services: dmd: Strip the default list of modules.

[jackhill/guix/guix.git] / doc / 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 The GNU Guix Developers

@page
@vskip 0pt plus 1filll
Edition @value{EDITION} @*
@value{UPDATED} @*

@insertcopying
@end titlepage

@contents

@c *********************************************************************
@node Top
@top GNU Guix

This document describes GNU Guix version @value{VERSION}, a functional
package management tool written for the GNU system.

@menu
* Introduction::                What is Guix about?
* Installation::                Installing Guix.
* Package Management::          Package installation, upgrade, etc.
* Emacs Interface::             Using Guix from Emacs.
* Programming Interface::       Using Guix in Scheme.
* Utilities::                   Package management commands.
* GNU Distribution::            Software for your friendly GNU system.
* Contributing::                Your help needed!

* Acknowledgments::             Thanks!
* GNU Free Documentation License::  The license of this manual.
* Concept Index::               Concepts.
* Programming Index::           Data types, functions, and variables.

@detailmenu
 --- The Detailed Node Listing ---

Installation

* Binary Installation::         Getting Guix running in no time!
* Requirements::                Software needed to build and run Guix.
* Running the Test Suite::      Testing Guix.
* Setting Up the Daemon::       Preparing the build daemon's environment.
* Invoking guix-daemon::        Running the build daemon.
* Application Setup::           Application-specific setup.

Setting Up the Daemon

* Build Environment Setup::     Preparing the isolated build environment.
* Daemon Offload Setup::        Offloading builds to remote machines.

Package Management

* Features::                    How Guix will make your life brighter.
* Invoking guix package::       Package installation, removal, etc.
* Substitutes::                 Downloading pre-built binaries.
* Packages with Multiple Outputs::  Single source package, multiple outputs.
* Invoking guix gc::            Running the garbage collector.
* Invoking guix pull::          Fetching the latest Guix and distribution.
* Invoking guix archive::       Exporting and importing store files.

Emacs Interface

* Initial Setup: Emacs Initial Setup.	Preparing @file{~/.emacs}.
* Package Management: Emacs Package Management.	Managing packages and generations.
* Popup Interface: Emacs Popup Interface.	Magit-like interface for guix commands.
* Prettify Mode: Emacs Prettify.	Abbreviating @file{/gnu/store/@dots{}} file names.
* Build Log Mode: Emacs Build Log.	Highlighting Guix build logs.
* Completions: Emacs Completions.	Completing @command{guix} shell command.
* Development: Emacs Development.	Tools for Guix developers.

Programming Interface

* Defining Packages::           Defining new packages.
* Build Systems::               Specifying how packages are built.
* The Store::                   Manipulating the package store.
* Derivations::                 Low-level interface to package derivations.
* The Store Monad::             Purely functional interface to the store.
* G-Expressions::               Manipulating build expressions.

Defining Packages

* package Reference::           The package data type.
* origin Reference::            The origin data type.

Utilities

* Invoking guix build::         Building packages from the command line.
* Invoking guix edit::          Editing package definitions.
* Invoking guix download::      Downloading a file and printing its hash.
* Invoking guix hash::          Computing the cryptographic hash of a file.
* Invoking guix import::        Importing package definitions.
* Invoking guix refresh::       Updating package definitions.
* Invoking guix lint::          Finding errors in package definitions.
* Invoking guix size::          Profiling disk usage.
* Invoking guix graph::         Visualizing the graph of packages.
* Invoking guix environment::   Setting up development environments.
* Invoking guix publish::       Sharing substitutes.
* Invoking guix challenge::     Challenging substitute servers.
* Invoking guix container::     Process isolation.

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.
* Web Services::                Web servers.
* Various Services::            Other services.

Defining Services

* Service Composition::         The model for composing services.
* Service Types and Services::  Types and services.
* Service Reference::           API reference.
* dmd Services::                A particular type of service.

Packaging Guidelines

* Software Freedom::            What may go into the distribution.
* Package Naming::              What's in a name?
* Version Numbers::             When the name is not enough.
* Synopses and Descriptions::   Helping users find the right package.
* Python Modules::              Taming the snake.
* Perl Modules::                Little pearls.
* 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 pioneered by Nix (@pxref{Acknowledgments}).
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.

When this option is used, @command{guix gc --list-failures} can be used
to query the set of store items marked as failed; @command{guix gc
--clear-failures} removes store items from the set of cached failures.
@xref{Invoking guix gc}.

@item --cores=@var{n}
@itemx -c @var{n}
Use @var{n} CPU cores to build each derivation; @code{0} means as many
as available.

The default value is @code{0}, but it may be overridden by clients, such
as the @code{--cores} option of @command{guix build} (@pxref{Invoking
guix build}).

The effect is to define the @code{NIX_BUILD_CORES} environment variable
in the build process, which can then use it to exploit internal
parallelism---for instance, by running @code{make -j$NIX_BUILD_CORES}.

@item --max-jobs=@var{n}
@itemx -M @var{n}
Allow at most @var{n} build jobs in parallel.  The default value is
@code{1}.  Setting it to @code{0} means that no builds will be performed
locally; instead, the daemon will offload builds (@pxref{Daemon Offload
Setup}), or simply fail.

@item --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
so-called @dfn{foreign distro}---a few additional steps are needed to
get everything in place.  Here are some of them.

@subsection Locales

@anchor{locales-and-locpath}
@cindex locales, when not on GuixSD
@vindex LOCPATH
@vindex GUIX_LOCPATH
Packages installed @i{via} Guix will not use the host system's locale
data.  Instead, you must first install one of the locale packages
available with Guix and then define the @code{GUIX_LOCPATH} environment
variable:

@example
$ guix package -i glibc-locales
$ export GUIX_LOCPATH=$HOME/.guix-profile/lib/locale
@end example

Note that the @code{glibc-locales} package contains data for all the
locales supported by the GNU@tie{}libc and weighs in at around
110@tie{}MiB.  Alternately, the @code{glibc-utf8-locales} is smaller but
limited to a few UTF-8 locales.

The @code{GUIX_LOCPATH} variable plays a role similar to @code{LOCPATH}
(@pxref{Locale Names, @code{LOCPATH},, libc, The GNU C Library Reference
Manual}).  There are two important differences though:

@enumerate
@item
@code{GUIX_LOCPATH} is honored only by Guix's libc, and not by the libc
provided by foreign distros.  Thus, using @code{GUIX_LOCPATH} allows you
to make sure the the foreign distro's programs will not end up loading
incompatible locale data.

@item
libc suffixes each entry of @code{GUIX_LOCPATH} with @code{/X.Y}, where
@code{X.Y} is the libc version---e.g., @code{2.22}.  This means that,
should your Guix profile contain a mixture of programs linked against
different libc version, each libc version will only try to load locale
data in the right format.
@end enumerate

This is important because the locale data format used by different libc
versions may be incompatible.

@subsection X11 Fonts

The majority of graphical applications use Fontconfig to locate and
load fonts and perform X11-client-side rendering.  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}.

To display text written in Chinese languages, Japanese, or Korean in
graphical applications, consider installing
@code{font-adobe-source-han-sans} or @code{font-wqy-zenhei}.  The former
has multiple outputs, one per language family (@pxref{Packages with
Multiple Outputs}).  For instance, the following command installs fonts
for Chinese languages:

@example
guix package -i font-adobe-source-han-sans:cn
@end example

@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 described below
(@pxref{Invoking guix package, @code{guix package}}), as well as a visual user
interface in Emacs described in a subsequent chapter (@pxref{Emacs Interface}).

@menu
* Features::                    How Guix will make your life brighter.
* Invoking guix package::       Package installation, removal, etc.
* Substitutes::                 Downloading pre-built binaries.
* Packages with Multiple Outputs::  Single source package, multiple outputs.
* Invoking guix gc::            Running the garbage collector.
* Invoking guix pull::          Fetching the latest Guix and distribution.
* Invoking guix archive::       Exporting and importing store files.
@end menu

@node Features
@section Features

When using Guix, each package ends up in the @dfn{package store}, in its
own directory---something that resembles
@file{/gnu/store/xxx-package-1.2}, where @code{xxx} is a base32 string
(note that Guix comes with an Emacs extension to shorten those file
names, @pxref{Emacs Prettify}.)

Instead of referring to these directories, users have their own
@dfn{profile}, which points to the packages that they actually want to
use.  These profiles are stored within each user's home directory, at
@code{$HOME/.guix-profile}.

For example, @code{alice} installs GCC 4.7.2.  As a result,
@file{/home/alice/.guix-profile/bin/gcc} points to
@file{/gnu/store/@dots{}-gcc-4.7.2/bin/gcc}.  Now, on the same machine,
@code{bob} had already installed GCC 4.8.0.  The profile of @code{bob}
simply continues to point to
@file{/gnu/store/@dots{}-gcc-4.8.0/bin/gcc}---i.e., both versions of GCC
coexist on the same system without any interference.

The @command{guix package} command is the central tool to manage
packages (@pxref{Invoking guix package}).  It operates on 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 --install-from-file=@var{file}
@itemx -f @var{file}
Install the package that the code within @var{file} evaluates to.

As an example, @var{file} might contain a definition like this
(@pxref{Defining Packages}):

@example
@verbatiminclude package-hello.scm
@end example

Developers may find it useful to include such a @file{package.scm} file
in the root of their project's source tree that can be used to test
development snapshots and create reproducible development environments
(@pxref{Invoking guix environment}).

@item --remove=@var{package} @dots{}
@itemx -r @var{package} @dots{}
Remove the specified @var{package}s.

As for @code{--install}, each @var{package} may specify a version number
and/or output name in addition to the package name.  For instance,
@code{-r glibc:debug} would remove the @code{debug} output of
@code{glibc}.

@item --upgrade[=@var{regexp} @dots{}]
@itemx -u [@var{regexp} @dots{}]
Upgrade all the installed packages.  If one or more @var{regexp}s are
specified, upgrade only installed packages whose name matches a
@var{regexp}.  Also see the @code{--do-not-upgrade} option below.

Note that this upgrades package to the latest version of packages found
in the distribution currently installed.  To update your distribution,
you should regularly run @command{guix pull} (@pxref{Invoking guix
pull}).

@item --do-not-upgrade[=@var{regexp} @dots{}]
When used together with the @code{--upgrade} option, do @emph{not}
upgrade any packages whose name matches a @var{regexp}.  For example, to
upgrade all packages in the current profile except those containing the
substring ``emacs'':

@example
$ guix package --upgrade . --do-not-upgrade emacs
@end example

@item @anchor{profile-manifest}--manifest=@var{file}
@itemx -m @var{file}
@cindex profile declaration
@cindex profile manifest
Create a new generation of the profile from the manifest object
returned by the Scheme code in @var{file}.

This allows you to @emph{declare} the profile's contents rather than
constructing it through a sequence of @code{--install} and similar
commands.  The advantage is that @var{file} can be put under version
control, copied to different machines to reproduce the same profile, and
so on.

@c FIXME: Add reference to (guix profile) documentation when available.
@var{file} must return a @dfn{manifest} object, which is roughly a list
of packages:

@findex packages->manifest
@example
(use-package-modules guile emacs)

(packages->manifest
 (list emacs
       guile-2.0
       ;; Use a specific package output.
       (list guile-2.0 "debug")))
@end example

@item --roll-back
Roll back to the previous @dfn{generation} of the profile---i.e., undo
the last transaction.

When combined with options such as @code{--install}, roll back occurs
before any other actions.

When rolling back from the first generation that actually contains
installed packages, the profile is made to point to the @dfn{zeroth
generation}, which contains no files apart from its own 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}.

This option can also be used to compute the @emph{combined} search paths
of several profiles.  Consider this example:

@example
$ guix package -p foo -i guile
$ guix package -p bar -i guile-json
$ guix package -p foo -p bar --search-paths
@end example

The last command above reports about the @code{GUILE_LOAD_PATH}
variable, even though, taken individually, neither @file{foo} nor
@file{bar} would lead to that recommendation.


@item --profile=@var{profile}
@itemx -p @var{profile}
Use @var{profile} instead of the user's default profile.

@item --verbose
Produce verbose output.  In particular, emit the 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}).

@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.  The @command{guix challenge} command aims to
help users assess substitute servers, and to assist developers in
finding out about non-deterministic package builds (@pxref{Invoking guix
challenge}).

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}).
@command{guix graph} can also be helpful (@pxref{Invoking guix graph}).

There are several such multiple-output packages in the GNU distribution.
Other conventional output names include @code{lib} for libraries and
possibly header files, @code{bin} for stand-alone programs, and
@code{debug} for debugging information (@pxref{Installing Debugging
Files}).  The outputs of a packages are listed in the third column of
the output of @command{guix package --list-available} (@pxref{Invoking
guix package}).


@node Invoking guix gc
@section Invoking @command{guix gc}

@cindex garbage collector
Packages that are installed but not used may be @dfn{garbage-collected}.
The @command{guix gc} command allows users to explicitly run the garbage
collector to reclaim space from the @file{/gnu/store} directory.  It is
the @emph{only} way to remove files from @file{/gnu/store}---removing
files or directories manually may break it beyond repair!

The garbage collector has a set of known @dfn{roots}: any file under
@file{/gnu/store} reachable from a root is considered @dfn{live} and
cannot be deleted; any other file is considered @dfn{dead} and may be
deleted.  The set of garbage collector roots includes default user
profiles, and may be augmented with @command{guix build --root}, for
example (@pxref{Invoking guix build}).

Prior to running @code{guix gc --collect-garbage} to make space, it is
often useful to remove old generations from user profiles; that way, old
package builds referenced by those generations can be reclaimed.  This
is achieved by running @code{guix package --delete-generations}
(@pxref{Invoking guix package}).

The @command{guix gc} command has three modes of operation: it can be
used to garbage-collect any dead files (the default), to delete specific
files (the @code{--delete} option), to print garbage-collector
information, or for more advanced queries.  The garbage collection
options are as follows:

@table @code
@item --collect-garbage[=@var{min}]
@itemx -C [@var{min}]
Collect garbage---i.e., unreachable @file{/gnu/store} files and
sub-directories.  This is the default operation when no option is
specified.

