
    @end menu
    
    @node Software Freedom
    @subsection Software Freedom
    
    @c Adapted from http://www.gnu.org/philosophy/philosophy.html.
    
    The GNU operating system has been developed so that users can have
    freedom in their computing.  GNU is @dfn{free software}, meaning that
    users have the @url{http://www.gnu.org/philosophy/free-sw.html,four
    essential freedoms}: to run the program, to study and change the program
    in source code form, to redistribute exact copies, and to distribute
    modified versions.  Packages found in the GNU distribution provide only
    software that conveys these four freedoms.
    
    In addition, the GNU distribution follow the
    @url{http://www.gnu.org/distros/free-system-distribution-guidelines.html,free
    software distribution guidelines}.  Among other things, these guidelines
    reject non-free firmware, recommendations of non-free software, and
    discuss ways to deal with trademarks and patents.
    
    Some otherwise free upstream package sources contain a small and optional
    subset that violates the above guidelines, for instance because this subset
    is itself non-free code.  When that happens, the offending items are removed
    with appropriate patches or code snippets in the @code{origin} form of the
    package (@pxref{Defining Packages}).  This way, @code{guix
    build --source} returns the ``freed'' source rather than the unmodified
    upstream source.
    
    
    @node Package Naming
    @subsection Package Naming
    
    A package has actually two names associated with it:
    First, there is the name of the @emph{Scheme variable}, the one following
    @code{define-public}.  By this name, the package can be made known in the
    Scheme code, for instance as input to another package.  Second, there is
    the string in the @code{name} field of a package definition.  This name
    is used by package management commands such as
    @command{guix package} and @command{guix build}.
    
    Both are usually the same and correspond to the lowercase conversion of
    the project name chosen upstream, with underscores replaced with
    hyphens.  For instance, GNUnet is available as @code{gnunet}, and
    SDL_net as @code{sdl-net}.
    
    We do not add @code{lib} prefixes for library packages, unless these are
    already part of the official project name.  But @pxref{Python
    Modules} and @ref{Perl Modules} for special rules concerning modules for
    the Python and Perl languages.
    
    Font package names are handled differently, @pxref{Fonts}.
    
    
    @node Version Numbers
    @subsection Version Numbers
    
    We usually package only the latest version of a given free software
    project.  But sometimes, for instance for incompatible library versions,
    two (or more) versions of the same package are needed.  These require
    different Scheme variable names.  We use the name as defined
    in @ref{Package Naming}
    for the most recent version; previous versions use the same name, suffixed
    by @code{-} and the smallest prefix of the version number that may
    distinguish the two versions.
    
    The name inside the package definition is the same for all versions of a
    package and does not contain any version number.
    
    For instance, the versions 2.24.20 and 3.9.12 of GTK+ may be packaged as follows:
    
    @example
    (define-public gtk+
      (package
        (name "gtk+")
        (version "3.9.12")
        ...))
    (define-public gtk+-2
      (package
        (name "gtk+")
        (version "2.24.20")
        ...))
    @end example
    If we also wanted GTK+ 3.8.2, this would be packaged as
    @example
    (define-public gtk+-3.8
      (package
        (name "gtk+")
        (version "3.8.2")
        ...))
    @end example
    
    @c See <https://lists.gnu.org/archive/html/guix-devel/2016-01/msg00425.html>,
    @c for a discussion of what follows.
    @cindex version number, for VCS snapshots
    Occasionally, we package snapshots of upstream's version control system
    (VCS) instead of formal releases.  This should remain exceptional,
    because it is up to upstream developers to clarify what the stable
    release is.  Yet, it is sometimes necessary.  So, what should we put in
    the @code{version} field?
    
    Clearly, we need to make the commit identifier of the VCS snapshot
    visible in the version string, but we also need to make sure that the
    version string is monotonically increasing so that @command{guix package
    --upgrade} can determine which version is newer.  Since commit
    identifiers, notably with Git, are not monotonically increasing, we add
    a revision number that we increase each time we upgrade to a newer
    snapshot.  The resulting version string looks like this:
    
    @example
    2.0.11-3.cabba9e
      ^    ^    ^
      |    |    `-- upstream commit ID
      |    |
      |    `--- Guix package revision
      |
    latest upstream version
    @end example
    
    It is a good idea to strip commit identifiers in the @code{version}
    field to, say, 7 digits.  It avoids an aesthetic annoyance (assuming
    aesthetics have a role to play here) as well as problems related to OS
    limits such as the maximum shebang length (127 bytes for the Linux
    kernel.)  It is best to use the full commit identifiers in
    @code{origin}s, though, to avoid ambiguities.  A typical package
    definition may look like this:
    
