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guix.texi

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    Ludovic Courtès authored
    * guix/scripts/lint.scm (report-tabulations, report-trailing-white-space,
      report-long-line, report-formatting-issues, check-formatting): New
      procedures.
      (%formatting-reporters): New variable.
      (%checkers): Add 'formatting' checker.
    * tests/lint.scm ("formatting: tabulation", "formatting: trailing white
      space", "formatting: long line", "formatting: alright"): New tests.
    * doc/guix.texi (Invoking guix lint): Mention the 'formatting' checker.
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    guix.texi 274.70 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 Ludovic Courtès@*
    Copyright @copyright{} 2013, 2014 Andreas Enge@*
    Copyright @copyright{} 2013 Nikita Karetnikov@*
    Copyright @copyright{} 2015 Mathieu Lirzin@*
    Copyright @copyright{} 2014 Pierre-Antoine Rault@*
    Copyright @copyright{} 2015 Taylan Ulrich Bayırlı/Kammer
    
    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 Package management
    @direntry
    * guix: (guix).       Guix, the functional package manager.
    * guix package: (guix)Invoking guix package
                          Managing packages with Guix.
    * guix build: (guix)Invoking guix build
                          Building packages with Guix.
    * guix system: (guix)Invoking guix system
                          Managing the operating system configuration.
    @end direntry
    
    @dircategory Software development
    @direntry
    * guix environment: (guix)Invoking guix environment
                          Building development environments with Guix.
    @end direntry
    
    @titlepage
    @title GNU Guix Reference Manual
    @subtitle Using the GNU Guix Functional Package Manager
    @author Ludovic Courtès
    @author Andreas Enge
    @author Nikita Karetnikov
    
    @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.
    * 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.
    * Emacs Interface::             Package management from Emacs.
    * 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.
    
    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 environment::   Setting up development environments.
    * Invoking guix publish::       Sharing substitutes.
    
    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 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.
    * Defining Services::           Adding new service definitions.
    
    Services
    
    * Base Services::               Essential system services.
    * Networking Services::         Network setup, SSH daemon, etc.
    * X Window::                    Graphical display.
    * Desktop Services::            D-Bus and desktop services.
    * Database Services::           SQL databases.
    * Various Services::            Other services.
    
    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.
    * Python Modules::              Taming the snake.
    * Perl Modules::                Little pearls.
    * 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
    
    GNU Guix@footnote{``Guix'' is pronounced like ``geeks'', or ``ɡiːks''
    using the international phonetic alphabet (IPA).} is a functional
    package management tool for the GNU system.  Package management consists
    of all activities that relate to building packages from sources,
    honoring their build-time and run-time dependencies,
    installing packages in user environments, upgrading installed packages
    to new versions or rolling back to a previous set, removing unused
    software packages, etc.
    
    @cindex functional package management
    The term @dfn{functional} refers to a specific package management
    discipline.  In Guix, the package build and installation process is seen
    as a 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 system's environment 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 of Guix's salient features: support for
    transactional package upgrade and rollback, per-user installation, and
    garbage collection of packages (@pxref{Features}).
    
    Guix has a command-line interface, which allows users to build, install,
    upgrade, and remove packages, as well as a Scheme programming interface.
    
    @cindex Guix System Distribution
    @cindex GuixSD
    Last but not least, Guix is used to build a distribution of the GNU
    system, with many GNU and non-GNU free software packages.  The Guix
    System Distribution, or GNU@tie{}GuixSD, takes advantage of the core
    properties of Guix at the system level.  With GuixSD, users
    @emph{declare} all aspects of the operating system configuration, and
    Guix takes care of instantiating that configuration in a reproducible,
    stateless fashion.  @xref{GNU Distribution}.
    
    @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}.
    
    @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}@footnote{As
    usual, make sure to download the associated @file{.sig} file and to
    verify the authenticity of the tarball against it!}, where @var{system}
    is @code{x86_64-linux} for an @code{x86_64} machine already running the
    kernel Linux, and so on.
    
    @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:
    
    @example
    # ~root/.guix-profile/bin/guix-daemon --build-users-group=guixbuild
    @end example
    
    On hosts using the systemd init system, drop
    @file{~root/.guix-profile/lib/systemd/system/guix-daemon.service} in
    @file{/etc/systemd/system}.
    
    @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
    
    @item
    To use substitutes from @code{hydra.gnu.org} (@pxref{Substitutes}),
    authorize them:
    
    @example
    # guix archive --authorize < ~root/.guix-profile/share/guix/hydra.gnu.org.pub
    @end example
    @end enumerate
    
    And that's it!  For additional tips and tricks, @pxref{Application
    Setup}.
    
    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.
    