When @var{min} is given, stop once @var{min} bytes have been collected.
@var{min} may be a number of bytes, or it may include a unit as a
suffix, such as @code{MiB} for mebibytes and @code{GB} for gigabytes
(@pxref{Block size, size specifications,, coreutils, GNU Coreutils}).

When @var{min} is omitted, collect all the garbage.

@item --delete
@itemx -d
Attempt to delete all the store files and directories specified as
arguments.  This fails if some of the files are not in the store, or if
they are still live.

@item --list-failures
List store items corresponding to cached build failures.

This prints nothing unless the daemon was started with
@option{--cache-failures} (@pxref{Invoking guix-daemon,
@option{--cache-failures}}).

@item --clear-failures
Remove the specified store items from the failed-build cache.

Again, this option only makes sense when the daemon is started with
@option{--cache-failures}.  Otherwise, it does nothing.

@item --list-dead
Show the list of dead files and directories still present in the
store---i.e., files and directories no longer reachable from any root.

@item --list-live
Show the list of live store files and directories.

@end table

In addition, the references among existing store files can be queried:

@table @code

@item --references
@itemx --referrers
List the references (respectively, the referrers) of store files given
as arguments.

@item --requisites
@itemx -R
@cindex closure
List the requisites of the store files passed as arguments.  Requisites
include the store files themselves, their references, and the references
of these, recursively.  In other words, the returned list is the
@dfn{transitive closure} of the store files.

@xref{Invoking guix size}, for a tool to profile the size of an
element's closure.  @xref{Invoking guix graph}, for a tool to visualize
the graph of references.

@end table

Lastly, the following options allow you to check the integrity of the
store and to control disk usage.

@table @option

@item --verify[=@var{options}]
@cindex integrity, of the store
@cindex integrity checking
Verify the integrity of the store.

By default, make sure that all the store items marked as valid in the
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@footnote{Under the hood, @command{guix pull} updates
the @file{~/.config/guix/latest} symbolic link to point to the latest
Guix, and the @command{guix} command loads code from there.}.

The @command{guix pull} command is usually invoked with no arguments,
but it supports the following options:

@table @code
@item --verbose
Produce verbose output, writing build logs to the standard error output.

@item --url=@var{url}
Download the source tarball of Guix from @var{url}.

By default, the tarball is taken from its canonical address at
@code{gnu.org}, for the stable branch of Guix.

@item --bootstrap
Use the bootstrap Guile to build the latest Guix.  This option is only
useful to Guix developers.
@end table


@node Invoking guix archive
@section Invoking @command{guix archive}

The @command{guix archive} command allows users to @dfn{export} files
from the store into a single archive, and to later @dfn{import} them.
In particular, it allows store files to be transferred from one machine
to 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)}.

@item --extract=@var{directory}
@itemx -x @var{directory}
Read a single-item archive as served by substitute servers
(@pxref{Substitutes}) and extract it to @var{directory}.  This is a
low-level operation needed in only very narrow use cases; see below.

For example, the following command extracts the substitute for Emacs
served by @code{hydra.gnu.org} to @file{/tmp/emacs}:

@example
$ wget -O - \
  http://hydra.gnu.org/nar/@dots{}-emacs-24.5 \
  | bunzip2 | guix archive -x /tmp/emacs
@end example

Single-item archives are different from multiple-item archives produced
by @command{guix archive --export}; they contain a single store item,
and they do @emph{not} embed a signature.  Thus this operation does
@emph{no} signature verification and its output should be considered
unsafe.

The primary purpose of this operation is to facilitate inspection of
archive contents coming from possibly untrusted substitute servers.

@end table

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 *********************************************************************
@include emacs.texi

@c *********************************************************************
@node Programming Interface
@chapter Programming Interface

GNU Guix provides several Scheme programming interfaces (APIs) to
define, build, and query packages.  The first interface allows users to
write high-level package definitions.  These definitions refer to
familiar packaging concepts, such as the name and version of a package,
its build system, and its dependencies.  These definitions can then be
turned into concrete build actions.

Build actions are performed by the Guix daemon, on behalf of users.  In a
standard setup, the daemon has write access to the store---the
@file{/gnu/store} directory---whereas users do not.  The recommended
setup also has the daemon perform builds in chroots, under a specific
build users, to minimize interference with the rest of the system.

@cindex derivation
Lower-level APIs are available to interact with the daemon and the
store.  To instruct the daemon to perform a build action, users actually
provide it with a @dfn{derivation}.  A derivation is a low-level
representation of the build actions to be taken, and the environment in
which they should occur---derivations are to package definitions what
assembly is to C programs.  The term ``derivation'' comes from the fact
that build results @emph{derive} from them.

This chapter describes all these APIs in turn, starting from high-level
package definitions.

@menu
* Defining Packages::           Defining new packages.
* Build Systems::               Specifying how packages are built.
* The Store::                   Manipulating the package store.
* Derivations::                 Low-level interface to package derivations.
* The Store Monad::             Purely functional interface to the store.
* G-Expressions::               Manipulating build expressions.
@end menu

@node Defining Packages
@section Defining Packages

The high-level interface to package definitions is implemented in the
@code{(guix packages)} and @code{(guix build-system)} modules.  As an
example, the package definition, or @dfn{recipe}, for the GNU Hello
package looks like this:

@example
(define-module (gnu packages hello)
  #:use-module (guix packages)
  #:use-module (guix download)
  #:use-module (guix build-system gnu)
  #:use-module (guix licenses)
  #:use-module (gnu packages gawk))

(define-public hello
  (package
    (name "hello")
    (version "2.10")
    (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnu/hello/hello-" version
                                  ".tar.gz"))
              (sha256
               (base32
                "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
    (build-system gnu-build-system)
    (arguments `(#:configure-flags '("--enable-silent-rules")))
    (inputs `(("gawk" ,gawk)))
    (synopsis "Hello, GNU world: An example GNU package")
    (description "Guess what GNU Hello prints!")
    (home-page "http://www.gnu.org/software/hello/")
    (license gpl3+)))
@end example

@noindent
Without being a Scheme expert, the reader may have guessed the meaning
of the various fields here.  This expression binds 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} r-build-system
This variable is exported by @code{(guix build-system r)}.  It
implements the build procedure used by @uref{http://r-project.org, R}
packages, which essentially is little more than running @code{R CMD
INSTALL --library=/gnu/store/@dots{}} in an environment where
@code{R_LIBS_SITE} contains the paths to all R package inputs.  Tests
are run after installation using the R function
@code{tools::testInstalledPackage}.
@end defvr

@defvr {Scheme Variable} ruby-build-system
This variable is exported by @code{(guix build-system ruby)}.  It
implements the RubyGems build procedure used by Ruby packages, which
involves running @code{gem build} followed by @code{gem install}.

The @code{source} field of a package that uses this build system
typically references a gem archive, since this is the format that Ruby
developers use when releasing their software.  The build system unpacks
the gem archive, potentially patches the source, runs the test suite,
repackages the gem, and installs it.  Additionally, directories and
tarballs may be referenced to allow building unreleased gems from Git or
a traditional source release tarball.

Which Ruby package is used can be specified with the @code{#:ruby}
parameter.  A list of additional flags to be passed to the @command{gem}
command can be specified with the @code{#:gem-flags} parameter.
@end defvr

@defvr {Scheme Variable} waf-build-system
This variable is exported by @code{(guix build-system waf)}.  It
implements a build procedure around the @code{waf} script.  The common
phases---@code{configure}, @code{build}, and @code{install}---are
implemented by passing their names as arguments to the @code{waf}
script.

The @code{waf} script is executed by the Python interpreter.  Which
Python package is used to run the script can be specified with the
@code{#:python} parameter.
@end defvr

@defvr {Scheme Variable} haskell-build-system
This variable is exported by @code{(guix build-system haskell)}.  It
implements the Cabal build procedure used by Haskell packages, which
involves running @code{runhaskell Setup.hs configure
--prefix=/gnu/store/@dots{}} and @code{runhaskell Setup.hs build}.
Instead of installing the package by running @code{runhaskell Setup.hs
install}, to avoid trying to register libraries in the read-only
compiler store directory, the build system uses @code{runhaskell
Setup.hs copy}, followed by @code{runhaskell Setup.hs register}.  In
addition, the build system generates the package documentation by
running @code{runhaskell Setup.hs haddock}, unless @code{#:haddock? #f}
is passed.  Optional Haddock parameters can be passed with the help of
the @code{#:haddock-flags} parameter.  If the file @code{Setup.hs} is
not found, the build system looks for @code{Setup.lhs} instead.

Which Haskell compiler is used can be specified with the @code{#:haskell}
parameter which defaults to @code{ghc}.
@end defvr

@defvr {Scheme Variable} emacs-build-system
This variable is exported by @code{(guix build-system emacs)}.  It
implements an installation procedure similar to the 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

@cindex lowering, of high-level objects in gexps
This mechanism is not limited to package and derivation
objects: @dfn{compilers} able to ``lower'' other high-level objects to
derivations or files in the store can be defined,
such that these objects can also be inserted
into gexps.  For example, a useful type of high-level 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] @
       [#:script-name (string-append @var{name} "-builder")] @
       [#:local-build? #f] [#:substitutable? #t] [#:guile-for-build #f]
Return a derivation @var{name} that runs @var{exp} (a gexp) with
@var{guile-for-build} (a derivation) on @var{system}; @var{exp} is
stored in a file called @var{script-name}.  When @var{target} is true,
it is used as the cross-compilation target triplet for packages referred
to by @var{exp}.

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}, @code{plain-file}, @code{computed-file},
@code{program-file}, and @code{scheme-file} procedures below return
@dfn{file-like objects}.  That is, when unquoted in a G-expression,
these objects lead to a file in the store.  Consider this G-expression:

@example
#~(system* (string-append #$glibc "/sbin/nscd") "-f"
           #$(local-file "/tmp/my-nscd.conf"))
@end example

The effect here is to ``intern'' @file{/tmp/my-nscd.conf} by copying it
to the store.  Once expanded, for instance @i{via}
@code{gexp->derivation}, the G-expression refers to that copy under
@file{/gnu/store}; thus, modifying or removing the file in @file{/tmp}
does not have any effect on what the G-expression does.
@code{plain-file} can be used similarly; it differs in that the file
content is directly passed as a string.

@deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
   [#:recursive? #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 {Scheme Procedure} computed-file @var{name} @var{gexp} @
          [#:modules '()] [#:options '(#:local-build? #t)]
Return an object representing the store item @var{name}, a file or
directory computed by @var{gexp}.  @var{modules} specifies the set of
modules visible in the execution context of @var{gexp}.  @var{options}
is a list of additional arguments to pass to @code{gexp->derivation}.

This is the declarative counterpart of @code{gexp->derivation}.
@end deffn

@deffn {Monadic Procedure} gexp->script @var{name} @var{exp}
Return an executable script @var{name} that runs @var{exp} using
@var{guile} with @var{modules} in its search path.

The example below builds a script that simply invokes the @command{ls}
command:

@example
(use-modules (guix gexp) (gnu packages base))

(gexp->script "list-files"
              #~(execl (string-append #$coreutils "/bin/ls")
                       "ls"))
@end example

When ``running'' it through the store (@pxref{The Store Monad,
@code{run-with-store}}), we obtain a derivation that produces an
executable file @file{/gnu/store/@dots{}-list-files} along these lines:

@example
#!/gnu/store/@dots{}-guile-2.0.11/bin/guile -ds
!#
(execl (string-append "/gnu/store/@dots{}-coreutils-8.22"/bin/ls")
       "ls")
@end example
@end deffn

@deffn {Scheme Procedure} program-file @var{name} @var{exp} @
          [#:modules '()] [#:guile #f]
Return an object representing the executable store item @var{name} that
runs @var{gexp}.  @var{guile} is the Guile package used to execute that
script, and @var{modules} is the list of modules visible to that script.

This is the declarative counterpart of @code{gexp->script}.
@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 {Scheme Procedure} scheme-file @var{name} @var{exp}
Return an object representing the Scheme file @var{name} that contains
@var{exp}.

This is the declarative counterpart of @code{gexp->file}.
@end deffn

@deffn {Monadic Procedure} text-file* @var{name} @var{text} @dots{}
Return as a monadic value a derivation that builds a text file
containing all of @var{text}.  @var{text} may list, in addition to
strings, objects of any type that can be used in a gexp: packages,
derivations, local file objects, etc.  The resulting store file holds
references to all these.

This variant should be preferred over @code{text-file} anytime the file
to create will reference items from the store.  This is typically the
case when building a configuration file that embeds store file names,
like this:

@example
(define (profile.sh)
  ;; Return the name of a shell script in the store that
  ;; initializes the 'PATH' environment variable.
  (text-file* "profile.sh"
              "export PATH=" coreutils "/bin:"
              grep "/bin:" sed "/bin\n"))
@end example

In this example, the resulting @file{/gnu/store/@dots{}-profile.sh} file
will references @var{coreutils}, @var{grep}, and @var{sed}, thereby
preventing them from being garbage-collected during its lifetime.
@end deffn

@deffn {Scheme Procedure} mixed-text-file @var{name} @var{text} @dots{}
Return an object representing store file @var{name} containing
@var{text}.  @var{text} is a sequence of strings and file-like objects,
as in:

@example
(mixed-text-file "profile"
                 "export PATH=" coreutils "/bin:" grep "/bin")
@end example

This is the declarative counterpart of @code{text-file*}.
@end deffn

Of course, in addition to gexps embedded in ``host'' code, there are
also modules containing build tools.  To make it clear that they are
meant to be used in the build stratum, these modules are kept in the
@code{(guix build @dots{})} name space.