    @example
    (define my-package
      (let ((commit "c3f29bc928d5900971f65965feaae59e1272a3f7"))
        (package
          (version (string-append "0.9-1."
                                  (string-take commit 7)))
          (source (origin
                    (method git-fetch)
                    (uri (git-reference
                          (url "git://example.org/my-package.git")
                          (commit commit)))
                    (sha256 (base32 "1mbikn@dots{}"))
                    (file-name (string-append "my-package-" version
                                              "-checkout"))))
          ;; @dots{}
          )))
    @end example
    
    @node Synopses and Descriptions
    @subsection Synopses and Descriptions
    
    As we have seen before, each package in GNU@tie{}Guix includes a
    synopsis and a description (@pxref{Defining Packages}).  Synopses and
    descriptions are important: They are what @command{guix package
    --search} searches, and a crucial piece of information to help users
    determine whether a given package suits their needs.  Consequently,
    packagers should pay attention to what goes into them.
    
    Synopses must start with a capital letter and must not end with a
    period.  They must not start with ``a'' or ``the'', which usually does
    not bring anything; for instance, prefer ``File-frobbing tool'' over ``A
    tool that frobs files''.  The synopsis should say what the package
    is---e.g., ``Core GNU utilities (file, text, shell)''---or what it is
    used for---e.g., the synopsis for GNU@tie{}grep is ``Print lines
    matching a pattern''.
    
    Keep in mind that the synopsis must be meaningful for a very wide
    audience.  For example, ``Manipulate alignments in the SAM format''
    might make sense for a seasoned bioinformatics researcher, but might be
    fairly unhelpful or even misleading to a non-specialized audience.  It
    is a good idea to come up with a synopsis that gives an idea of the
    application domain of the package.  In this example, this might give
    something like ``Manipulate nucleotide sequence alignments'', which
    hopefully gives the user a better idea of whether this is what they are
    looking for.
    
    Descriptions should take between five and ten lines.  Use full
    sentences, and avoid using acronyms without first introducing them.
    Please avoid marketing phrases such as ``world-leading'',
    ``industrial-strength'', and ``next-generation'', and avoid superlatives
    like ``the most advanced''---they are not helpful to users looking for a
    package and may even sound suspicious.  Instead, try to be factual,
    mentioning use cases and features.
    
    @cindex Texinfo markup, in package descriptions
    Descriptions can include Texinfo markup, which is useful to introduce
    ornaments such as @code{@@code} or @code{@@dfn}, bullet lists, or
    hyperlinks (@pxref{Overview,,, texinfo, GNU Texinfo}).  However you
    should be careful when using some characters for example @samp{@@} and
    curly braces which are the basic special characters in Texinfo
    (@pxref{Special Characters,,, texinfo, GNU Texinfo}).  User interfaces
    such as @command{guix package --show} take care of rendering it
    appropriately.
    
    Synopses and descriptions are translated by volunteers
    @uref{http://translationproject.org/domain/guix-packages.html, at the
    Translation Project} so that as many users as possible can read them in
    their native language.  User interfaces search them and display them in
    the language specified by the current locale.
    
    Translation is a lot of work so, as a packager, please pay even more
    attention to your synopses and descriptions as every change may entail
    additional work for translators.  In order to help them, it is possible
    to make recommendations or instructions visible to them by inserting
    special comments like this (@pxref{xgettext Invocation,,, gettext, GNU
    Gettext}):
    
    @example
    ;; TRANSLATORS: "X11 resize-and-rotate" should not be translated.
    (description "ARandR is designed to provide a simple visual front end
    for the X11 resize-and-rotate (RandR) extension. @dots{}")
    @end example
    
    
    @node Python Modules
    @subsection Python Modules
    
    We currently package Python 2 and Python 3, under the Scheme variable names
    @code{python-2} and @code{python} as explained in @ref{Version Numbers}.
    To avoid confusion and naming clashes with other programming languages, it
    seems desirable that the name of a package for a Python module contains
    the word @code{python}.
    
    Some modules are compatible with only one version of Python, others with both.
    If the package Foo compiles only with Python 3, we name it
    @code{python-foo}; if it compiles only with Python 2, we name it
    @code{python2-foo}. If it is compatible with both versions, we create two
    packages with the corresponding names.
    
    If a project already contains the word @code{python}, we drop this;
    for instance, the module python-dateutil is packaged under the names
    @code{python-dateutil} and @code{python2-dateutil}.  If the project name
    starts with @code{py} (e.g. @code{pytz}), we keep it and prefix it as
    described above.
    