    The tarball in question 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
    @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.
    @item
    Installing @uref{http://gnutls.org/, GnuTLS-Guile} will
    allow you to access @code{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.  This is primarily
    of interest to developers.  @xref{Guile Preparations, how to install the
    GnuTLS bindings for Guile,, gnutls-guile, GnuTLS-Guile}.
    @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 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.
    
    Upon failure, please email @email{bug-guix@@gnu.org} and attach the
    @file{test-suite.log} file.  When @file{tests/@var{something}.scm}
    fails, please also attach the @file{@var{something}.log} file available
    in the top-level build directory.  Please specify the Guix version being
    used as well as version numbers of the dependencies
    (@pxref{Requirements}) in your message.
    
    @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.  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}}).  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.}:
    
    @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 container's processes
    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
    
    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}; anytime a build is requested, for
    instance via @code{guix build}, the daemon attempts to offload it to one
    of the machines that satisfies the derivation's constraints, 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 the daemon may offload builds
    to.  The important fields are:
    
    @table @code
    
    @item name
    The remote machine's host name.
    
    @item system
    The remote machine's system type---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 the machine's SSH server (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's 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{http://hydra.gnu.org}
    is used.
    
    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.
    
    @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 --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
    
    When using Guix on top of GNU/Linux distribution other than GuixSD, a
    few additional steps are needed to get everything in place.  Here are
    some of them.
    
    @subsection Locales
    
    @cindex locales, when not on GuixSD
    Packages installed @i{via} Guix will not use the host system's locale
    data.  Instead, you must first install one of the locale packages
    available with Guix and then define the @code{LOCPATH} environment
    variable (@pxref{Locale Names, @code{LOCPATH},, libc, The GNU C Library
    Reference Manual}):
    
    @example
    $ guix package -i glibc-locales
    $ export 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.  Alternately, the @code{glibc-utf8-locales} is smaller but
    limited to a few UTF-8 locales.
    
    @subsection X11 Fonts
    
    The majority of graphical applications use Fontconfig to locate and
    load fonts and perform X11-client-side rendering.  Guix's
    @code{fontconfig} package looks for fonts in @file{$HOME/.guix-profile}
    by default.  Thus, to allow graphical applications installed with Guix
    to display fonts, you will have to install fonts with Guix as well.
    Essential font packages include @code{gs-fonts}, @code{font-dejavu}, and
    @code{font-gnu-freefont-ttf}.
    
    @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
    procedure 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
    (@pxref{Invoking guix package, @code{guix package}}), and a visual user
    interface in Emacs (@pxref{Emacs Interface}).
    
    @menu
    * Features::                    How Guix will make your life brighter.
    * Invoking guix package::       Package installation, removal, etc.
    * Emacs Interface::             Package management from Emacs.
    * 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 those 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 is subject to transactional upgrades and roll-back
    (@pxref{Using the Configuration System}).
    
    All those packages in the package store may be @emph{garbage-collected}.
    Guix can determine which packages are still referenced by the 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}).
    
    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 package's dependencies
    in 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 a hyphen 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 independently.
    
    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.
    
    @c XXX: keep me up-to-date
    Finally, when installing a GNU package, the tool reports the
    availability of a newer upstream version.  In the future, it may provide
    the option of installing directly from the upstream version, even if
    that version is not yet in the distribution.
    
    @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 --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 meta-data.
    
    Installing, removing, or upgrading packages from a generation that has
    been rolled back to overwrites previous future generations.  Thus, the
    history of a profile's generations 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}.
    
    @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 environment's build log
    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}
    List the available packages whose name, synopsis, or description matches
    @var{regexp}.  Print all the meta-data 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
    
    @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 lesser 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 roll-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 that @command{guix
    build} supports (@pxref{Invoking guix build, common build options}).
    
    @include emacs.texi
    
    @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.  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}).
    
    @cindex security
    @cindex digital signatures
    To allow Guix to download substitutes from @code{hydra.gnu.org}, 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.
    
    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.
    
    
    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.
    
    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 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 install 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}).
    
    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 --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-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 an
    element's closure.
    
    @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
    daemon's database actually exist in @file{/gnu/store}.
    
    When provided, @var{options} must a comma-separated list containing one
    or more of @code{contents} and @code{repair}.
    
    When passing @option{--verify=contents}, the daemon will compute the
    content hash of each store item and compare 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.
    
    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 another machine's store.  For example, 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 target
    machine's store.  The @code{--missing} option can help figure out which
    items are missing from the target's 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 meta-data 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 daemons.  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.
    Alternately, @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)}.
    @end table
    
    To export store files as an archive to the 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
    same options that can be passed to the @command{guix build} command
    (@pxref{Invoking guix build, common build options}).
    