@cindex lowering, of high-level objects in gexps
Internally, high-level objects are @dfn{lowered}, using their compiler,
to either derivations or store items.  For instance, lowering a package
yields a derivation, and lowering a @code{plain-file} yields a store
item.  This is achieved using the @code{lower-object} monadic procedure.

@deffn {Monadic Procedure} lower-object @var{obj} [@var{system}] @
           [#:target #f]
Return as a value in @var{%store-monad} the derivation or store item
corresponding to @var{obj} for @var{system}, cross-compiling for
@var{target} if @var{target} is true.  @var{obj} must be an object that
has an associated gexp compiler, such as a @code{<package>}.
@end deffn


@c *********************************************************************
@node Utilities
@chapter Utilities

This section describes 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 graph::         Visualizing the graph of packages.
* Invoking guix environment::   Setting up development environments.
* Invoking guix publish::       Sharing substitutes.
* Invoking guix challenge::     Challenging substitute servers.
* Invoking guix container::     Process isolation.
@end menu

@node Invoking guix build
@section Invoking @command{guix build}

The @command{guix build} command builds packages or derivations and
their dependencies, and prints the resulting store paths.  Note that it
does not modify the user's profile---this is the job of the
@command{guix package} command (@pxref{Invoking guix package}).  Thus,
it is mainly useful for distribution developers.

The general syntax is:

@example
guix build @var{options} @var{package-or-derivation}@dots{}
@end example

@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 --file=@var{file}
@itemx -f @var{file}

Build the package or derivation that the code within @var{file}
evaluates to.

As an example, @var{file} might contain a package definition like this
(@pxref{Defining Packages}):

@example
@verbatiminclude package-hello.scm
@end example

@item --expression=@var{expr}
@itemx -e @var{expr}
Build the package or derivation @var{expr} evaluates to.

For example, @var{expr} may be @code{(@@ (gnu packages guile)
guile-1.8)}, which unambiguously designates this specific variant of
version 1.8 of Guile.

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 or URLs 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

If a log is unavailable locally, and unless @code{--no-substitutes} is
passed, the command looks for a corresponding log on one of the
substitute servers (as specified with @code{--substitute-urls}.)

So for instance, let's say you want to see the build log of GDB on MIPS
but you're actually on an @code{x86_64} machine:

@example
$ guix build --log-file gdb -s mips64el-linux 
http://hydra.gnu.org/log/@dots{}-gdb-7.10
@end example

You can freely access a huge library of build logs!
@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{VISUAL} or 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 gem
@cindex gem
Import meta-data from @uref{https://rubygems.org/,
RubyGems}@footnote{This functionality requires Guile-JSON to be
installed.  @xref{Requirements}.}.  Information is taken from the
JSON-formatted description available at @code{rubygems.org} and includes
most relevant information, including runtime dependencies.  There are
some caveats, however.  The meta-data doesn't distinguish between
synopses and descriptions, so the same string is used for both fields.
Additionally, the details of non-Ruby dependencies required to build
native extensions is unavailable and left as an exercise to the
packager.

The command below imports meta-data for the @code{rails} Ruby package:

@example
guix import gem rails
@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 cran
@cindex CRAN
Import meta-data from @uref{http://cran.r-project.org/, CRAN}, the
central repository for the @uref{http://r-project.org, GNU@tie{}R
statistical and graphical environment}.

Information is extracted from the HTML package description.

The command command below imports meta-data for the @code{Cairo}
R package:

@example
guix import cran Cairo
@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.

Usually, you will first need to do:

@example
export NIX_REMOTE=daemon
@end example

@noindent
so that @command{nix-instantiate} does not try to open the Nix database.

As an example, the command below imports the package definition of
LibreOffice (more precisely, it imports the definition of the package
bound to the @code{libreoffice} top-level attribute):

@example
guix import nix ~/path/to/nixpkgs libreoffice
@end example

@item hackage
@cindex hackage
Import 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.  The command
knows how to update specific types of packages: GNU packages, ELPA
packages, etc.---see the documentation for @option{--type} below.  The
are many packages, though, for which it lacks a method to determine
whether a new upstream release is available.  However, the mechanism is
extensible, so feel free to get in touch with us to add a new method!

When passed @code{--update}, it modifies distribution source files to
update the version numbers and source tarball hashes of those 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 --expression=@var{expr}
@itemx -e @var{expr}
Consider the package @var{expr} evaluates to.

This is useful to precisely refer to a package, as in this example:

@example
guix refresh -l -e '(@@@@ (gnu packages commencement) glibc-final)'
@end example

This command lists the dependents of the ``final'' libc (essentially all
the packages.)

@item --update
@itemx -u
Update distribution source files (package recipes) in place.  This is
usually run from a checkout of the Guix source tree (@pxref{Running
Guix Before It Is Installed}):

@example
$ ./pre-inst-env guix refresh -s non-core
@end example

@xref{Defining Packages}, for more information on package definitions.

@item --select=[@var{subset}]
@itemx -s @var{subset}
Select all the packages in @var{subset}, one of @code{core} or
@code{non-core}.

The @code{core} subset refers to all the packages at the core of the
distribution---i.e., packages that are used to build ``everything
else''.  This includes GCC, libc, Binutils, Bash, etc.  Usually,
changing one of these packages in the distribution entails a rebuild of
all the others.  Thus, such updates are an inconvenience to users in
terms of build time or bandwidth used to achieve the upgrade.

The @code{non-core} subset refers to the remaining packages.  It is
typically useful in cases where an update of the core packages would be
inconvenient.

@item --type=@var{updater}
@itemx -t @var{updater}
Select only packages handled by @var{updater} (may be a comma-separated
list of updaters).  Currently, @var{updater} may be one of:

@table @code
@item gnu
the updater for GNU packages;
@item elpa
the updater for @uref{http://elpa.gnu.org/, ELPA} packages;
@item cran
the updater for @uref{http://cran.r-project.org/, CRAN} packages;
@item pypi
the updater for @uref{https://pypi.python.org, PyPI} packages.
@end table

For instance, the following commands only checks for updates of Emacs
packages hosted at @code{elpa.gnu.org} and updates of CRAN packages:

@example
$ guix refresh --type=elpa,cran
gnu/packages/statistics.scm:819:13: r-testthat would be upgraded from 0.10.0 to 0.11.0
gnu/packages/emacs.scm:856:13: emacs-auctex would be upgraded from 11.88.6 to 11.88.9
@end example

@end table

In addition, @command{guix refresh} can be passed one or more package
names, as in this example:

@example
$ ./pre-inst-env guix refresh -u emacs idutils gcc-4.8.4
@end example

@noindent
The command above specifically updates the @code{emacs} and
@code{idutils} packages.  The @code{--select} option would have no
effect in this case.

When considering whether to upgrade a package, it is sometimes
convenient to know which packages would be affected by the upgrade and
should be checked for compatibility.  For this the following option may
be used when passing @command{guix refresh} one or more package names:

@table @code

@item --list-updaters
@itemx -L
List available updaters and exit (see @option{--type} above.)

@item --list-dependent
@itemx -l
List top-level dependent packages that would need to be rebuilt as a
result of upgrading one or more packages.

@end table

Be aware that the @code{--list-dependent} option only
@emph{approximates} the rebuilds that would be required as a result of
an upgrade.  More rebuilds might be required under some circumstances.

@example
$ guix refresh --list-dependent flex
Building the following 120 packages would ensure 213 dependent packages are rebuilt:
hop-2.4.0 geiser-0.4 notmuch-0.18 mu-0.9.9.5 cflow-1.4 idutils-4.6 @dots{}
@end example

The command above lists a set of packages that could be built to check
for compatibility with an upgraded @code{flex} package.

The following options can be used to customize GnuPG operation:

@table @code

@item --gpg=@var{command}
Use @var{command} as the GnuPG 2.x command.  @var{command} is searched
for in @code{$PATH}.

@item --key-download=@var{policy}
Handle missing OpenPGP keys according to @var{policy}, which may be one
of:

@table @code
@item always
Always download missing OpenPGP keys from the key server, and add them
to the user's GnuPG keyring.

@item never
Never try to download missing OpenPGP keys.  Instead just bail out.

@item interactive
When a package signed with an unknown OpenPGP key is encountered, ask
the user whether to download it or not.  This is the default behavior.
@end table

@item --key-server=@var{host}
Use @var{host} as the OpenPGP key server when importing a public key.

@end table

@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
@itemx source-file-name
Probe @code{home-page} and @code{source} URLs and report those that are
invalid.  Check that the source file name is meaningful, e.g. is not
just a version number or ``git-checkout'', and should not have a
@code{file-name} declared (@pxref{origin Reference}).

@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 graph
@section Invoking @command{guix graph}

@cindex DAG
Packages and their dependencies form a @dfn{graph}, specifically a
directed acyclic graph (DAG).  It can quickly become difficult to have a
mental model of the package DAG, so the @command{guix graph} command is
here to provide a visual representation of the DAG.  @command{guix
graph} emits a DAG representation in the input format of
@uref{http://www.graphviz.org/, Graphviz}, so its output can be passed
directly to Graphviz's @command{dot} command, for instance.  The general
syntax is:

@example
guix graph @var{options} @var{package}@dots{}
@end example

For example, the following command generates a PDF file representing the
package DAG for the GNU@tie{}Core Utilities, showing its build-time
dependencies:

@example
guix graph coreutils | dot -Tpdf > dag.pdf
@end example

The output looks like this:

@image{images/coreutils-graph,2in,,Dependency graph of the GNU Coreutils}

Nice little graph, no?

But there's more than one graph!  The one above is concise: it's the
graph of package objects, omitting implicit inputs such as GCC, libc,
grep, etc.  It's often useful to have such a concise graph, but
sometimes you want to see more details.  @command{guix graph} supports
several types of graphs, allowing you to choose the level of details:

@table @code
@item package
This is the default type, the one we used above.  It shows the DAG of
package objects, excluding implicit dependencies.  It is concise, but
filters out many details.

@item bag-emerged
This is the package DAG, @emph{including} implicit inputs.

For instance, the following command:

@example
guix graph --type=bag-emerged coreutils | dot -Tpdf > dag.pdf
@end example

... yields this bigger graph:

@image{images/coreutils-bag-graph,,5in,Detailed dependency graph of the GNU Coreutils}

At the bottom of the graph, we see all the implicit inputs of
@var{gnu-build-system} (@pxref{Build Systems, @code{gnu-build-system}}).

Now, note that the dependencies of those implicit inputs---that is, the
@dfn{bootstrap dependencies} (@pxref{Bootstrapping})---are not shown
here, for conciseness.

@item bag
Similar to @code{bag-emerged}, but this time including all the bootstrap
dependencies.

@item bag-with-origins
Similar to @code{bag}, but also showing origins and their dependencies.

@item derivations
This is the most detailed representation: It shows the DAG of
derivations (@pxref{Derivations}) and plain store items.  Compared to
the above representation, many additional nodes are visible, including
builds scripts, patches, Guile modules, etc.

@end table

All the above types correspond to @emph{build-time dependencies}.  The
following graph type represents the @emph{run-time dependencies}:

@table @code
@item references
This is the graph of @dfn{references} of a package output, as returned
by @command{guix gc --references} (@pxref{Invoking guix gc}).

If the given package output is not available in the store, @command{guix
graph} attempts to obtain dependency information from substitutes.
@end table

The available options are the following:

@table @option
@item --type=@var{type}
@itemx -t @var{type}
Produce a graph output of @var{type}, where @var{type} must be one of
the values listed above.

@item --list-types
List the supported graph types.

@item --expression=@var{expr}
@itemx -e @var{expr}
Consider the package @var{expr} evaluates to.

This is useful to precisely refer to a package, as in this example:

@example
guix graph -e '(@@@@ (gnu packages commencement) gnu-make-final)'
@end example
@end table


@node Invoking guix environment
@section Invoking @command{guix environment}

@cindex reproducible build environments
@cindex development environments
The purpose of @command{guix environment} is to assist hackers in
creating reproducible development environments without polluting their
package profile.  The @command{guix environment} tool takes one or more
packages, builds all of 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.  An arbitrary
command may be invoked by placing the @code{--} token to separate the
command from the rest of the arguments:

@example
guix environment guile -- make -j4
@end example

In other situations, it is more convenient to specify the list of
packages needed in the environment.  For example, the following command
runs @command{python} from an environment containing Python@tie{}2.7 and
NumPy:

@example
guix environment --ad-hoc python2-numpy python-2.7 -- python
@end example

Furthermore, one might want the dependencies of a package and also some
additional packages that are not build-time or runtime dependencies, but
are useful when developing nonetheless.  Because of this, the
@code{--ad-hoc} flag is positional.  Packages appearing before
@code{--ad-hoc} are interpreted as packages whose dependencies will be
added to the environment.  Packages appearing after are interpreted as
packages that will be added to the environment directly.  For example,
the following command creates a Guix development environment that
additionally includes Git and strace:

@example
guix environment guix --ad-hoc git strace
@end example

Sometimes it is desirable to isolate the environment as much as
possible, for maximal purity and reproducibility.  In particular, when
using Guix on a host distro that is not GuixSD, it is desirable to
prevent access to @file{/usr/bin} and other system-wide resources from
the development environment.  For example, the following command spawns
a Guile REPL in a ``container'' where only the store and the current
working directory are mounted:

@example
guix environment --ad-hoc --container guile -- guile
@end example

@quotation Note
The @code{--container} option requires Linux-libre 3.19 or newer.
@end quotation

The available options are summarized below.

@table @code
@item --expression=@var{expr}
@itemx -e @var{expr}
Create an environment for the package or list of packages that
@var{expr} evaluates to.

For example, running:

@example
guix environment -e '(@@ (gnu packages maths) petsc-openmpi)'
@end example

starts a shell with the environment for this specific variant of the
PETSc package.

Running:

@example
guix environment --ad-hoc -e '(@@ (gnu) %base-packages)'
@end example

starts a shell with all the GuixSD base packages available.

@item --load=@var{file}
@itemx -l @var{file}
Create an environment for the package or list of packages that the code
within @var{file} evaluates to.

