guix.texi

\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 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.

@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
Run the daemon:

@example
# ~root/.guix-profile/bin/guix-daemon --build-users-group=guixbuild
@end example

@item
Make the @command{guix} command available to other users on the machine,
for instance with:

@example
# mkdir -p /usr/local/bin
# cd /usr/local/bin
# ln -s /var/guix/profiles/per-user/root/guix-profile/bin/guix
@end example

@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!

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:

@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")
                        "/.ssh/id-rsa-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.

@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.

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 need to generate a
key pair 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
Thus, when receiving files, a machine's 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}
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 increasde
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 uses 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}.

@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}).