    
    @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))
    
    (define-public hello
      (package
        (name "hello")
        (version "2.8")
        (source (origin
                 (method url-fetch)
                 (uri (string-append "mirror://gnu/hello/hello-" version
                                     ".tar.gz"))
                 (sha256
                  (base32 "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))))
        (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 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 into 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.
    
    Eventually, 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 origin object telling how the source code for the package should be
    acquired (@pxref{origin Reference}).
    
    @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{'()})
    Package or derivation inputs to the build.  This is a list of lists,
    where each list has the name of the input (a string) as its first
    element, a package or derivation object as its second element, and
    optionally the name of the output of the package or derivation that
    should be used, which defaults to @code{"out"}.
    
    @item @anchor{package-propagated-inputs}@code{propagated-inputs} (default: @code{'()})
    @cindex propagated inputs
    This field is like @code{inputs}, but the specified packages will be
    force-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 library needs headers of another
    library to compile, or needs another shared library to be linked
    alongside itself when a program wants to link to it.
    
    @item @code{native-inputs} (default: @code{'()})
    This field is like @code{inputs}, but in case of a cross-compilation it
    will be ensured that packages for the architecture of the build machine
    are present, such that executables from them can be used during the
    build.
    
    This is typically where you would 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}).
    
    @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 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)}.
    
    @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's 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 will handle the URI.
    
    Examples include:
    
    @table @asis
    @item @var{url-fetch} from @code{(guix download)}
    download a file 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's 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 quoted piece of code that will be run in the source directory to make
    any modifications, which is 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{imported-modules} (default: @code{'()})
    The list of Guile modules to import in the patch derivation, for use by
    the @code{snippet}.
    
    @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 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 packages 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 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-builds?} 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 (alist-delete 'configure %standard-phases)
    @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 don't
    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} 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 found under
    @file{bin/} are able to find GLib's ``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 GLib's
    @uref{https://developer.gnome.org/gio/stable/glib-compile-schemas.html,
    GSettings schemas} 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 their @code{PYTHONPATH}
    environment variable points to all the Python libraries they depend on.
    
    Which Python package is used can be specified with the @code{#:python}
    parameter.
    @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} 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 is
    expected to reference a gem archive instead of a traditional tarball,
    since this is the format that all 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.
    
    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 one of Emacs' own
    packaging system (@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's 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 paths
    
    Conceptually, the @dfn{store} is where derivations that have been
    successfully built are stored---by default, under @file{/gnu/store}.
    Sub-directories in the store are referred to as @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 paths---paths that result from a successful build.
    
    The store is 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 it requests, and
    read the result---these are remote procedure calls, or RPCs.
    
    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}
    Return @code{#t} when @var{path} is a valid store path.
    @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 contain 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] [#: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.
    
    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] @
           [#: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{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 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 Guile's 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]
    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.
    
    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}).
    Those 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 those 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 Scheme's homoiconicity---code has a direct
    representation as data---comes in handy for that.  But we need more than
    Scheme's normal @code{quasiquote} mechanism 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 in three syntactic forms: @code{gexp},
    @code{ungexp}, and @code{ungexp-splicing} (or simply: @code{#~},
    @code{#$}, and @code{#$@@}), which are comparable respectively to
    @code{quasiquote}, @code{unquote}, and @code{unquote-splicing}
    (@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
    
    This mechanism is not limited to package and derivation
    objects: @dfn{compilers} able to ``lower'' other high-level objects to
    derivations can be defined, such that these objects can also be inserted
    into gexps.  For example, a useful type of high-level object that can be
    inserted in a gexp is ``file-like objects'', which make it easy to
    add files to the store and 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 derivation's output
    directory name.
    
    @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.
    
    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 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] @
           [#:leaked-env-vars #f] @
           [#: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}.  When @var{target}
    is true, it is used as the cross-compilation target triplet for packages
    referred to by @var{exp}.
    
    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.
    
    The other arguments are as for @code{derivation} (@pxref{Derivations}).
    @end deffn
    
    @cindex file-like objects
    The @code{local-file} and @code{plain-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? #t]
    Return an object representing local file @var{file} to add to the store; this
    object can be used in a gexp.  @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.
    
    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 {Monadic Procedure} gexp->script @var{name} @var{exp}
    Return an executable script @var{name} that runs @var{exp} using
    @var{guile} with @var{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 {Monadic Procedure} gexp->file @var{name} @var{exp}
    Return a derivation that builds a file @var{name} containing @var{exp}.
    
    The resulting file holds references to all the dependencies of @var{exp}
    or a subset thereof.
    @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 references @var{coreutils}, @var{grep}, and @var{sed}, thereby
    preventing them from being garbage-collected during its lifetime.
    @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.
    