As an example, @var{file} might contain a definition like this
(@pxref{Defining Packages}):

@example
@verbatiminclude environment-gdb.scm
@end example

@item --ad-hoc
Include all specified packages in the resulting environment, as if an
@i{ad hoc} package were defined with them as inputs.  This option is
useful for quickly creating an environment without having to write a
package expression to contain the desired inputs.

For instance, the command:

@example
guix environment --ad-hoc guile guile-sdl -- guile
@end example

runs @command{guile} in an environment where Guile and Guile-SDL are
available.

Note that this example implicitly asks for the default output of
@code{guile} and @code{guile-sdl} but it is possible to ask for a
specific output---e.g., @code{glib:bin} asks for the @code{bin} output
of @code{glib} (@pxref{Packages with Multiple Outputs}).

This option may be composed with the default behavior of @command{guix
environment}.  Packages appearing before @code{--ad-hoc} are interpreted
as packages whose dependencies will be added to the environment, the
default behavior.  Packages appearing after are interpreted as packages
that will be added to the environment directly.

@item --pure
Unset existing environment variables when building the new environment.
This has the effect of creating an environment in which search paths
only contain package inputs.

@item --search-paths
Display the environment variable definitions that make up the
environment.

@item --system=@var{system}
@itemx -s @var{system}
Attempt to build for @var{system}---e.g., @code{i686-linux}.

@item --container
@itemx -C
@cindex container
Run @var{command} within an isolated container.  The current working
directory outside the container is mapped to @file{/env} inside the
container.  Additionally, the spawned process runs as the current user
outside the container, but has root privileges in the context of the
container.

@item --network
@itemx -N
For containers, share the network namespace with the host system.
Containers created without this flag only have access to the loopback
device.

@item --expose=@var{source}[=@var{target}]
For containers, expose the file system @var{source} from the host system
as the read-only file system @var{target} within the container.  If
@var{target} is not specified, @var{source} is used as the target mount
point in the container.

The example below spawns a Guile REPL in a container in which the user's
home directory is accessible read-only via the @file{/exchange}
directory:

@example
guix environment --container --expose=$HOME=/exchange guile -- guile
@end example

@item --share=@var{source}[=@var{target}]
For containers, share the file system @var{source} from the host system
as the writable file system @var{target} within the container.  If
@var{target} is not specified, @var{source} is used as the target mount
point in the container.

The example below spawns a Guile REPL in a container in which the user's
home directory is accessible for both reading and writing via the
@file{/exchange} directory:

@example
guix environment --container --share=$HOME=/exchange guile -- guile
@end example
@end table

It also supports all of the common build options that @command{guix
build} supports (@pxref{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 signing key pair must be generated before @command{guix publish} is
launched, using @command{guix archive --generate-key} (@pxref{Invoking
guix archive}).

The general syntax is:

@example
guix publish @var{options}@dots{}
@end example

Running @command{guix publish} without any additional arguments will
spawn an HTTP server on port 8080:

@example
guix publish
@end example

Once a publishing server has been authorized (@pxref{Invoking guix
archive}), the daemon may download substitutes from it:

@example
guix-daemon --substitute-urls=http://example.org:8080
@end example

The following options are available:

@table @code
@item --port=@var{port}
@itemx -p @var{port}
Listen for HTTP requests on @var{port}.

@item --listen=@var{host}
Listen on the network interface for @var{host}.  The default is to
accept connections from any interface.

@item --user=@var{user}
@itemx -u @var{user}
Change privileges to @var{user} as soon as possible---i.e., once the
server socket is open and the signing key has been read.

@item --repl[=@var{port}]
@itemx -r [@var{port}]
Spawn a Guile REPL server (@pxref{REPL Servers,,, guile, GNU Guile
Reference Manual}) on @var{port} (37146 by default).  This is used
primarily for debugging a running @command{guix publish} server.
@end table

Enabling @command{guix publish} on a GuixSD system is a one-liner: just
add a call to @code{guix-publish-service} in the @code{services} field
of the @code{operating-system} declaration (@pxref{guix-publish-service,
@code{guix-publish-service}}).


@node Invoking guix challenge
@section Invoking @command{guix challenge}

@cindex reproducible builds
@cindex verifiable builds

Do the binaries provided by this server really correspond to the source
code it claims to build?  Is this package's build process deterministic?
These are the questions the @command{guix challenge} command attempts to
answer.

The former is obviously an important question: Before using a substitute
server (@pxref{Substitutes}), you'd rather @emph{verify} that it
provides the right binaries, and thus @emph{challenge} it.  The latter
is what enables the former: If package builds are deterministic, then
independent builds of the package should yield the exact same result,
bit for bit; if a server provides a binary different from the one
obtained locally, it may be either corrupt or malicious.

We know that the hash that shows up in @file{/gnu/store} file names is
the hash of all the inputs of the process that built the file or
directory---compilers, libraries, build scripts,
etc. (@pxref{Introduction}).  Assuming deterministic build processes,
one store file name should map to exactly one build output.
@command{guix challenge} checks whether there is, indeed, a single
mapping by comparing the build outputs of several independent builds of
any given store item.

The command's output looks like this:

@smallexample
$ guix challenge --substitute-urls="http://hydra.gnu.org http://guix.example.org"
updating list of substitutes from 'http://hydra.gnu.org'... 100.0%
updating list of substitutes from 'http://guix.example.org'... 100.0%
/gnu/store/@dots{}-openssl-1.0.2d contents differ:
  local hash: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q
  http://hydra.gnu.org/nar/@dots{}-openssl-1.0.2d: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q
  http://guix.example.org/nar/@dots{}-openssl-1.0.2d: 1zy4fmaaqcnjrzzajkdn3f5gmjk754b43qkq47llbyak9z0qjyim
/gnu/store/@dots{}-git-2.5.0 contents differ:
  local hash: 00p3bmryhjxrhpn2gxs2fy0a15lnip05l97205pgbk5ra395hyha
  http://hydra.gnu.org/nar/@dots{}-git-2.5.0: 069nb85bv4d4a6slrwjdy8v1cn4cwspm3kdbmyb81d6zckj3nq9f
  http://guix.example.org/nar/@dots{}-git-2.5.0: 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73
/gnu/store/@dots{}-pius-2.1.1 contents differ:
  local hash: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
  http://hydra.gnu.org/nar/@dots{}-pius-2.1.1: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
  http://guix.example.org/nar/@dots{}-pius-2.1.1: 1cy25x1a4fzq5rk0pmvc8xhwyffnqz95h2bpvqsz2mpvlbccy0gs
@end smallexample

@noindent
In this example, @command{guix challenge} first scans the store to
determine the set of locally-built derivations---as opposed to store
items that were downloaded from a substitute server---and then queries
all the substitute servers.  It then reports those store items for which
the servers obtained a result different from the local build.

@cindex non-determinism, in package builds
As an example, @code{guix.example.org} always gets a different answer.
Conversely, @code{hydra.gnu.org} agrees with local builds, except in the
case of Git.  This might indicate that the build process of Git is
non-deterministic, meaning that its output varies as a function of
various things that Guix does not fully control, in spite of building
packages in isolated environments (@pxref{Features}).  Most common
sources of non-determinism include the addition of timestamps in build
results, the inclusion of random numbers, and directory listings sorted
by inode number.  See @uref{http://reproducible.debian.net/howto/}, for
more information.

To find out what's wrong with this Git binary, we can do something along
these lines (@pxref{Invoking guix archive}):

@example
$ wget -q -O - http://hydra.gnu.org/nar/@dots{}-git-2.5.0 \
   | guix archive -x /tmp/git
$ diff -ur --no-dereference /gnu/store/@dots{}-git.2.5.0 /tmp/git
@end example

This command shows the difference between the files resulting from the
local build, and the files resulting from the build on
@code{hydra.gnu.org} (@pxref{Overview, Comparing and Merging Files,,
diffutils, Comparing and Merging Files}).  The @command{diff} command
works great for text files.  When binary files differ, a better option
is @uref{http://diffoscope.org/, Diffoscope}, a tool that helps
visualize differences for all kinds of files.

Once you've done that work, you can tell whether the differences are due
to a non-deterministic build process or to a malicious server.  We try
hard to remove sources of non-determinism in packages to make it easier
to verify substitutes, but of course, this is a process, one that
involves not just Guix but a large part of the free software community.
In the meantime, @command{guix challenge} is one tool to help address
the problem.

If you are writing packages for Guix, you are encouraged to check
whether @code{hydra.gnu.org} and other substitute servers obtain the
same build result as you did with:

@example
$ guix challenge @var{package}
@end example

@noindent
... where @var{package} is a package specification such as
@code{guile-2.0} or @code{glibc:debug}.

The general syntax is:

@example
guix challenge @var{options} [@var{packages}@dots{}]
@end example

The one option that matters is:

@table @code

@item --substitute-urls=@var{urls}
Consider @var{urls} the whitespace-separated list of substitute source
URLs to compare to.

@end table


@node Invoking guix container
@section Invoking @command{guix container}
@cindex container

@quotation Note
As of version @value{VERSION}, this tool is experimental.  The interface
is subject to radical change in the future.
@end quotation

The purpose of @command{guix container} is to manipulate processes
running within an isolated environment, commonly known as a
``container'', typically created by the @command{guix environment}
(@pxref{Invoking guix environment}) and @command{guix system container}
(@pxref{Invoking guix system}) commands.

The general syntax is:

@example
guix container @var{action} @var{options}@dots{}
@end example

@var{action} specifies the operation to perform with a container, and
@var{options} specifies the context-specific arguments for the action.

The following actions are available:

@table @code
@item exec
Execute a command within the context of a running container.

The syntax is:

@example
guix container exec @var{pid} @var{program} @var{arguments}@dots{}
@end example

@var{pid} specifies the process ID of the running container.
@var{program} specifies an executable file name within the container's
root file system.  @var{arguments} are the additional options that will
be passed to @var{program}.

The following command launches an interactive login shell inside a
GuixSD container, started by @command{guix system container}, and whose
process ID is 9001:

@example
guix container exec 9001 /run/current-system/profile/bin/bash --login
@end example

Note that the @var{pid} cannot be the parent process of a container.  It
must be the container's PID 1 or one of its child processes.

@end table

@c *********************************************************************
@node GNU Distribution
@chapter GNU Distribution

@cindex Guix System Distribution
@cindex GuixSD
Guix comes with a distribution of the GNU system consisting entirely of
free software@footnote{The term ``free'' here refers to the
@url{http://www.gnu.org/philosophy/free-sw.html,freedom provided to
users of that software}.}.  The
distribution can be installed on its own (@pxref{System Installation}),
but it is also possible to install Guix as a package manager on top of
an installed GNU/Linux system (@pxref{Installation}).  To distinguish
between the two, we refer to the standalone distribution as the Guix
System Distribution, or GuixSD.

The distribution provides core GNU packages such as GNU libc, GCC, and
Binutils, as well as many GNU and non-GNU applications.  The complete
list of available packages can be browsed
@url{http://www.gnu.org/software/guix/packages,on-line} or by
running @command{guix package} (@pxref{Invoking guix package}):

@example
guix package --list-available
@end example

Our goal 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}).  The installation system runs the GPM mouse daemon,
which allows you to select text with the left mouse button and to paste
it with the middle button.

To install the system, you would:

@enumerate

@item
Configure the network, by running:

@example
ifconfig eno1 up && dhclient eno1
@end example

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 then format
the target partition.

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.

@c FIXME: Uncomment this once GRUB fully supports encrypted roots.
@c A typical command sequence may be:
@c
@c @example
@c # fdisk /dev/sdX
@c @dots{} Create partitions etc.@dots{}
@c # cryptsetup luksFormat /dev/sdX1
@c # cryptsetup open --type luks /dev/sdX1 my-partition
@c # mkfs.ext4 -L my-root /dev/mapper/my-partition
@c @end example

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
tcpdump to those, taken from the @code{(gnu packages admin)} 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 (@pxref{Networking Services,
@code{lsh-service}}).  Under the hood,
@code{lsh-service} arranges so that @code{lshd} is started with the
right command-line options, possibly with supporting configuration files
generated as needed (@pxref{Defining Services}).

@cindex customization, of services
@findex modify-services
Occasionally, instead of using the base services as is, you will want to
customize them.  For instance, to change the configuration of
@code{guix-daemon} and Mingetty (the console log-in), you may write the
following instead of @var{%base-services}:

@lisp
(modify-services %base-services
  (guix-service-type config =>
                     (guix-configuration
                      (inherit config)
                      (use-substitutes? #f)
                      (extra-options '("--gc-keep-outputs"))))
  (mingetty-service-type config =>
                         (mingetty-configuration
                          (inherit config)
                          (motd (plain-file "motd" "Hi there!")))))
@end lisp

@noindent
The effect here is to change the options passed to @command{guix-daemon}
when it is started, as well as the ``message of the day'' that appears
when logging in at the console.  @xref{Service Reference,
@code{modify-services}}, for more on that.

The configuration for a typical ``desktop'' usage, with the X11 display
server, a desktop environment, network management, power management, 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.
@xref{operating-system Reference}, for details about all the available
@code{operating-system} fields.

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 @command{guix system reconfigure}.  One should never
have to touch files in @command{/etc} or to run commands that modify the
system state such as @command{useradd} or @command{grub-install}.  In
fact, you must avoid that since that would not only void your warranty
but also prevent you from rolling back to previous versions of your
system, should you ever need to.

@cindex roll-back, of the operating system
Speaking of roll-back, each time you run @command{guix system
reconfigure}, a new @dfn{generation} of the system is created---without
modifying or deleting previous generations.  Old system generations get
an entry in the GRUB boot menu, allowing you to boot them in case
something went wrong with the latest generation.  Reassuring, no?  The
@command{guix system list-generations} command lists the system
generations available on disk.