    
    @node Perl Modules
    @subsection Perl Modules
    
    Perl programs standing for themselves are named as any other package,
    using the lowercase upstream name.
    For Perl packages containing a single class, we use the lowercase class name,
    replace all occurrences of @code{::} by dashes and prepend the prefix
    @code{perl-}.
    So the class @code{XML::Parser} becomes @code{perl-xml-parser}.
    Modules containing several classes keep their lowercase upstream name and
    are also prepended by @code{perl-}.  Such modules tend to have the word
    @code{perl} somewhere in their name, which gets dropped in favor of the
    prefix.  For instance, @code{libwww-perl} becomes @code{perl-libwww}.
    
    
    @node Java Packages
    @subsection Java Packages
    
    Java programs standing for themselves are named as any other package,
    using the lowercase upstream name.
    
    To avoid confusion and naming clashes with other programming languages,
    it is desirable that the name of a package for a Java package is
    prefixed with @code{java-}.  If a project already contains the word
    @code{java}, we drop this; for instance, the package @code{ngsjava} is
    packaged under the name @code{java-ngs}.
    
    For Java packages containing a single class or a small class hierarchy,
    we use the lowercase class name, replace all occurrences of @code{.} by
    dashes and prepend the prefix @code{java-}.  So the class
    @code{apache.commons.cli} becomes package
    @code{java-apache-commons-cli}.
    
    
    @node Fonts
    @subsection Fonts
    
    For fonts that are in general not installed by a user for typesetting
    purposes, or that are distributed as part of a larger software package,
    we rely on the general packaging rules for software; for instance, this
    applies to the fonts delivered as part of the X.Org system or fonts that
    are part of TeX Live.
    
    To make it easier for a user to search for fonts, names for other packages
    containing only fonts are constructed as follows, independently of the
    upstream package name.
    
    The name of a package containing only one font family starts with
    @code{font-}; it is followed by the foundry name and a dash @code{-}
    if the foundry is known, and the font family name, in which spaces are
    replaced by dashes (and as usual, all upper case letters are transformed
    to lower case).
    For example, the Gentium font family by SIL is packaged under the name
    @code{font-sil-gentium}.
    
    For a package containing several font families, the name of the collection
    is used in the place of the font family name.
    For instance, the Liberation fonts consist of three families,
    Liberation Sans, Liberation Serif and Liberation Mono.
    These could be packaged separately under the names
    @code{font-liberation-sans} and so on; but as they are distributed together
    under a common name, we prefer to package them together as
    @code{font-liberation}.
    
    In the case where several formats of the same font family or font collection
    are packaged separately, a short form of the format, prepended by a dash,
    is added to the package name.  We use @code{-ttf} for TrueType fonts,
    @code{-otf} for OpenType fonts and @code{-type1} for PostScript Type 1
    fonts.
    
    
    
    @node Bootstrapping
    @section Bootstrapping
    
    @c Adapted from the ELS 2013 paper.
    
    @cindex bootstrapping
    
    Bootstrapping in our context refers to how the distribution gets built
    ``from nothing''.  Remember that the build environment of a derivation
    contains nothing but its declared inputs (@pxref{Introduction}).  So
    there's an obvious chicken-and-egg problem: how does the first package
    get built?  How does the first compiler get compiled?  Note that this is
    a question of interest only to the curious hacker, not to the regular
    user, so you can shamelessly skip this section if you consider yourself
    a ``regular user''.
    
    @cindex bootstrap binaries
    The GNU system is primarily made of C code, with libc at its core.  The
    GNU build system itself assumes the availability of a Bourne shell and
    command-line tools provided by GNU Coreutils, Awk, Findutils, `sed', and
    `grep'.  Furthermore, build programs---programs that run
    @code{./configure}, @code{make}, etc.---are written in Guile Scheme
    (@pxref{Derivations}).  Consequently, to be able to build anything at
    all, from scratch, Guix relies on pre-built binaries of Guile, GCC,
    Binutils, libc, and the other packages mentioned above---the
    @dfn{bootstrap binaries}.
    
    These bootstrap binaries are ``taken for granted'', though we can also
    re-create them if needed (more on that later).
    