    
    @c *********************************************************************
    @node Utilities
    @chapter Utilities
    
    This section describes tools primarily targeted at developers and users
    who write new package definitions.  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 environment::   Setting up development environments.
    * Invoking guix publish::       Sharing substitutes.
    @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
    
    @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
    disambiguation among several same-named packages or package variants is
    needed.
    
    The @var{options} may be zero or more of the following:
    
    @table @code
    
    @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.
    
    Alternately, @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 packages' source derivations, 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 GCC's source tarball.
    
    The returned source tarball is the result of applying any patches and
    code snippets specified in the package's @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 all packages' source derivations, 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 all packages' source derivations, as well as all source
    derivations for packages' transitive inputs.  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 host's system type.
    
    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}).
    
    @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 one specified on the
    command line whose name 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's @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 --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 --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 for the given
    @var{package-or-derivation}s, 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
    
    
    @end table
    
    @cindex common build options
    In addition, 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 fail, 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 --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}).
    
    @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-build-hook
    Do not attempt to offload builds @i{via} the daemon's ``build hook''
    (@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 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-4.8 vim
    @end example
    
    @noindent
    launches the program specified in the @code{EDITOR} environment variable
    to edit the recipe of GCC@tie{}4.8.4 and that of Vim.
    
    If you are using Emacs, note that the Emacs user interface provides
    similar functionality in the ``package info'' and ``package list''
    buffers created by @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
    the package's 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 @var{file}'s
    meta-data 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.  Meta-data 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 willing to add a
    package to the distribution but who'd rather do as little work as
    possible to get there---a legitimate demand.  The command knows of a few
    repositories from which it can ``import'' package meta-data.  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
    meta-data, and @var{options} specifies a package identifier and other
    options specific to @var{importer}.  Currently, the available
    ``importers'' are:
    
    @table @code
    @item gnu
    Import meta-data 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's 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's signature.  @xref{Invoking guix
    refresh, @code{--key-download}}.
    @end table
    
    @item pypi
    @cindex pypi
    Import meta-data 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.
    
    The command below imports meta-data for the @code{itsdangerous} Python
    package:
    
    @example
    guix import pypi itsdangerous
    @end example
    
    @item cpan
    @cindex CPAN
    Import meta-data from @uref{https://www.metacpan.org/, MetaCPAN}.
    Information is taken from the JSON-formatted meta-data 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 meta-data for the @code{Acme::Boolean}
    Perl module:
    
    @example
    guix import cpan Acme::Boolean
    @end example
    
    @item nix
    Import meta-data 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.
    
    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 meta-data from 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 the standard input.
    @item --no-test-dependencies
    @itemx -t
    Do not include dependencies required by the test suites only.
    @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 meta-data 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 a hyphen 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 meta-data 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 each package's FTP directory and determining the
    highest version number of the source tarballs
    therein@footnote{Currently, this only works for GNU packages.}.
    
    When passed @code{--update}, it modifies distribution source files to
    update the version numbers and source tarball hashes of those packages'
    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 it's 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 --update
    @itemx -u
    Update distribution source files (package recipes) in place.
    @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.
    
    @end table
    
    In addition, @command{guix refresh} can be passed one or more package
    names, as in this example:
    
    @example
    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-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
    
    @node Invoking guix lint
    @section Invoking @command{guix lint}
    The @command{guix lint} 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
    Probe @code{home-page} and @code{source} URLs and report those that are
    invalid.
    
    @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 --checkers
    @itemx -c
    Only enable the checkers specified in a comma-separated list using the
    names returned by @code{--list-checkers}.
    
    @item --list-checkers
    @itemx -l
    List and describe all the available checkers that will be run on packages
    and exit.
    
    @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}).  These 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 3 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 item's size 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 information about the available substitutes
    (@pxref{Substitutes}).  This allows it to profile disk usage of store
    items that are not even on disk, only available remotely.
    
    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 to @var{file} a graphical map of disk usage as a PNG 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 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 the necessary 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 specified packages are not built yet, @command{guix environment}
    automatically builds them.  The new shell's environment 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 spaws.  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.  The
    @code{--exec} option can be used to specify the command to run instead.
    
    @example
    guix environment guile --exec=make
    @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 -E python
    @end example
    
    The available options are summarized below.
    
    @table @code
    @item --expression=@var{expr}
    @itemx -e @var{expr}
    Create an environment for the package 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.
    
    @item --load=@var{file}
    @itemx -l @var{file}
    Create an environment for 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 environment-gdb.scm
    @end example
    
    
    @item --exec=@var{command}
    @item -E @var{command}
    Execute @var{command} in the new environment.
    
    @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 -E 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}).
    
    @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}.
    @end table
    
    It also supports all of the common build options that @command{guix
    build} supports (@pxref{Invoking guix build, 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, which 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 system's signing key, 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 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 --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
    
    @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 has been 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 ABI, and Linux-Libre kernel.
    
    @item mips64el-linux
    little-endian 64-bit MIPS processors, specifically the Loongson series,
    n32 application binary interface (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,
    @xref{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
    @c This paragraph is for people reading this from tty2 of the
    @c installation image.
    You're 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{Help,,, info, Info: An Introduction}.  Hit
    @kbd{l} afterwards to come back here.
    @end ifinfo
    
    @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
    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
    The system does not yet provide full GNOME and KDE desktops.  Xfce and
    Enlightenment are available though, if graphical desktop environments
    are your thing, as well as a number of X11 window managers.
    
    @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 2,000 packages are available, but you may
    occasionally find that a useful package is missing.
    @end itemize
    
    You've been warned.  But more than a disclaimer, this is an invitation
    to report issues (and success stories!), and join us in improving it.
    @xref{Contributing}, for more info.
    
    @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 in your machine, and determine
    its device name.  Assuming that 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.
    
    @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{Help,,, info, Info: An
    Introduction}).
    
    To install the system, you would:
    
    @enumerate
    
    @item
    Configure the network, by running @command{ifconfig eno1 up && dhclient
    eno1} (to get an automatically assigned IP address from the wired
    network interface controller@footnote{
    @c http://cgit.freedesktop.org/systemd/systemd/tree/src/udev/udev-builtin-net_id.c#n20
    The name @code{eno1} is for the first on-board Ethernet controller.  The
    interface name for an Ethernet controller that is in the first slot of
    the first PCI bus, for instance, would be @code{enp1s0}.  Use
    @command{ifconfig -a} to list all the available network interfaces.}),
    or using the @command{ifconfig} command.
    
    The system automatically loads drivers for your network interface
    controllers.
    
    Setting up network access is almost always a requirement because the
    image does not contain all the software and tools that may be needed.
    
    @item
    Unless this has already been done, you must partition and format the
    target partitions.
    
    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.
    
    The installation image includes Parted (@pxref{Overview,,, parted, GNU
    Parted User Manual}), @command{fdisk}, Cryptsetup/LUKS for disk
    encryption, and e2fsprogs, the suite of tools to manipulate
    ext2/ext3/ext4 file systems.
    
    @item
    Once that is done, mount the target root partition under @file{/mnt}.
    
    @item
    Lastly, run @code{deco start cow-store /mnt}.
    
    This will make @file{/gnu/store} copy-on-write, such that packages added
    to it during the installation phase will be written to the target disk
    rather than kept in memory.
    
    @end enumerate
    
    
    @subsection Proceeding with the Installation
    
    With the target partitions ready, you now 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.
    It is better to store that file on the target root file system, say, as
    @file{/mnt/etc/config.scm}.
    
    @xref{Using the Configuration System}, for examples of operating system
    configurations.  These examples are available under
    @file{/etc/configuration} in the installation image, so you can copy
    them and use them as a starting point for your own configuration.
    
    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 will copy all the necessary files, and install 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.
    
    @subsection Building the Installation Image
    
    The installation image described above was built using the @command{guix
    system} command, specifically:
    
    @example
    guix system disk-image --image-size=850MiB gnu/system/install.scm
    @end example
    
    @xref{Invoking guix system}, for more information.  See
    @file{gnu/system/install.scm} in the source tree 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
    one 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 system's own tools.
    @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.
    * 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.
    
    @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
    Emacs to those, taken from the @code{(gnu packages emacs)} module
    (@pxref{Package Modules}).
    
    @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, and allowing remote @code{root} logins
    (@pxref{Invoking lshd,,, lsh, GNU lsh Manual}).  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}).  @xref{operating-system
    Reference}, for details about the available @code{operating-system}
    fields.
    
    The configuration for a typical ``desktop'' usage, with the X11 display
    server, a desktop environment, network management, an SSH server, and
    more, would look like this:
    
    @lisp
    @include os-config-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.
    
    Assuming the above snippet 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's configuration is by updating this file and re-running the
    @command{guix system} command.
    
    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
    
    @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 kernel's command-line---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}.)
    
    @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 monadic list of pairs of target file name and files.  These are the
    files that will be used as skeletons as new accounts are created.
    
    For instance, a valid value may look like this:
    
    @example
    (mlet %store-monad ((bashrc (text-file "bashrc" "\
         export PATH=$HOME/.guix-profile/bin")))
      (return `((".bashrc" ,bashrc))))
    @end example
    
    @item @code{issue} (default: @var{%default-issue})
    A string denoting the contents of the @file{/etc/issue} file, which is
    what 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, but 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"}.
    
    @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{name-service-switch} (default: @var{%default-nss})
    Configuration of libc's 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 monadic values 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's 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, 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 a file system's source 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{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{%devtmpfs-file-system} and @var{%immutable-store} (see
    below.)  Operating system declarations should always contain at least
    these.
    @end defvr
    
    @defvr {Scheme Variable} %devtmpfs-file-system
    The @code{devtmpfs} file system to be mounted on @file{/dev}.  This is a
    requirement for udev (@pxref{Base Services, @code{udev-service}}).
    @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
    
    @noindent
    @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.  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 same-named package.  This 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's owner 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 group's name.
    
    @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 group's password.
    
    @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{charset}}---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}}).
    
    That locale must be among the @dfn{locale definitions} that are known to
    the system---and these are specified in the @code{locale-definitions}
    slot of @code{operating-system}.  The default value includes locale
    definition for some widely used locales, but not for all the available
    locales, in order to save space.
    
    If the locale specified in the @code{locale} field is not among the
    definitions listed in @code{locale-definitions}, @command{guix system}
    raises an error.  In that case, you should add the locale definition to
    the @code{locale-definitions} field.  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
    
    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
    An arbitrary list of commonly used locales, used as the default value of
    the @code{locale-definitions} field of @code{operating-system}
    declarations.
    @end defvr
    
    @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 GNU@tie{}dmd (@pxref{Introduction,,, dmd, GNU
    dmd Manual}).  On a running system, the @command{deco} command allows
    you to list the available services, show their status, start and stop
    them, or do other specific operations (@pxref{Jump Start,,, dmd, GNU dmd
    Manual}).  For example:
    
    @example
    # deco status dmd
    @end example
    
    The above command, run as @code{root}, lists the currently defined
    services.  The @command{deco doc} command shows a synopsis of the given
    service:
    
    @example
    # deco 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
    # deco stop nscd
    Service nscd has been stopped.
    # deco 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.
    * Networking Services::         Network setup, SSH daemon, etc.
    * X Window::                    Graphical display.
    * Desktop Services::            D-Bus and desktop services.
    * Database Services::           SQL databases.
    * 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@footnote{Technically,
    this is a list of monadic services.  @xref{The Store Monad}.} one would
    expect from the system: a login service (mingetty) on each tty, syslogd,
    libc's 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 {Monadic Procedure} host-name-service @var{name}
    Return a service that sets the host name to @var{name}.
    @end deffn
    
    @deffn {Monadic Procedure} mingetty-service @var{tty} [#:motd] @
           [#:auto-login #f] [#:login-program] [#:login-pause? #f] @
           [#:allow-empty-passwords? #f]
    Return a service to run mingetty on @var{tty}.
    
    When @var{allow-empty-passwords?} is true, allow empty log-in password.  When
    @var{auto-login} is true, it must be a user name under which to log-in
    automatically.  @var{login-pause?} can be set to @code{#t} in conjunction with
    @var{auto-login}, in which case the user will have to press a key before the
    login shell is launched.
    
    When true, @var{login-program} is a gexp or a monadic gexp denoting the name
    of the log-in program (the default is the @code{login} program from the Shadow
    tool suite.)
    
    @var{motd} is a monadic value containing a text file to use as
    the ``message of the day''.
    @end deffn
    
    @cindex name service cache daemon
    @cindex nscd
    @deffn {Monadic Procedure} nscd-service [@var{config}] [#:glibc glibc] @
                    [#:name-services '()]
    Return a service that runs libc's name service cache daemon (nscd) with
    the given @var{config}---an @code{<nscd-configuration>} object.
    Optionally, @code{#:name-services} is a list of packages that provide
    name service switch (NSS) modules needed by nscd.  @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}.  This uses the caches defined by
    @var{%nscd-default-caches}; see below.
    @end defvr
    
    @deftp {Data Type} nscd-configuration
    This is the type representing the name service cache daemon (nscd)
    configuration.
    
    @table @asis
    
    @item @code{log-file} (default: @code{"/var/log/nscd.log"})
    Name of nscd's 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 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 {Monadic Procedure} syslog-service [#:config-file #f]
    Return a service that runs @code{syslogd}.  If configuration file name
    @var{config-file} is not specified, use some reasonable default
    settings.
    @end deffn
    
    @deffn {Monadic Procedure} guix-service [#:guix guix] @
           [#:builder-group "guixbuild"] [#:build-accounts 10] @
           [#:authorize-hydra-key? #t] [#:use-substitutes? #t] @
           [#:extra-options '()]
    Return a service that runs the build daemon from @var{guix}, and has
    @var{build-accounts} user accounts available under @var{builder-group}.
    
    When @var{authorize-hydra-key?} is true, the @code{hydra.gnu.org} public key
    provided by @var{guix} is authorized upon activation, meaning that substitutes
    from @code{hydra.gnu.org} are used by default.
    
    If @var{use-substitutes?} is false, the daemon is run with
    @option{--no-substitutes} (@pxref{Invoking guix-daemon,
    @option{--no-substitutes}}).
    
    Finally, @var{extra-options} is a list of additional command-line options
    passed to @command{guix-daemon}.
    @end deffn
    
    @deffn {Monadic Procedure} udev-service [#:udev udev]
    Run @var{udev}, which populates the @file{/dev} directory dynamically.
    @end deffn
    
    @deffn {Monadic Procedure} console-keymap-service @var{file}
    Return a service to load console keymap from @var{file} using
    @command{loadkeys} command.
    @end deffn
    
    
    @node Networking Services
    @subsubsection Networking Services
    
    The @code{(gnu services networking)} module provides services to configure
    the network interface.
    
    @cindex DHCP, networking service
    @deffn {Monadic 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 {Monadic Procedure} static-networking-service @var{interface} @var{ip} @
           [#:gateway #f] [#:name-services @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
    @deffn {Monadic Procedure} wicd-service [#:wicd @var{wicd}]
    Return a service that runs @url{https://launchpad.net/wicd,Wicd}, a
    network manager that aims to simplify wired and wireless networking.
    @end deffn
    
    @deffn {Monadic 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 {Monadic Procedure} tor-service [#:tor tor]
    Return a service to run the @uref{https://torproject.org,Tor} daemon.
    
    The daemon runs with the default settings (in particular the default exit
    policy) as the @code{tor} unprivileged user.
    @end deffn
    
    @deffn {Monadic 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 {Monadic 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 {Monadic Procedure} avahi-service [#:avahi @var{avahi}] @
              [#:host-name #f] [#:publish? #t] [#:ipv4? #t] @
              [#:ipv6? #t] [#:wide-area? #f] @
              [#:domains-to-browse '()]
    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/}).
    
    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 {Monadic 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}, the 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 {Monadic 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 {Monadic 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
    
    @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.
    
    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 adjust services for a typical ``desktop'' setup.
    
    In particular, it adds a graphical login manager (@pxref{X Window,
    @code{slim-service}}), a network management tool (@pxref{Networking
    Services, @code{wicd-service}}), energy and color management services,
    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}}).
    
    The actual service definitions provided by @code{(gnu services desktop)}
    are described below.
    
    @deffn {Monadic Procedure} dbus-service @var{services} @
                             [#:dbus @var{dbus}]
    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 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 {Monadic 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 {Monadic 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
    
    @node Database Services
    @subsubsection Database Services
    
    The @code{(gnu services databases)} module provides the following service.
    
    @deffn {Monadic 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
    
    @node Various Services
    @subsubsection Various Services
    
    The @code{(gnu services lirc)} module provides the following service.
    
    @deffn {Monadic 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
    
    
    @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 mechanisms.)
    
    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 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.
    
    
    @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 libc's @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
    
    Don't worry: the @code{%mdns-host-lookup-nss} variable (see below)
    contains this configuration, so you won't 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,
    @code{nscd-service} must be told where to find the @code{nss-mdns}
    shared library (@pxref{Base Services, @code{nscd-service}}).  Since the
    @code{nscd} service is part of @var{%base-services}, you may want to
    customize it by adding this snippet in the operating system
    configuration file:
    
    @example
    (use-modules (guix) (gnu))
    
    (define %my-base-services
      ;; Replace the default nscd service with one that knows
      ;; about nss-mdns.
      (map (lambda (mservice)
             ;; "Bind" the MSERVICE monadic value to inspect it.
             (mlet %store-monad ((service mservice))
               (if (member 'nscd (service-provision service))
                   (nscd-service (nscd-configuration)
                                 #:name-services (list nss-mdns))
                   mservice)))
           %base-services))
    @end example
    
    @noindent
    @dots{} and then refer to @var{%my-base-services} instead of
    @var{%base-services} in the @code{operating-system} declaration.
    Lastly, this relies on the availability of the Avahi service
    (@pxref{Networking Services, @code{avahi-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 C library's configuration file format, so
    please refer to the C library manual for more information (@pxref{NSS
    Configuration File,,, libc, The GNU C Library Reference Manual}).
    Compared to libc's NSS configuration file format, it has the advantage
    not only of adding this warm parenthetic feel that we like, but also
    static checks: you'll 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 whose
    root file system is volatile.
    
    @deffn {Monadic Procedure} base-initrd @var{file-systems} @
           [#:qemu-networking? #f] [#:virtio? #f] [#: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 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"] @
           [#:modules '()]
    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.
    
    @var{modules} is a list of Guile module names to be embedded in 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 @code{grub-configuration} declarations.  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 current
    system's entry.
    
    @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 instantiate.  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 GNU.}.
    
    This effects all the configuration specified in @var{file}: user
    accounts, system services, global package list, setuid programs, etc.
    
    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.
    
    @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.
    
    @item build
    Build the operating system's derivation, 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
    Build a virtual machine that contain the operating system declared in
    @var{file}, and return a script to run that virtual machine (VM).
    Arguments given to the script are passed as is 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 the host's @file{$HOME/tmp}:
    
    @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
    host's store 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
    image's size.
    
    @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.
    
    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 on it
    using the following command:
    
    @example
    # dd if=$(guix system disk-image my-os.scm) of=/dev/sdc
    @end example
    
    @end table
    
    @var{options} can contain any of the common build options provided by
    @command{guix build} (@pxref{Invoking guix build}).  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's system type.
    This works as per @command{guix build} (@pxref{Invoking guix build}).
    
    @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 program's
    state.  @xref{Debug Commands,,, guile, GNU Guile Reference Manual}, for
    a list of available debugging commands.
    @end table
    @end table
    
    Note that all the actions above, except @code{build} and @code{init},
    rely on KVM support in the Linux-Libre kernel.  Specifically, the
    machine should have 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 daemon's
    build users.
    
    @node Defining Services
    @subsection Defining Services
    
    The @code{(gnu services @dots{})} modules define several procedures that allow
    users to declare the operating system's services (@pxref{Using the
    Configuration System}).  These procedures are @emph{monadic
    procedures}---i.e., procedures that return a monadic value in the store
    monad (@pxref{The Store Monad}).  For examples of such procedures,
    @xref{Services}.
    
    @cindex service definition
    The monadic value returned by those procedures is a @dfn{service
    definition}---a structure as returned by the @code{service} form.
    Service definitions specifies the inputs the service depends on, and an
    expression to start and stop the service.  Behind the scenes, service
    definitions are ``translated'' into the form suitable for the
    configuration file of dmd, the init system (@pxref{Services,,, dmd, GNU
    dmd Manual}).
    
    As an example, here is what the @code{nscd-service} procedure looks
    like:
    
    @lisp
    (define (nscd-service)
      (with-monad %store-monad
        (return (service
                 (documentation "Run libc's name service cache daemon.")
                 (provision '(nscd))
                 (activate #~(begin
                               (use-modules (guix build utils))
                               (mkdir-p "/var/run/nscd")))
                 (start #~(make-forkexec-constructor
                           (string-append #$glibc "/sbin/nscd")
                           "-f" "/dev/null" "--foreground"))
                 (stop #~(make-kill-destructor))
                 (respawn? #f)))))
    @end lisp
    
    @noindent
    The @code{activate}, @code{start}, and @code{stop} fields are G-expressions
    (@pxref{G-Expressions}).  The @code{activate} field contains a script to
    run at ``activation'' time; it makes sure that the @file{/var/run/nscd}
    directory exists before @command{nscd} is started.
    
    The @code{start} and @code{stop} fields refer to dmd's facilities to
    start and stop processes (@pxref{Service De- and Constructors,,, dmd,
    GNU dmd Manual}).  The @code{provision} field specifies the name under
    which this service is known to dmd, and @code{documentation} specifies
    on-line documentation.  Thus, the commands @command{deco start ncsd},
    @command{deco stop nscd}, and @command{deco doc nscd} will do what you
    would expect (@pxref{Invoking deco,,, dmd, GNU dmd Manual}).
    
    
    @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 those packages
    whose definition explicitly declares 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
    
    @quotation Note
    As of version @value{VERSION}, the feature described in this section is
    experimental.
    @end quotation
    
    @cindex security updates
    Occasionally, important security vulnerabilities are discovered in core
    software packages and must be patched.  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 that, 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 that
    is installed will automatically be ``rewritten'' to refer to
    @var{bash-fixed} instead of @var{bash}.  This grafting process takes
    time proportional to the size of the package, but expect less than a
    minute for an ``average'' package on a recent machine.
    
    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.
    
    
    @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.  Thus, users can invoke commands such as
    @command{guix package} and @command{guix build} have to be used 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 package
    modules.  Directories listed in this variable take precedence over the
    distribution's own modules.
    @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 meta-data} 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.
    * Python Modules::              Taming the snake.
    * Perl Modules::                Little pearls.
    * 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 packages 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 package definition's
    @code{origin} form (@pxref{Defining Packages}).  That 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
    
    
    @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}.
    
    
    @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 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.  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
    
    @c TODO: Add a package-level dependency graph generated from (gnu
    @c packages base).
    
    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.
    
    @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 Make, 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 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-system.am} 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 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
    
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