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{locale-libcs} (default: @code{(list @var{glibc})})
The list of GNU@tie{}libc packages whose locale data and tools are used
to build the locale definitions.  @xref{Locales}, for compatibility
considerations that justify this option.

@item @code{name-service-switch} (default: @var{%default-nss})
Configuration of 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 service objects denoting system services.  @xref{Services}.

@item @code{pam-services} (default: @code{(base-pam-services)})
@cindex PAM
@cindex pluggable authentication modules
Linux @dfn{pluggable authentication module} (PAM) services.
@c FIXME: Add xref to PAM services section.

@item @code{setuid-programs} (default: @var{%setuid-programs})
List of string-valued G-expressions denoting setuid programs.
@xref{Setuid Programs}.

@item @code{sudoers-file} (default: @var{%sudoers-specification})
@cindex sudoers file
The contents of the @file{/etc/sudoers} file as a file-like object
(@pxref{G-Expressions, @code{local-file} and @code{plain-file}}).

This file specifies which users can use the @command{sudo} command, what
they are allowed to do, and what privileges they may gain.  The default
is that only @code{root} and members of the @code{wheel} group may use
@code{sudo}.

@end table
@end deftp

@node File Systems
@subsection File Systems

The list of file systems to be mounted is specified in the
@code{file-systems} field of the operating system'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{%pseudo-terminal-file-system} and @var{%immutable-store} (see
below.)  Operating system declarations should always contain at least
these.
@end defvr

@defvr {Scheme Variable} %pseudo-terminal-file-system
This is the file system to be mounted as @file{/dev/pts}.  It supports
@dfn{pseudo-terminals} created @i{via} @code{openpty} and similar
functions (@pxref{Pseudo-Terminals,,, libc, The GNU C Library Reference
Manual}).  Pseudo-terminals are used by terminal emulators such as
@command{xterm}.
@end defvr

@defvr {Scheme Variable} %shared-memory-file-system
This file system is mounted as @file{/dev/shm} and is used to support
memory sharing across processes (@pxref{Memory-mapped I/O,
@code{shm_open},, libc, The GNU C Library Reference Manual}).
@end defvr

@defvr {Scheme Variable} %immutable-store
This file system performs a read-only ``bind mount'' of
@file{/gnu/store}, making it read-only for all the users including
@code{root}.  This prevents against accidental modification by software
running as @code{root} or by system administrators.

The daemon itself is still able to write to the store: it remounts it
read-write in its own ``name space.''
@end defvr

@defvr {Scheme Variable} %binary-format-file-system
The @code{binfmt_misc} file system, which allows handling of arbitrary
executable file types to be delegated to user space.  This requires the
@code{binfmt.ko} kernel module to be loaded.
@end defvr

@defvr {Scheme Variable} %fuse-control-file-system
The @code{fusectl} file system, which allows unprivileged users to mount
and unmount user-space FUSE file systems.  This requires the
@code{fuse.ko} kernel module to be loaded.
@end defvr

@node Mapped Devices
@subsection Mapped Devices

@cindex device mapping
@cindex mapped devices
The Linux kernel has a notion of @dfn{device mapping}: a block device,
such as a hard disk partition, can be @dfn{mapped} into another device,
with additional processing over the data that flows through
it@footnote{Note that the GNU@tie{}Hurd makes no difference between the
concept of a ``mapped device'' and that of a file system: both boil down
to @emph{translating} input/output operations made on a file to
operations on its backing store.  Thus, the Hurd implements mapped
devices, like file systems, using the generic @dfn{translator} mechanism
(@pxref{Translators,,, hurd, The GNU Hurd Reference Manual}).}.  A
typical example is encryption device mapping: all writes to the mapped
device are encrypted, and all reads are deciphered, transparently.

Mapped devices are declared using the @code{mapped-device} form:

@example
(mapped-device
  (source "/dev/sda3")
  (target "home")
  (type luks-device-mapping))
@end example

@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{codeset}}---e.g.,
@code{fr_LU.utf8} designates the locale for the French language, with
cultural conventions from Luxembourg, and using the UTF-8 encoding.

@cindex locale definition
Usually, you will want to specify the default locale for the machine
using the @code{locale} field of the @code{operating-system} declaration
(@pxref{operating-system Reference, @code{locale}}).

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

@vindex LOCPATH
The compiled locale definitions are available at
@file{/run/current-system/locale/X.Y}, where @code{X.Y} is the libc
version, which is the default location where the GNU@tie{}libc provided
by Guix looks for locale data.  This can be overridden using the
@code{LOCPATH} environment variable (@pxref{locales-and-locpath,
@code{LOCPATH} and locale packages}).

The @code{locale-definition} form is provided by the @code{(gnu system
locale)} module.  Details are given below.

@deftp {Data Type} locale-definition
This is the data type of a locale definition.

@table @asis

@item @code{name}
The name of the locale.  @xref{Locale Names,,, libc, The GNU C Library
Reference Manual}, for more information on locale names.

@item @code{source}
The name of the source for that locale.  This is typically the
@code{@var{language}_@var{territory}} part of the locale name.

@item @code{charset} (default: @code{"UTF-8"})
The ``character set'' or ``code set'' for that locale,
@uref{http://www.iana.org/assignments/character-sets, as defined by
IANA}.

@end table
@end deftp

@defvr {Scheme Variable} %default-locale-definitions
An arbitrary list of commonly used UTF-8 locales, used as the default
value of the @code{locale-definitions} field of @code{operating-system}
declarations.

@cindex locale name
@cindex normalized codeset in locale names
These locale definitions use the @dfn{normalized codeset} for the part
that follows the dot in the name (@pxref{Using gettextized software,
normalized codeset,, libc, The GNU C Library Reference Manual}).  So for
instance it has @code{uk_UA.utf8} but @emph{not}, say,
@code{uk_UA.UTF-8}.
@end defvr

@subsubsection Locale Data Compatibility Considerations

@cindex incompatibility, of locale data
@code{operating-system} declarations provide a @code{locale-libcs} field
to specify the GNU@tie{}libc packages that are used to compile locale
declarations (@pxref{operating-system Reference}).  ``Why would I
care?'', you may ask.  Well, it turns out that the binary format of
locale data is occasionally incompatible from one libc version to
another.

@c See <https://sourceware.org/ml/libc-alpha/2015-09/msg00575.html>
@c and <https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00737.html>.
For instance, a program linked against libc version 2.21 is unable to
read locale data produced with libc 2.22; worse, that program
@emph{aborts} instead of simply ignoring the incompatible locale
data@footnote{Versions 2.23 and later of GNU@tie{}libc will simply skip
the incompatible locale data, which is already an improvement.}.
Similarly, a program linked against libc 2.22 can read most, but not
all, the locale data from libc 2.21 (specifically, @code{LC_COLLATE}
data is incompatible); thus calls to @code{setlocale} may fail, but
programs will not abort.

The ``problem'' in GuixSD is that users have a lot of freedom: They can
choose whether and when to upgrade software in their profiles, and might
be using a libc version different from the one the system administrator
used to build the system-wide locale data.

Fortunately, unprivileged users can also install their own locale data
and define @var{GUIX_LOCPATH} accordingly (@pxref{locales-and-locpath,
@code{GUIX_LOCPATH} and locale packages}).

Still, it is best if the system-wide locale data at
@file{/run/current-system/locale} is built for all the libc versions
actually in use on the system, so that all the programs can access
it---this is especially crucial on a multi-user system.  To do that, the
administrator can specify several libc packages in the
@code{locale-libcs} field of @code{operating-system}:

@example
(use-package-modules base)

(operating-system
  ;; @dots{}
  (locale-libcs (list glibc-2.21 (canonical-package glibc))))
@end example

This example would lead to a system containing locale definitions for
both libc 2.21 and the current version of libc in
@file{/run/current-system/locale}.


@node Services
@subsection Services

@cindex system services
An important part of preparing an @code{operating-system} declaration is
listing @dfn{system services} and their configuration (@pxref{Using the
Configuration System}).  System services are typically daemons launched
when the system boots, or other actions needed at that time---e.g.,
configuring network access.

Services are managed by 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.
* Web Services::                Web servers.
* Various Services::            Other services.
@end menu

@node Base Services
@subsubsection Base Services

The @code{(gnu services base)} module provides definitions for the basic
services that one expects from the system.  The services exported by
this module are listed below.

@defvr {Scheme Variable} %base-services
This variable contains a list of basic services@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 {Scheme Procedure} host-name-service @var{name}
Return a service that sets the host name to @var{name}.
@end deffn

@deffn {Scheme Procedure} mingetty-service @var{config}
Return a service to run mingetty according to @var{config}, a
@code{<mingetty-configuration>} object, which specifies the tty to run, among
other things.
@end deffn

@deftp {Data Type} mingetty-configuration
This is the data type representing the configuration of Mingetty, which
implements console log-in.

@table @asis

@item @code{tty}
The name of the console this Mingetty runs on---e.g., @code{"tty1"}.

@item @code{motd}
A file-like object containing the ``message of the day''.

@item @code{auto-login} (default: @code{#f})
When true, this field must be a string denoting the user name under
which the the system automatically logs in.  When it is @code{#f}, a
user name and password must be entered to log in.

@item @code{login-program} (default: @code{#f})
This must be either @code{#f}, in which case the default log-in program
is used (@command{login} from the Shadow tool suite), or a gexp denoting
the name of the log-in program.

@item @code{login-pause?} (default: @code{#f})
When set to @code{#t} in conjunction with @var{auto-login}, the user
will have to press a key before the log-in shell is launched.

@item @code{mingetty} (default: @var{mingetty})
The Mingetty package to use.

@end table
@end deftp

@cindex name service cache daemon
@cindex nscd
@deffn {Scheme Procedure} nscd-service [@var{config}] [#:glibc glibc] @
                [#:name-services '()]
Return a service that runs libc's name service cache daemon (nscd) with the
given @var{config}---an @code{<nscd-configuration>} object.  @xref{Name
Service Switch}, for an example.
@end deffn

@defvr {Scheme Variable} %nscd-default-configuration
This is the default @code{<nscd-configuration>} value (see below) used
by @code{nscd-service}.  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{name-services} (default: @code{'()})
List of packages denoting @dfn{name services} that must be visible to
the nscd---e.g., @code{(list @var{nss-mdns})}.

@item @code{glibc} (default: @var{glibc})
Package object denoting the GNU C Library providing the @command{nscd}
command.

@item @code{log-file} (default: @code{"/var/log/nscd.log"})
Name of 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 {Scheme 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

@anchor{guix-configuration-type}
@deftp {Data Type} guix-configuration
This data type represents the configuration of the Guix build daemon.
@xref{Invoking guix-daemon}, for more information.

@table @asis
@item @code{guix} (default: @var{guix})
The Guix package to use.

@item @code{build-group} (default: @code{"guixbuild"})
Name of the group for build user accounts.

@item @code{build-accounts} (default: @code{10})
Number of build user accounts to create.

@item @code{authorize-key?} (default: @code{#t})
Whether to authorize the substitute key for @code{hydra.gnu.org}
(@pxref{Substitutes}).

@item @code{use-substitutes?} (default: @code{#t})
Whether to use substitutes.

@item @code{substitute-urls} (default: @var{%default-substitute-urls})
The list of URLs where to look for substitutes by default.

@item @code{extra-options} (default: @code{'()})
List of extra command-line options for @command{guix-daemon}.

@item @code{lsof} (default: @var{lsof})
@itemx @code{lsh} (default: @var{lsh})
The lsof and lsh packages to use.

@end table
@end deftp

@deffn {Scheme Procedure} guix-service @var{config}
Return a service that runs the Guix build daemon according to
@var{config}.
@end deffn

@deffn {Scheme Procedure} udev-service [#:udev udev]
Run @var{udev}, which populates the @file{/dev} directory dynamically.
@end deffn

@deffn {Scheme Procedure} console-keymap-service @var{file}
Return a service to load console keymap from @var{file} using
@command{loadkeys} command.
@end deffn

@deffn {Scheme Procedure} gpm-service-type [#:gpm @var{gpm}] @
          [#:options]
Run @var{gpm}, the general-purpose mouse daemon, with the given
command-line @var{options}.  GPM allows users to use the mouse in the console,
notably to select, copy, and paste text.  The default value of @var{options}
uses the @code{ps2} protocol, which works for both USB and PS/2 mice.

This service is not part of @var{%base-services}.
@end deffn

@anchor{guix-publish-service}
@deffn {Scheme Procedure} guix-publish-service [#:guix @var{guix}] @
           [#:port 80] [#:host "localhost"]
Return a service that runs @command{guix publish} listening on @var{host}
and @var{port} (@pxref{Invoking guix publish}).

This assumes that @file{/etc/guix} already contains a signing key pair as
created by @command{guix archive --generate-key} (@pxref{Invoking guix
archive}).  If that is not the case, the service will fail to start.
@end deffn


@node Networking Services
@subsubsection Networking Services

The @code{(gnu services networking)} module provides services to configure
the network interface.

@cindex DHCP, networking service
@deffn {Scheme Procedure} dhcp-client-service [#:dhcp @var{isc-dhcp}]
Return a service that runs @var{dhcp}, a Dynamic Host Configuration
Protocol (DHCP) client, on all the non-loopback network interfaces.
@end deffn

@deffn {Scheme Procedure} static-networking-service @var{interface} @var{ip} @
       [#:gateway #f] [#:name-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
@cindex network management
@deffn {Scheme Procedure} wicd-service [#:wicd @var{wicd}]
Return a service that runs @url{https://launchpad.net/wicd,Wicd}, a network
management daemon that aims to simplify wired and wireless networking.

This service adds the @var{wicd} package to the global profile, providing
several commands to interact with the daemon and configure networking:
@command{wicd-client}, a graphical user interface, and the @command{wicd-cli}
and @command{wicd-curses} user interfaces.
@end deffn

@deffn {Scheme Procedure} ntp-service [#:ntp @var{ntp}] @
  [#:name-service @var{%ntp-servers}]
Return a service that runs the daemon from @var{ntp}, the
@uref{http://www.ntp.org, Network Time Protocol package}.  The daemon will
keep the system clock synchronized with that of @var{servers}.
@end deffn

@defvr {Scheme Variable} %ntp-servers
List of host names used as the default NTP servers.
@end defvr

@deffn {Scheme Procedure} tor-service [@var{config-file}] [#:tor @var{tor}]
Return a service to run the @uref{https://torproject.org, Tor} anonymous
networking daemon.

The daemon runs as the @code{tor} unprivileged user.  It is passed
@var{config-file}, a file-like object, with an additional @code{User tor}
line.  Run @command{man tor} for information about the configuration file.
@end deffn

@deffn {Scheme Procedure} bitlbee-service [#:bitlbee bitlbee] @
         [#:interface "127.0.0.1"] [#:port 6667] @
         [#:extra-settings ""]
Return a service that runs @url{http://bitlbee.org,BitlBee}, a daemon that
acts as a gateway between IRC and chat networks.

The daemon will listen to the interface corresponding to the IP address
specified in @var{interface}, on @var{port}.  @code{127.0.0.1} means that only
local clients can connect, whereas @code{0.0.0.0} means that connections can
come from any networking interface.

In addition, @var{extra-settings} specifies a string to append to the
configuration file.
@end deffn

Furthermore, @code{(gnu services ssh)} provides the following service.

@deffn {Scheme Procedure} lsh-service [#:host-key "/etc/lsh/host-key"] @
       [#:daemonic? #t] [#:interfaces '()] [#:port-number 22] @
       [#:allow-empty-passwords? #f] [#:root-login? #f] @
       [#:syslog-output? #t] [#:x11-forwarding? #t] @
       [#:tcp/ip-forwarding? #t] [#:password-authentication? #t] @
       [#:public-key-authentication? #t] [#:initialize? #t]
Run the @command{lshd} program from @var{lsh} to listen on port @var{port-number}.
@var{host-key} must designate a file containing the host key, and readable
only by root.

When @var{daemonic?} is true, @command{lshd} will detach from the
controlling terminal and log its output to syslogd, unless one sets
@var{syslog-output?} to false.  Obviously, it also makes lsh-service
depend on existence of syslogd service.  When @var{pid-file?} is true,
@command{lshd} writes its PID to the file called @var{pid-file}.

When @var{initialize?} is true, automatically create the seed and host key
upon service activation if they do not exist yet.  This may take long and
require interaction.

When @var{initialize?} is false, it is up to the user to initialize the
randomness generator (@pxref{lsh-make-seed,,, lsh, LSH Manual}), and to create
a key pair with the private key stored in file @var{host-key} (@pxref{lshd
basics,,, lsh, LSH Manual}).

When @var{interfaces} is empty, lshd listens for connections on all the
network interfaces; otherwise, @var{interfaces} must be a list of host names
or addresses.

@var{allow-empty-passwords?} specifies whether to accept log-ins with empty
passwords, and @var{root-login?} specifies whether to accept log-ins as
root.

The other options should be self-descriptive.
@end deffn

@defvr {Scheme Variable} %facebook-host-aliases
This variable contains a string for use in @file{/etc/hosts}
(@pxref{Host Names,,, libc, The GNU C Library Reference Manual}).  Each
line contains a entry that maps a known server name of the Facebook
on-line service---e.g., @code{www.facebook.com}---to the local
host---@code{127.0.0.1} or its IPv6 equivalent, @code{::1}.

This variable is typically used in the @code{hosts-file} field of an
@code{operating-system} declaration (@pxref{operating-system Reference,
@file{/etc/hosts}}):

@example
(use-modules (gnu) (guix))

(operating-system
  (host-name "mymachine")
  ;; ...
  (hosts-file
    ;; Create a /etc/hosts file with aliases for "localhost"
    ;; and "mymachine", as well as for Facebook servers.
    (plain-file "hosts"
                (string-append (local-host-aliases host-name)
                               %facebook-host-aliases))))
@end example

This mechanism can prevent programs running locally, such as Web
browsers, from accessing Facebook.
@end defvr

The @code{(gnu services avahi)} provides the following definition.

@deffn {Scheme Procedure} avahi-service [#:avahi @var{avahi}] @
          [#:host-name #f] [#:publish? #t] [#:ipv4? #t] @
          [#:ipv6? #t] [#:wide-area? #f] @
          [#:domains-to-browse '()]
Return a service that runs @command{avahi-daemon}, a system-wide
mDNS/DNS-SD responder that allows for service discovery and
"zero-configuration" host name lookups (see @uref{http://avahi.org/}), and
extends the name service cache daemon (nscd) so that it can resolve
@code{.local} host names using
@uref{http://0pointer.de/lennart/projects/nss-mdns/, nss-mdns}.  Additionally,
add the @var{avahi} package to the system profile so that commands such as
@command{avahi-browse} are directly usable.

If @var{host-name} is different from @code{#f}, use that as the host name to
publish for this machine; otherwise, use the machine's actual host name.

When @var{publish?} is true, publishing of host names and services is allowed;
in particular, avahi-daemon will publish the machine's host name and IP
address via mDNS on the local network.

When @var{wide-area?} is true, DNS-SD over unicast DNS is enabled.

Boolean values @var{ipv4?} and @var{ipv6?} determine whether to use IPv4/IPv6
sockets.
@end deffn


@node X Window
@subsubsection X Window

Support for the X Window graphical display system---specifically
Xorg---is provided by the @code{(gnu services xorg)} module.  Note that
there is no @code{xorg-service} procedure.  Instead, the X server is
started by the @dfn{login manager}, currently SLiM.

@deffn {Scheme Procedure} slim-service [#:allow-empty-passwords? #f] @
  [#:auto-login? #f] [#:default-user ""] [#:startx] @
  [#:theme @var{%default-slim-theme}] @
  [#:theme-name @var{%default-slim-theme-name}]
Return a service that spawns the SLiM graphical login manager, which in
turn starts the X display server with @var{startx}, a command as returned by
@code{xorg-start-command}.

@cindex X session

SLiM automatically looks for session types described by the @file{.desktop}
files in @file{/run/current-system/profile/share/xsessions} and allows users
to choose a session from the log-in screen using @kbd{F1}.  Packages such as
@var{xfce}, @var{sawfish}, and @var{ratpoison} provide @file{.desktop} files;
adding them to the system-wide set of packages automatically makes them
available at the log-in screen.

In addition, @file{~/.xsession} files are honored.  When available,
@file{~/.xsession} must be an executable that starts a window manager
and/or other X clients.

When @var{allow-empty-passwords?} is true, allow logins with an empty
password.  When @var{auto-login?} is true, log in automatically as
@var{default-user}.

If @var{theme} is @code{#f}, 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 {Scheme Procedure} xorg-start-command [#:guile] @
  [#:configuration-file #f] [#:xorg-server @var{xorg-server}]
Return a derivation that builds a @var{guile} script to start the X server
from @var{xorg-server}.  @var{configuration-file} is the server configuration
file or a derivation that builds it; when omitted, the result of
@code{xorg-configuration-file} is used.

Usually the X server is started by a login manager.
@end deffn

@deffn {Scheme Procedure} xorg-configuration-file @
  [#:drivers '()] [#:resolutions '()] [#:extra-config '()]
Return a configuration file for the Xorg server containing search paths for
all the common drivers.

@var{drivers} must be either the empty list, in which case Xorg chooses a
graphics driver automatically, or a list of driver names that will be tried in
this order---e.g., @code{(\"modesetting\" \"vesa\")}.

Likewise, when @var{resolutions} is the empty list, Xorg chooses an
appropriate screen resolution; otherwise, it must be a list of
resolutions---e.g., @code{((1024 768) (640 480))}.

Last, @var{extra-config} is a list of strings or objects appended to the
@code{text-file*} argument list.  It is used to pass extra text to be added
verbatim to the configuration file.
@end deffn

@deffn {Scheme Procedure} screen-locker-service @var{package} [@var{name}]
Add @var{package}, a package for a screen-locker or screen-saver whose
command is @var{program}, to the set of setuid programs and add a PAM entry
for it.  For example:

@lisp
(screen-locker-service xlockmore "xlock")
@end lisp

makes the good ol' XlockMore usable.
@end deffn


@node Desktop Services
@subsubsection Desktop Services

The @code{(gnu services desktop)} module provides services that are
usually useful in the context of a ``desktop'' setup---that is, on a
machine running a graphical display server, possibly with graphical user
interfaces, etc.

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}}), screen lockers,
a network management tool (@pxref{Networking
Services, @code{wicd-service}}), energy and color management services,
the @code{elogind} login and seat manager, the Polkit privilege service,
the GeoClue location service, an NTP client (@pxref{Networking
Services}), the Avahi daemon, and has the name service switch service
configured to be able to use @code{nss-mdns} (@pxref{Name Service
Switch, mDNS}).
@end defvr

The @var{%desktop-services} variable can be used as the @code{services}
field of an @code{operating-system} declaration (@pxref{operating-system
Reference, @code{services}}).

The actual service definitions provided by @code{(gnu services dbus)}
and @code{(gnu services desktop)} are described below.

@deffn {Scheme Procedure} dbus-service [#:dbus @var{dbus}] [#:services '()]
Return a service that runs the ``system bus'', using @var{dbus}, with
support for @var{services}.

@uref{http://dbus.freedesktop.org/, D-Bus} is an inter-process communication
facility.  Its system bus is used to allow system services to communicate
and be notified of system-wide events.

@var{services} must be a list of packages that provide an
@file{etc/dbus-1/system.d} directory containing additional D-Bus configuration
and policy files.  For example, to allow avahi-daemon to use the system bus,
@var{services} must be equal to @code{(list avahi)}.
@end deffn

@deffn {Scheme Procedure} elogind-service [#:config @var{config}]
Return a service that runs the @code{elogind} login and
seat management daemon.  @uref{https://github.com/andywingo/elogind,
Elogind} exposes a D-Bus interface that can be used to know which users
are logged in, know what kind of sessions they have open, suspend the
system, inhibit system suspend, reboot the system, and other tasks.

Elogind handles most system-level power events for a computer, for
example suspending the system when a lid is closed, or shutting it down
when the power button is pressed.

The @var{config} keyword argument specifies the configuration for
elogind, and should be the result of a @code{(elogind-configuration
(@var{parameter} @var{value})...)} invocation.  Available parameters and
their default values are:

@table @code
@item kill-user-processes?
@code{#f}
@item kill-only-users
@code{()}
@item kill-exclude-users
@code{("root")}
@item inhibit-delay-max-seconds
@code{5}
@item handle-power-key
@code{poweroff}
@item handle-suspend-key
@code{suspend}
@item handle-hibernate-key
@code{hibernate}
@item handle-lid-switch
@code{suspend}
@item handle-lid-switch-docked
@code{ignore}
@item power-key-ignore-inhibited?
@code{#f}
@item suspend-key-ignore-inhibited?
@code{#f}
@item hibernate-key-ignore-inhibited?
@code{#f}
@item lid-switch-ignore-inhibited?
@code{#t}
@item holdoff-timeout-seconds
@code{30}
@item idle-action
@code{ignore}
@item idle-action-seconds
@code{(* 30 60)}
@item runtime-directory-size-percent
@code{10}
@item runtime-directory-size
@code{#f}
@item remove-ipc?
@code{#t}
@item suspend-state
@code{("mem" "standby" "freeze")}
@item suspend-mode
@code{()}
@item hibernate-state
@code{("disk")}
@item hibernate-mode
@code{("platform" "shutdown")}
@item hybrid-sleep-state
@code{("disk")}
@item hybrid-sleep-mode
@code{("suspend" "platform" "shutdown")}
@end table
@end deffn

@deffn {Scheme Procedure} polkit-service @
                         [#:polkit @var{polkit}]
Return a service that runs the
@uref{http://www.freedesktop.org/wiki/Software/polkit/, Polkit privilege
management service}, which allows system administrators to grant access to
privileged operations in a structured way.  By querying the Polkit service, a
privileged system component can know when it should grant additional
capabilities to ordinary users.  For example, an ordinary user can be granted
the capability to suspend the system if the user is logged in locally.
@end deffn

@deffn {Scheme Procedure} upower-service [#:upower @var{upower}] @
                         [#:watts-up-pro? #f] @
                         [#:poll-batteries? #t] @
                         [#:ignore-lid? #f] @
                         [#:use-percentage-for-policy? #f] @
                         [#:percentage-low 10] @
                         [#:percentage-critical 3] @
                         [#:percentage-action 2] @
                         [#:time-low 1200] @
                         [#:time-critical 300] @
                         [#:time-action 120] @
                         [#:critical-power-action 'hybrid-sleep]
Return a service that runs @uref{http://upower.freedesktop.org/,
@command{upowerd}}, a system-wide monitor for power consumption and battery
levels, with the given configuration settings.  It implements the
@code{org.freedesktop.UPower} D-Bus interface, and is notably used by
GNOME.
@end deffn

@deffn {Scheme Procedure} udisks-service [#:udisks @var{udisks}]
Return a service for @uref{http://udisks.freedesktop.org/docs/latest/,
UDisks}, a @dfn{disk management} daemon that provides user interfaces with
notifications and ways to mount/unmount disks.  Programs that talk to UDisks
include the @command{udisksctl} command, part of UDisks, and GNOME Disks.
@end deffn

@deffn {Scheme Procedure} colord-service [#:colord @var{colord}]
Return a service that runs @command{colord}, a system service with a D-Bus
interface to manage the color profiles of input and output devices such as
screens and scanners.  It is notably used by the GNOME Color Manager graphical
tool.  See @uref{http://www.freedesktop.org/software/colord/, the colord web
site} for more information.
@end deffn

@deffn {Scheme Procedure} geoclue-application name [#:allowed? #t] [#:system? #f] [#:users '()]
Return an configuration allowing an application to access GeoClue
location data.  @var{name} is the Desktop ID of the application, without
the @code{.desktop} part.  If @var{allowed?} is true, the application
will have access to location information by default.  The boolean
@var{system?}  value indicates that an application is a system component
or not.  Finally @var{users} is a list of UIDs of all users for which
this application is allowed location info access.  An empty users list
means that all users are allowed.
@end deffn

@defvr {Scheme Variable} %standard-geoclue-applications
The standard list of well-known GeoClue application configurations,
granting authority to GNOME's date-and-time utility to ask for the
current location in order to set the time zone, and allowing the Firefox
(IceCat) and Epiphany web browsers to request location information.
Firefox and Epiphany both query the user before allowing a web page to
know the user's location.
@end defvr

@deffn {Scheme Procedure} geoclue-service [#:colord @var{colord}] @
                         [#:whitelist '()] @
                         [#:wifi-geolocation-url "https://location.services.mozilla.com/v1/geolocate?key=geoclue"] @
                         [#:submit-data? #f]
                         [#:wifi-submission-url "https://location.services.mozilla.com/v1/submit?key=geoclue"] @
                         [#:submission-nick "geoclue"] @
                         [#:applications %standard-geoclue-applications]
Return a service that runs the GeoClue location service.  This service
provides a D-Bus interface to allow applications to request access to a
user's physical location, and optionally to add information to online
location databases.  See
@uref{https://wiki.freedesktop.org/www/Software/GeoClue/, the GeoClue
web site} for more information.
@end deffn

@node Database Services
@subsubsection Database Services

The @code{(gnu services databases)} module provides the following service.

@deffn {Scheme Procedure} postgresql-service [#:postgresql postgresql] @
       [#:config-file] [#:data-directory ``/var/lib/postgresql/data'']
Return a service that runs @var{postgresql}, the PostgreSQL database
server.

The PostgreSQL daemon loads its runtime configuration from
@var{config-file} and stores the database cluster in
@var{data-directory}.
@end deffn

@node Web Services
@subsubsection Web Services

The @code{(gnu services web)} module provides the following service:

@deffn {Scheme Procedure} nginx-service [#:nginx nginx] @
       [#:log-directory ``/var/log/nginx''] @
       [#:run-directory ``/var/run/nginx''] @
       [#:config-file]

Return a service that runs @var{nginx}, the nginx web server.

The nginx daemon loads its runtime configuration from @var{config-file}.
Log files are written to @var{log-directory} and temporary runtime data
files are written to @var{run-directory}.  For proper operation, these
arguments should match what is in @var{config-file} to ensure that the
directories are created when the service is activated.

@end deffn

@node Various Services
@subsubsection Various Services

The @code{(gnu services lirc)} module provides the following service.

@deffn {Scheme Procedure} lirc-service [#:lirc lirc] @
       [#:device #f] [#:driver #f] [#:config-file #f] @
       [#:extra-options '()]
Return a service that runs @url{http://www.lirc.org,LIRC}, a daemon that
decodes infrared signals from remote controls.

Optionally, @var{device}, @var{driver} and @var{config-file}
(configuration file name) may be specified.  See @command{lircd} manual
for details.

Finally, @var{extra-options} is a list of additional command-line options
passed to @command{lircd}.
@end deffn


@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,
you also need to use @code{avahi-service} (@pxref{Networking Services,
@code{avahi-service}}), or @var{%desktop-services}, which includes it
(@pxref{Desktop Services}).  Doing this makes @code{nss-mdns} accessible
to the name service cache daemon (@pxref{Base Services,
@code{nscd-service}}).

For convenience, the following variables provide typical NSS
configurations.

@defvr {Scheme Variable} %default-nss
This is the default name service switch configuration, a
@code{name-service-switch} object.
@end defvr

@defvr {Scheme Variable} %mdns-host-lookup-nss
This is the name service switch configuration with support for host name
lookup over multicast DNS (mDNS) for host names ending in @code{.local}.
@end defvr

The reference for name service switch configuration is given below.  It
is a direct mapping of the 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 GuixSD.}.

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
@anchor{guix system 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

@item container
Return a script to run the operating system declared in @var{file}
within a container.  Containers are a set of lightweight isolation
mechanisms provided by the kernel Linux-libre.  Containers are
substantially less resource-demanding than full virtual machines since
the kernel, shared objects, and other resources can be shared with the
host system; this also means they provide thinner isolation.

Currently, the script must be run as root in order to support more than
a single user and group.  The container shares its store with the host
system.

As with the @code{vm} action (@pxref{guix system vm}), additional file
systems to be shared between the host and container can be specified
using the @option{--share} and @option{--expose} options:

@example
guix system container my-config.scm \
   --expose=$HOME --share=$HOME/tmp=/exchange
@end example

@quotation Note
This option requires Linux-libre 3.19 or newer.
@end quotation

@end table

@var{options} can contain any of the common build options 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 --derivation
@itemx -d
Return the derivation file name of the given operating system without
building anything.

@item --image-size=@var{size}
For the @code{vm-image} and @code{disk-image} actions, create an image
of the given @var{size}.  @var{size} may be a number of bytes, or it may
include a unit as a suffix (@pxref{Block size, size specifications,,
coreutils, GNU Coreutils}).

@item --on-error=@var{strategy}
Apply @var{strategy} when an error occurs when reading @var{file}.
@var{strategy} may be one of the following:

@table @code
@item nothing-special
Report the error concisely and exit.  This is the default strategy.

@item backtrace
Likewise, but also display a backtrace.

@item debug
Report the error and enter Guile's debugger.  From there, you can run
commands such as @code{,bt} to get a backtrace, @code{,locals} to
display local variable values, and more generally inspect the 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.

Once you have built, configured, re-configured, and re-re-configured
your GuixSD installation, you may find it useful to list the operating
system generations available on disk---and that you can choose from the
GRUB boot menu:

@table @code

@item list-generations
List a summary of each generation of the operating system available on
disk, in a human-readable way.  This is similar to the
@option{--list-generations} option of @command{guix package}
(@pxref{Invoking guix package}).

Optionally, one can specify a pattern, with the same syntax that is used
in @command{guix package --list-generations}, to restrict the list of
generations displayed.  For instance, the following command displays
generations up to 10-day old:

@example
$ guix system list-generations 10d
@end example

@end table

The @command{guix system} command has even more to offer!  The following
sub-commands allow you to visualize how your system services relate to
each other:

@anchor{system-extension-graph}
@table @code

@item extension-graph
Emit in Dot/Graphviz format to standard output the @dfn{service
extension graph} of the operating system defined in @var{file}
(@pxref{Service Composition}, for more information on service
extensions.)

The command:

@example
$ guix system extension-graph @var{file} | dot -Tpdf > services.pdf
@end example

produces a PDF file showing the extension relations among services.

@anchor{system-dmd-graph}
@item dmd-graph
Emit in Dot/Graphviz format to standard output the @dfn{dependency
graph} of dmd services of the operating system defined in @var{file}.
@xref{dmd Services}, for more information and for an example graph.

@end table


@node Defining Services
@subsection Defining Services

The previous sections show the available services and how one can combine
them in an @code{operating-system} declaration.  But how do we define
them in the first place?  And what is a service anyway?

@menu
* Service Composition::         The model for composing services.
* Service Types and Services::  Types and services.
* Service Reference::           API reference.
* dmd Services::                A particular type of service.
@end menu

@node Service Composition
@subsubsection Service Composition

@cindex services
@cindex daemons
Here we define a @dfn{service} as, broadly, something that extends the
operating system's functionality.  Often a service is a process---a
@dfn{daemon}---started when the system boots: a secure shell server, a
Web server, the Guix build daemon, etc.  Sometimes a service is a daemon
whose execution can be triggered by another daemon---e.g., an FTP server
started by @command{inetd} or a D-Bus service activated by
@command{dbus-daemon}.  Occasionally, a service does not map to a
daemon.  For instance, the ``account'' service collects user accounts
and makes sure they exist when the system runs; the ``udev'' service
collects device management rules and makes them available to the eudev
daemon; the @file{/etc} service populates the system's @file{/etc}
directory.

@cindex service extensions
GuixSD services are connected by @dfn{extensions}.  For instance, the
secure shell service @emph{extends} dmd---GuixSD's initialization system,
running as PID@tie{}1---by giving it the command lines to start and stop
the secure shell daemon (@pxref{Networking Services,
@code{lsh-service}}); the UPower service extends the D-Bus service by
passing it its @file{.service} specification, and extends the udev
service by passing it device management rules (@pxref{Desktop Services,
@code{upower-service}}); the Guix daemon service extends dmd by passing
it the command lines to start and stop the daemon, and extends the
account service by passing it a list of required build user accounts
(@pxref{Base Services}).

All in all, services and their ``extends'' relations form a directed
acyclic graph (DAG).  If we represent services as boxes and extensions
as arrows, a typical system might provide something like this:

@image{images/service-graph,,5in,Typical service extension graph.}

@cindex system service
At the bottom, we see the @dfn{system service}, which produces the
directory containing everything to run and boot the system, as returned
by the @command{guix system build} command.  @xref{Service Reference},
to learn about the other service types shown here.
@xref{system-extension-graph, the @command{guix system extension-graph}
command}, for information on how to generate this representation for a
particular operating system definition.

@cindex service types
Technically, developers can define @dfn{service types} to express these
relations.  There can be any number of services of a given type on the
system---for instance, a system running two instances of the GNU secure
shell server (lsh) has two instances of @var{lsh-service-type}, with
different parameters.

The following section describes the programming interface for service
types and services.

@node Service Types and Services
@subsubsection Service Types and Services

A @dfn{service type} is a node in the DAG described above.  Let us start
with a simple example, the service type for the Guix build daemon
(@pxref{Invoking guix-daemon}):

@example
(define guix-service-type
  (service-type
   (name 'guix)
   (extensions
    (list (service-extension dmd-root-service-type guix-dmd-service)
          (service-extension account-service-type guix-accounts)
          (service-extension activation-service-type guix-activation)))))
@end example

@noindent
It defines a two things:

@enumerate
@item
A name, whose sole purpose is to make inspection and debugging easier.

@item
A list of @dfn{service extensions}, where each extension designates the
target service type and a procedure that, given the service's
parameters, returns a list of object to extend the service of that type.

Every service type has at least one service extension.  The only
exception is the @dfn{boot service type}, which is the ultimate service.
@end enumerate

In this example, @var{guix-service-type} extends three services:

@table @var
@item dmd-root-service-type
The @var{guix-dmd-service} procedure defines how the dmd service is
extended.  Namely, it returns a @code{<dmd-service>} object that defines
how @command{guix-daemon} is started and stopped (@pxref{dmd Services}).

@item account-service-type
This extension for this service is computed by @var{guix-accounts},
which returns a list of @code{user-group} and @code{user-account}
objects representing the build user accounts (@pxref{Invoking
guix-daemon}).

@item activation-service-type
Here @var{guix-activation} is a procedure that returns a gexp, which is
a code snippet to run at ``activation time''---e.g., when the service is
booted.
@end table

A service of this type is instantiated like this:

@example
(service guix-service-type
         (guix-configuration
           (build-accounts 5)
           (use-substitutes? #f)))
@end example

The second argument to the @code{service} form is a value representing
the parameters of this specific service instance.
@xref{guix-configuration-type, @code{guix-configuration}}, for
information about the @code{guix-configuration} data type.

@var{guix-service-type} is quite simple because it extends other
services but is not extensible itself.

@c @subsubsubsection Extensible Service Types

The service type for an @emph{extensible} service looks like this:

@example
(define udev-service-type
  (service-type (name 'udev)
                (extensions
                 (list (service-extension dmd-root-service-type
                                          udev-dmd-service)))

                (compose concatenate)       ;concatenate the list of rules
                (extend (lambda (config rules)
                          (match config
                            (($ <udev-configuration> udev initial-rules)
                             (udev-configuration
                              (udev udev)   ;the udev package to use
                              (rules (append initial-rules rules)))))))))
@end example

This is the service type for the
@uref{https://wiki.gentoo.org/wiki/Project:Eudev, eudev device
management daemon}.  Compared to the previous example, in addition to an
extension of @var{dmd-root-service-type}, we see two new fields:

@table @code
@item compose
This is the procedure to @dfn{compose} the list of extensions to
services of this type.

Services can extend the udev service by passing it lists of rules; we
compose those extensions simply by concatenating them.

@item extend
This procedure defines how the service's value is @dfn{extended} with
the composition of the extensions.

Udev extensions are composed into a list of rules, but the udev service
value is itself a @code{<udev-configuration>} record.  So here, we
extend that record by appending the list of rules is contains to the
list of contributed rules.
@end table

There can be only one instance of an extensible service type such as
@var{udev-service-type}.  If there were more, the
@code{service-extension} specifications would be ambiguous.

Still here?  The next section provides a reference of the programming
interface for services.

@node Service Reference
@subsubsection Service Reference

We have seen an overview of service types (@pxref{Service Types and
Services}).  This section provides a reference on how to manipulate
services and service types.  This interface is provided by the
@code{(gnu services)} module.

@deffn {Scheme Procedure} service @var{type} @var{value}
Return a new service of @var{type}, a @code{<service-type>} object (see
below.)  @var{value} can be any object; it represents the parameters of
this particular service instance.
@end deffn

@deffn {Scheme Procedure} service? @var{obj}
Return true if @var{obj} is a service.
@end deffn

@deffn {Scheme Procedure} service-kind @var{service}
Return the type of @var{service}---i.e., a @code{<service-type>} object.
@end deffn

@deffn {Scheme Procedure} service-parameters @var{service}
Return the value associated with @var{service}.  It represents its
parameters.
@end deffn

Here is an example of how a service is created and manipulated:

@example
(define s
  (service nginx-service-type
           (nginx-configuration
            (nginx nginx)
            (log-directory log-directory)
            (run-directory run-directory)
            (file config-file))))

(service? s)
@result{} #t

(eq? (service-kind s) nginx-service-type)
@result{} #t
@end example

The @code{modify-services} form provides a handy way to change the
parameters of some of the services of a list such as
@var{%base-services} (@pxref{Base Services, @code{%base-services}}).  Of
course, you could always use standard list combinators such as
@code{map} and @code{fold} to do that (@pxref{SRFI-1, List Library,,
guile, GNU Guile Reference Manual}); @code{modify-services} simply
provides a more concise form for this common pattern.

@deffn {Scheme Syntax} modify-services @var{services} @
  (@var{type} @var{variable} => @var{body}) @dots{}

Modify the services listed in @var{services} according to the given
clauses.  Each clause has the form:

@example
(@var{type} @var{variable} => @var{body})
@end example

where @var{type} is a service type, such as @var{guix-service-type}, and
@var{variable} is an identifier that is bound within @var{body} to the
value of the service of that @var{type}.  @xref{Using the Configuration
System}, for an example.

This is a shorthand for:

@example
(map (lambda (service) @dots{}) @var{services})
@end example
@end deffn

Next comes the programming interface for service types.  This is
something you want to know when writing new service definitions, but not
necessarily when simply looking for ways to customize your
@code{operating-system} declaration.

@deftp {Data Type} service-type
@cindex service type
This is the representation of a @dfn{service type} (@pxref{Service Types
and Services}).

@table @asis
@item @code{name}
This is a symbol, used only to simplify inspection and debugging.

@item @code{extensions}
A non-empty list of @code{<service-extension>} objects (see below.)

@item @code{compose} (default: @code{#f})
If this is @code{#f}, then the service type denotes services that cannot
be extended---i.e., services that do not receive ``values'' from other
services.

Otherwise, it must be a one-argument procedure.  The procedure is called
by @code{fold-services} and is passed a list of values collected from
extensions.  It must return a value that is a valid parameter value for
the service instance.

@item @code{extend} (default: @code{#f})
If this is @code{#f}, services of this type cannot be extended.

Otherwise, it must be a two-argument procedure: @code{fold-services}
calls it, passing it the service's initial value as the first argument
and the result of applying @code{compose} to the extension values as the
second argument.
@end table

@xref{Service Types and Services}, for examples.
@end deftp

@deffn {Scheme Procedure} service-extension @var{target-type} @
                              @var{compute}
Return a new extension for services of type @var{target-type}.
@var{compute} must be a one-argument procedure: @code{fold-services}
calls it, passing it the value associated with the service that provides
the extension; it must return a valid value for the target service.
@end deffn

@deffn {Scheme Procedure} service-extension? @var{obj}
Return true if @var{obj} is a service extension.
@end deffn

At the core of the service abstraction lies the @code{fold-services}
procedure, which is responsible for ``compiling'' a list of services
down to a single directory that contains everything needed to boot and
run the system---the directory shown by the @command{guix system build}
command (@pxref{Invoking guix system}).  In essence, it propagates
service extensions down the service graph, updating each node parameters
on the way, until it reaches the root node.

@deffn {Scheme Procedure} fold-services @var{services} @
                            [#:target-type @var{system-service-type}]
Fold @var{services} by propagating their extensions down to the root of
type @var{target-type}; return the root service adjusted accordingly.
@end deffn

Lastly, the @code{(gnu services)} module also defines several essential
service types, some of which are listed below.

@defvr {Scheme Variable} system-service-type
This is the root of the service graph.  It produces the system directory
as returned by the @command{guix system build} command.
@end defvr

@defvr {Scheme Variable} boot-service-type
The type of the ``boot service'', which produces the @dfn{boot script}.
The boot script is what the initial RAM disk runs when booting.
@end defvr

@defvr {Scheme Variable} etc-service-type
The type of the @file{/etc} service.  This service can be extended by
passing it name/file tuples such as:

@example
(list `("issue" ,(plain-file "issue" "Welcome!\n")))
@end example

In this example, the effect would be to add an @file{/etc/issue} file
pointing to the given file.
@end defvr

@defvr {Scheme Variable} setuid-program-service-type
Type for the ``setuid-program service''.  This service collects lists of
executable file names, passed as gexps, and adds them to the set of
setuid-root programs on the system (@pxref{Setuid Programs}).
@end defvr

@defvr {Scheme Variable} profile-service-type
Type of the service that populates the @dfn{system profile}---i.e., the
programs under @file{/run/current-system/profile}.  Other services can
extend it by passing it lists of packages to add to the system profile.
@end defvr


@node dmd Services
@subsubsection dmd Services

@cindex PID 1
@cindex init system
The @code{(gnu services dmd)} provides a way to define services managed
by GNU@tie{}dmd, which is GuixSD initialization system---the first
process that is started when the system boots, aka. PID@tie{}1
(@pxref{Introduction,,, dmd, GNU dmd Manual}).

Services in dmd can depend on each other.  For instance, the SSH daemon
may need to be started after the syslog daemon has been started, which
in turn can only happen once all the file systems have been mounted.
The simple operating system defined earlier (@pxref{Using the
Configuration System}) results in a service graph like this:

@image{images/dmd-graph,,5in,Typical dmd service graph.}

You can actually generate such a graph for any operating system
definition using the @command{guix system dmd-graph} command
(@pxref{system-dmd-graph, @command{guix system dmd-graph}}).

The @var{%dmd-root-service} is a service object representing PID@tie{}1,
of type @var{dmd-root-service-type}; it can be extended by passing it
lists of @code{<dmd-service>} objects.

@deftp {Data Type} dmd-service
The data type representing a service managed by dmd.

@table @asis
@item @code{provision}
This is a list of symbols denoting what the service provides.

These are the names that may be passed to @command{deco start},
@command{deco status}, and similar commands (@pxref{Invoking deco,,,
dmd, GNU dmd Manual}).  @xref{Slots of services, the @code{provides}
slot,, dmd, GNU dmd Manual}, for details.

@item @code{requirements} (default: @code{'()})
List of symbols denoting the dmd services this one depends on.

@item @code{respawn?} (default: @code{#t})
Whether to restart the service when it stops, for instance when the
underlying process dies.

@item @code{start}
@itemx @code{stop} (default: @code{#~(const #f)})
The @code{start} and @code{stop} fields refer to dmd's facilities to
start and stop processes (@pxref{Service De- and Constructors,,, dmd,
GNU dmd Manual}).  They are given as G-expressions that get expanded in
the dmd configuration file (@pxref{G-Expressions}).

@item @code{documentation}
A documentation string, as shown when running:

@example
deco doc @var{service-name}
@end example

where @var{service-name} is one of the symbols in @var{provision}
(@pxref{Invoking deco,,, dmd, GNU dmd Manual}).

@item @code{modules} (default: @var{%default-modules})
This is the list of modules that must be in scope when @code{start} and
@code{stop} are evaluated.

@item @code{imported-modules} (default: @var{%default-imported-modules})
This is the list of modules to import in the execution environment of
dmd.

@end table
@end deftp

@defvr {Scheme Variable} dmd-root-service-type
The service type for the dmd ``root service''---i.e., PID@tie{}1.

This is the service type that extensions target when they want to create
dmd services (@pxref{Service Types and Services}, for an example).  Each
extension must pass a list of @code{<dmd-service>}.
@end defvr

@defvr {Scheme Variable} %dmd-root-service
This service represents PID@tie{}1.
@end defvr


@node Installing Debugging Files
@section Installing Debugging Files

@cindex debugging files
Program binaries, as produced by the GCC compilers for instance, are
typically written in the ELF format, with a section containing
@dfn{debugging information}.  Debugging information is what allows the
debugger, GDB, to map binary code to source code; it is required to
debug a compiled program in good conditions.

The problem with debugging information is that is takes up a fair amount
of disk space.  For example, debugging information for the GNU C Library
weighs in at more than 60 MiB.  Thus, as a user, keeping all the
debugging info of all the installed programs is usually not an option.
Yet, space savings should not come at the cost of an impediment to
debugging---especially in the GNU system, which should make it easier
for users to exert their computing freedom (@pxref{GNU Distribution}).

Thankfully, the GNU Binary Utilities (Binutils) and GDB provide a
mechanism that allows users to get the best of both worlds: debugging
information can be stripped from the binaries and stored in separate
files.  GDB is then able to load debugging information from those files,
when they are available (@pxref{Separate Debug Files,,, gdb, Debugging
with GDB}).

The GNU distribution takes advantage of this by storing debugging
information in the @code{lib/debug} sub-directory of a separate package
output unimaginatively called @code{debug} (@pxref{Packages with
Multiple Outputs}).  Users can choose to install the @code{debug} output
of a package when they need it.  For instance, the following command
installs the debugging information for the GNU C Library and for GNU
Guile:

@example
guix package -i glibc:debug guile:debug
@end example

GDB must then be told to look for debug files in the user's profile, by
setting the @code{debug-file-directory} variable (consider setting it
from the @file{~/.gdbinit} file, @pxref{Startup,,, gdb, Debugging with
GDB}):

@example
(gdb) set debug-file-directory ~/.guix-profile/lib/debug
@end example

From there on, GDB will pick up debugging information from the
@code{.debug} files under @file{~/.guix-profile/lib/debug}.

In addition, you will most likely want GDB to be able to show the source
code being debugged.  To do that, you will have to unpack the source
code of the package of interest (obtained with @code{guix build
--source}, @pxref{Invoking guix build}), and to point GDB to that source
directory using the @code{directory} command (@pxref{Source Path,
@code{directory},, gdb, Debugging with GDB}).

@c XXX: keep me up-to-date
The @code{debug} output mechanism in Guix is implemented by the
@code{gnu-build-system} (@pxref{Build Systems}).  Currently, it is
opt-in---debugging information is available only for 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.
* Synopses and Descriptions::   Helping users find the right package.
* 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 Synopses and Descriptions
@subsection Synopses and Descriptions

As we have seen before, each package in GNU@tie{}Guix includes a
synopsis and a description (@pxref{Defining Packages}).  Synopses and
descriptions are important: They are what @command{guix package
--search} searches, and a crucial piece of information to help users
determine whether a given package suits their needs.  Consequently,
packagers should pay attention to what goes into them.

Synopses must start with a capital letter and must not end with a
period.  They must not start with ``a'' or ``the'', which usually does
not bring anything; for instance, prefer ``File-frobbing tool'' over ``A
tool that frobs files''.  The synopsis should say what the package
is---e.g., ``Core GNU utilities (file, text, shell)''---or what it is
used for---e.g., the synopsis for GNU@tie{}grep is ``Print lines
matching a pattern''.

Keep in mind that the synopsis must be meaningful for a very wide
audience.  For example, ``Manipulate alignments in the SAM format''
might make sense for a seasoned bioinformatics researcher, but might be
fairly unhelpful or even misleading to a non-specialized audience.  It
is a good idea to come up with a synopsis that gives an idea of the
application domain of the package.  In this example, this might give
something like ``Manipulate nucleotide sequence alignments'', which
hopefully gives the user a better idea of whether this is what they are
looking for.

@cindex Texinfo markup, in package descriptions
Descriptions should take between five and ten lines.  Use full
sentences, and avoid using acronyms without first introducing them.
Descriptions can include Texinfo markup, which is useful to introduce
ornaments such as @code{@@code} or @code{@@dfn}, bullet lists, or
hyperlinks (@pxref{Overview,,, texinfo, GNU Texinfo}).  However you
should be careful when using some characters for example @samp{@@} and
curly braces which are the basic special characters in Texinfo
(@pxref{Special Characters,,, texinfo, GNU Texinfo}).  User interfaces
such as @command{guix package --show} take care of rendering it
appropriately.

Synopses and descriptions are translated by volunteers
@uref{http://translationproject.org/domain/guix-packages.html, at the
Translation Project} so that as many users as possible can read them in
their native language.  User interfaces search them and display them in
the language specified by the current locale.

Translation is a lot of work so, as a packager, please pay even more
attention to your synopses and descriptions as every change may entail
additional work for translators.  In order to help them, it is possible
to make recommendations or instructions visible to them by inserting
special comments like this (@pxref{xgettext Invocation,,, gettext, GNU
Gettext}):

@example
;; TRANSLATORS: "X11 resize-and-rotate" should not be translated.
(description "ARandR is designed to provide a simple visual front end
for the X11 resize-and-rotate (RandR) extension. @dots{}")
@end example


@node Python Modules
@subsection Python Modules

We currently package Python 2 and Python 3, under the Scheme variable names
@code{python-2} and @code{python} as explained in @ref{Version Numbers}.
To avoid confusion and naming clashes with other programming languages, it
seems desirable that the name of a package for a Python module contains
the word @code{python}.

Some modules are compatible with only one version of Python, others with both.
If the package Foo compiles only with Python 3, we name it
@code{python-foo}; if it compiles only with Python 2, we name it
@code{python2-foo}. If it is compatible with both versions, we create two
packages with the corresponding names.

If a project already contains the word @code{python}, we drop this;
for instance, the module python-dateutil is packaged under the names
@code{python-dateutil} and @code{python2-dateutil}.


@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 @uref{http://nixos.org/nix/, Nix package manager},
which was designed and
implemented by Eelco Dolstra, with contributions from other people (see
the @file{nix/AUTHORS} file in Guix.)  Nix pioneered functional package
management, and promoted unprecedented features, such as transactional
package upgrades and rollbacks, per-user profiles, and referentially
transparent build processes.  Without this work, Guix would not exist.

The Nix-based software distributions, Nixpkgs and NixOS, have also been
an inspiration for Guix.

GNU@tie{}Guix itself is a collective work with contributions from a
number of people.  See the @file{AUTHORS} file in Guix for more
information on these fine people.  The @file{THANKS} file lists people
who have helped by reporting bugs, taking care of the infrastructure,
providing artwork and themes, making suggestions, and more---thank you!


@c *********************************************************************
@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl-1.3.texi

@c *********************************************************************
@node Concept Index
@unnumbered Concept Index
@printindex cp

@node Programming Index
@unnumbered Programming Index
@syncodeindex tp fn
@syncodeindex vr fn
@printindex fn

@bye

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