    @unnumberedsubsec Preparing to Use the Bootstrap Binaries
    
    @c As of Emacs 24.3, Info-mode displays the image, but since it's a
    @c large image, it's hard to scroll.  Oh well.
    @image{images/bootstrap-graph,6in,,Dependency graph of the early bootstrap derivations}
    
    The figure above shows the very beginning of the dependency graph of the
    distribution, corresponding to the package definitions of the @code{(gnu
    packages bootstrap)} module.  A similar figure can be generated with
    @command{guix graph} (@pxref{Invoking guix graph}), along the lines of:
    
    @example
    guix graph -t derivation \
      -e '(@@@@ (gnu packages bootstrap) %bootstrap-gcc)' \
      | dot -Tps > t.ps
    @end example
    
    At this level of detail, things are
    slightly complex.  First, Guile itself consists of an ELF executable,
    along with many source and compiled Scheme files that are dynamically
    loaded when it runs.  This gets stored in the @file{guile-2.0.7.tar.xz}
    tarball shown in this graph.  This tarball is part of Guix's ``source''
    distribution, and gets inserted into the store with @code{add-to-store}
    (@pxref{The Store}).
    
    But how do we write a derivation that unpacks this tarball and adds it
    to the store?  To solve this problem, the @code{guile-bootstrap-2.0.drv}
    derivation---the first one that gets built---uses @code{bash} as its
    builder, which runs @code{build-bootstrap-guile.sh}, which in turn calls
    @code{tar} to unpack the tarball.  Thus, @file{bash}, @file{tar},
    @file{xz}, and @file{mkdir} are statically-linked binaries, also part of
    the Guix source distribution, whose sole purpose is to allow the Guile
    tarball to be unpacked.
    
    Once @code{guile-bootstrap-2.0.drv} is built, we have a functioning
    Guile that can be used to run subsequent build programs.  Its first task
    is to download tarballs containing the other pre-built binaries---this
    is what the @code{.tar.xz.drv} derivations do.  Guix modules such as
    @code{ftp-client.scm} are used for this purpose.  The
    @code{module-import.drv} derivations import those modules in a directory
    in the store, using the original layout.  The
    @code{module-import-compiled.drv} derivations compile those modules, and
    write them in an output directory with the right layout.  This
    corresponds to the @code{#:modules} argument of
    @code{build-expression->derivation} (@pxref{Derivations}).
    
    Finally, the various tarballs are unpacked by the
    derivations @code{gcc-bootstrap-0.drv}, @code{glibc-bootstrap-0.drv},
    etc., at which point we have a working C tool chain.
    
    
    @unnumberedsubsec Building the Build Tools
    
    Bootstrapping is complete when we have a full tool chain that does not
    depend on the pre-built bootstrap tools discussed above.  This
    no-dependency requirement is verified by checking whether the files of
    the final tool chain contain references to the @file{/gnu/store}
    directories of the bootstrap inputs.  The process that leads to this
    ``final'' tool chain is described by the package definitions found in
    the @code{(gnu packages commencement)} module.
    
    The @command{guix graph} command allows us to ``zoom out'' compared to
    the graph above, by looking at the level of package objects instead of
    individual derivations---remember that a package may translate to
    several derivations, typically one derivation to download its source,
    one to build the Guile modules it needs, and one to actually build the
    package from source.  The command:
    
    @example
    guix graph -t bag \
      -e '(@@@@ (gnu packages commencement)
              glibc-final-with-bootstrap-bash)' | dot -Tps > t.ps
    @end example
    
    @noindent
    produces the dependency graph leading to the ``final'' C
    library@footnote{You may notice the @code{glibc-intermediate} label,
    suggesting that it is not @emph{quite} final, but as a good
    approximation, we will consider it final.}, depicted below.
    
    @image{images/bootstrap-packages,6in,,Dependency graph of the early packages}
    
    @c See <http://lists.gnu.org/archive/html/gnu-system-discuss/2012-10/msg00000.html>.
    The first tool that gets built with the bootstrap binaries is
    GNU@tie{}Make---noted @code{make-boot0} above---which is a prerequisite
    for all the following packages.  From there Findutils and Diffutils get
    built.
    
    Then come the first-stage Binutils and GCC, built as pseudo cross
    tools---i.e., with @code{--target} equal to @code{--host}.  They are
    used to build libc.  Thanks to this cross-build trick, this libc is
    guaranteed not to hold any reference to the initial tool chain.
    
    From there the final Binutils and GCC (not shown above) are built.
    GCC uses @code{ld}
    from the final Binutils, and links programs against the just-built libc.
    This tool chain is used to build the other packages used by Guix and by
    the GNU Build System: Guile, Bash, Coreutils, etc.
    
    And voilà!  At this point we have the complete set of build tools that
    the GNU Build System expects.  These are in the @code{%final-inputs}
    variable of the @code{(gnu packages commencement)} module, and are
    implicitly used by any package that uses @code{gnu-build-system}
    (@pxref{Build Systems, @code{gnu-build-system}}).
    
    
    @unnumberedsubsec Building the Bootstrap Binaries
    
    Because the final tool chain does not depend on the bootstrap binaries,
    those rarely need to be updated.  Nevertheless, it is useful to have an
    automated way to produce them, should an update occur, and this is what
    the @code{(gnu packages make-bootstrap)} module provides.
    
    The following command builds the tarballs containing the bootstrap
    binaries (Guile, Binutils, GCC, libc, and a tarball containing a mixture
    of Coreutils and other basic command-line tools):
    
    @example
    guix build bootstrap-tarballs
    @end example
    
    The generated tarballs are those that should be referred to in the
    @code{(gnu packages bootstrap)} module mentioned at the beginning of
    this section.
    
    Still here?  Then perhaps by now you've started to wonder: when do we
    reach a fixed point?  That is an interesting question!  The answer is
    unknown, but if you would like to investigate further (and have
    significant computational and storage resources to do so), then let us
    know.
    
    @node Porting
    @section Porting to a New Platform
    
    As discussed above, the GNU distribution is self-contained, and
    self-containment is achieved by relying on pre-built ``bootstrap
    binaries'' (@pxref{Bootstrapping}).  These binaries are specific to an
    operating system kernel, CPU architecture, and application binary
    interface (ABI).  Thus, to port the distribution to a platform that is
    not yet supported, one must build those bootstrap binaries, and update
    the @code{(gnu packages bootstrap)} module to use them on that platform.
    
    Fortunately, Guix can @emph{cross compile} those bootstrap binaries.
    When everything goes well, and assuming the GNU tool chain supports the
    target platform, this can be as simple as running a command like this
    one:
    
    @example
    guix build --target=armv5tel-linux-gnueabi bootstrap-tarballs
    @end example
    
    For this to work, the @code{glibc-dynamic-linker} procedure in
    @code{(gnu packages bootstrap)} must be augmented to return the right
    file name for libc's dynamic linker on that platform; likewise,
    @code{system->linux-architecture} in @code{(gnu packages linux)} must be
    taught about the new platform.
    
    Once these are built, the @code{(gnu packages bootstrap)} module needs
    to be updated to refer to these binaries on the target platform.  That
    is, the hashes and URLs of the bootstrap tarballs for the new platform
    must be added alongside those of the currently supported platforms.  The
    bootstrap Guile tarball is treated specially: it is expected to be
    available locally, and @file{gnu/local.mk} has rules do download it for
    the supported architectures; a rule for the new platform must be added
    as well.
    
    In practice, there may be some complications.  First, it may be that the
    extended GNU triplet that specifies an ABI (like the @code{eabi} suffix
    above) is not recognized by all the GNU tools.  Typically, glibc
    recognizes some of these, whereas GCC uses an extra @code{--with-abi}
    configure flag (see @code{gcc.scm} for examples of how to handle this).
    Second, some of the required packages could fail to build for that
    platform.  Lastly, the generated binaries could be broken for some
    reason.
    
    @c *********************************************************************
    @include contributing.texi
    
    @c *********************************************************************
    @node Acknowledgments
    @chapter Acknowledgments
    
    Guix is based on the @uref{http://nixos.org/nix/, Nix package manager},
    which was designed and
    implemented by Eelco Dolstra, with contributions from other people (see
    the @file{nix/AUTHORS} file in Guix.)  Nix pioneered functional package
    management, and promoted unprecedented features, such as transactional
    package upgrades and rollbacks, per-user profiles, and referentially
    transparent build processes.  Without this work, Guix would not exist.
    
    The Nix-based software distributions, Nixpkgs and NixOS, have also been
    an inspiration for Guix.
    
    GNU@tie{}Guix itself is a collective work with contributions from a
    number of people.  See the @file{AUTHORS} file in Guix for more
    information on these fine people.  The @file{THANKS} file lists people
    who have helped by reporting bugs, taking care of the infrastructure,
    providing artwork and themes, making suggestions, and more---thank you!
    
    
    @c *********************************************************************
    @node GNU Free Documentation License
    @appendix GNU Free Documentation License
    
    @include fdl-1.3.texi
    
    @c *********************************************************************
    @node Concept Index
    @unnumbered Concept Index
    @printindex cp
    
    @node Programming Index
    @unnumbered Programming Index
    @syncodeindex tp fn
    @syncodeindex vr fn
    @printindex fn
    
    @bye
    
    @c Local Variables:
    @c ispell-local-dictionary: "american";
    @c End: