guix.texi

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 locale data of the
host system.  Instead, you must first install one of the locale packages
available with Guix and then define the @code{GUIX_LOCPATH} environment
variable:

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

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

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

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

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

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

@subsection X11 Fonts

The majority of graphical applications use Fontconfig to locate and
load fonts and perform X11-client-side rendering.  The @code{fontconfig}
package in Guix looks for fonts in @file{$HOME/.guix-profile}
by default.  Thus, to allow graphical applications installed with Guix
to display fonts, you have to install fonts with Guix as well.
Essential font packages include @code{gs-fonts}, @code{font-dejavu}, and
@code{font-gnu-freefont-ttf}.

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

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

@subsection Emacs Packages

When you install Emacs packages with Guix, the elisp files may be placed
either in @file{$HOME/.guix-profile/share/emacs/site-lisp/} or in
sub-directories of
@file{$HOME/.guix-profile/share/emacs/site-lisp/guix.d/}.  The latter
directory exists because potentially there may exist thousands of Emacs
packages and storing all their files in a single directory may be not
reliable (because of name conflicts).  So we think using a separate
directory for each package is a good idea.  It is very similar to how
the Emacs package system organizes the file structure (@pxref{Package
Files,,, emacs, The GNU Emacs Manual}).

By default, Emacs (installed with Guix) ``knows'' where these packages
are placed, so you do not need to perform any configuration.  If, for
some reason, you want to avoid auto-loading Emacs packages installed
with Guix, you can do so by running Emacs with @code{--no-site-file}
option (@pxref{Init File,,, emacs, The GNU Emacs Manual}).

@c TODO What else?

@c *********************************************************************
@node Package Management
@chapter Package Management

The purpose of GNU Guix is to allow users to easily install, upgrade, and
remove software packages, without having to know about their build
procedures or dependencies.  Guix also goes beyond this obvious set of
features.

This chapter describes the main features of Guix, as well as the package
management tools it provides.  Two user interfaces are provided for
routine package management tasks: A command-line interface described below
(@pxref{Invoking guix package, @code{guix package}}), as well as a visual user
interface in Emacs described in a subsequent chapter (@pxref{Emacs Interface}).

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

@node Features
@section Features

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

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

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

The @command{guix package} command is the central tool to manage
packages (@pxref{Invoking guix package}).  It operates on the per-user
profiles, and can be used @emph{with normal user privileges}.

The command provides the obvious install, remove, and upgrade
operations.  Each invocation is actually a @emph{transaction}: either
the specified operation succeeds, or nothing happens.  Thus, if the
@command{guix package} process is terminated during the transaction,
or if a power outage occurs during the transaction, then the user's
profile remains in its previous state, and remains usable.

In addition, any package transaction may be @emph{rolled back}.  So, if,
for example, an upgrade installs a new version of a package that turns
out to have a serious bug, users may roll back to the previous instance
of their profile, which was known to work well.  Similarly, the global
system configuration on GuixSD is subject to
transactional upgrades and roll-back
(@pxref{Using the Configuration System}).

All packages in the package store may be @emph{garbage-collected}.
Guix can determine which packages are still referenced by user
profiles, and remove those that are provably no longer referenced
(@pxref{Invoking guix gc}).  Users may also explicitly remove old
generations of their profile so that the packages they refer to can be
collected.

@cindex reproducibility
@cindex reproducible builds
Finally, Guix takes a @dfn{purely functional} approach to package
management, as described in the introduction (@pxref{Introduction}).
Each @file{/gnu/store} package directory name contains a hash of all the
inputs that were used to build that package---compiler, libraries, build
scripts, etc.  This direct correspondence allows users to make sure a
given package installation matches the current state of their
distribution.  It also helps maximize @dfn{build reproducibility}:
thanks to the isolated build environments that are used, a given build
is likely to yield bit-identical files when performed on different
machines (@pxref{Invoking guix-daemon, container}).

@cindex substitutes
This foundation allows Guix to support @dfn{transparent binary/source
deployment}.  When a pre-built binary for a @file{/gnu/store} item is
available from an external source---a @dfn{substitute}, Guix just
downloads it and unpacks it;
otherwise, it builds the package from source, locally
(@pxref{Substitutes}).  Because build results are usually bit-for-bit
reproducible, users do not have to trust servers that provide
substitutes: they can force a local build and @emph{challenge} providers
(@pxref{Invoking guix challenge}).

Control over the build environment is a feature that is also useful for
developers.  The @command{guix environment} command allows developers of
a package to quickly set up the right development environment for their
package, without having to manually install the dependencies of the
package into their profile (@pxref{Invoking guix environment}).

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

The @command{guix package} command is the tool that allows users to
install, upgrade, and remove packages, as well as rolling back to
previous configurations.  It operates only on the user's own profile,
and works with normal user privileges (@pxref{Features}).  Its syntax
is:

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

Primarily, @var{options} specifies the operations to be performed during
the transaction.  Upon completion, a new profile is created, but
previous @dfn{generations} of the profile remain available, should the user
want to roll back.

For example, to remove @code{lua} and install @code{guile} and
@code{guile-cairo} in a single transaction:

@example
guix package -r lua -i guile guile-cairo
@end example

@command{guix package} also supports a @dfn{declarative approach}
whereby the user specifies the exact set of packages to be available and
passes it @i{via} the @option{--manifest} option
(@pxref{profile-manifest, @option{--manifest}}).

For each user, a symlink to the user's default profile is automatically
created in @file{$HOME/.guix-profile}.  This symlink always points to the
current generation of the user's default profile.  Thus, users can add
@file{$HOME/.guix-profile/bin} to their @code{PATH} environment
variable, and so on.
@cindex search paths
If you are not using the Guix System Distribution, consider adding the
following lines to your @file{~/.bash_profile} (@pxref{Bash Startup
Files,,, bash, The GNU Bash Reference Manual}) so that newly-spawned
shells get all the right environment variable definitions:

@example
GUIX_PROFILE="$HOME/.guix-profile" \
source "$HOME/.guix-profile/etc/profile"
@end example

In a multi-user setup, user profiles are stored in a place registered as
a @dfn{garbage-collector root}, which @file{$HOME/.guix-profile} points
to (@pxref{Invoking guix gc}).  That directory is normally
@code{@var{localstatedir}/profiles/per-user/@var{user}}, where
@var{localstatedir} is the value passed to @code{configure} as
@code{--localstatedir}, and @var{user} is the user name.  The
@file{per-user} directory is created when @command{guix-daemon} is
started, and the @var{user} sub-directory is created by @command{guix
package}.

The @var{options} can be among the following:

@table @code

@item --install=@var{package} @dots{}
@itemx -i @var{package} @dots{}
Install the specified @var{package}s.

Each @var{package} may specify either a simple package name, such as
@code{guile}, or a package name followed by an at-sign and version number,
such as @code{guile@@1.8.8} or simply @code{guile@@1.8} (in the latter
case, the newest version prefixed by @code{1.8} is selected.)

If no version number is specified, the
newest available version will be selected.  In addition, @var{package}
may contain a colon, followed by the name of one of the outputs of the
package, as in @code{gcc:doc} or @code{binutils@@2.22:lib}
(@pxref{Packages with Multiple Outputs}).  Packages with a corresponding
name (and optionally version) are searched for among the GNU
distribution modules (@pxref{Package Modules}).

@cindex propagated inputs
Sometimes packages have @dfn{propagated inputs}: these are dependencies
that automatically get installed along with the required package
(@pxref{package-propagated-inputs, @code{propagated-inputs} in
@code{package} objects}, for information about propagated inputs in
package definitions).

@anchor{package-cmd-propagated-inputs}
An example is the GNU MPC library: its C header files refer to those of
the GNU MPFR library, which in turn refer to those of the GMP library.
Thus, when installing MPC, the MPFR and GMP libraries also get installed
in the profile; removing MPC also removes MPFR and GMP---unless they had
also been explicitly installed by the user.

Besides, packages sometimes rely on the definition of environment
variables for their search paths (see explanation of
@code{--search-paths} below).  Any missing or possibly incorrect
environment variable definitions are reported here.

@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{guix.scm} file
in the root of their project source tree that can be used to test
development snapshots and create reproducible development environments
(@pxref{Invoking guix environment}).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

After having rolled back, installing, removing, or upgrading packages
overwrites previous future generations.  Thus, the history of the
generations in a profile is always linear.

@item --switch-generation=@var{pattern}
@itemx -S @var{pattern}
Switch to a particular generation defined by @var{pattern}.

@var{pattern} may be either a generation number or a number prefixed
with ``+'' or ``-''.  The latter means: move forward/backward by a
specified number of generations.  For example, if you want to return to
the latest generation after @code{--roll-back}, use
@code{--switch-generation=+1}.

The difference between @code{--roll-back} and
@code{--switch-generation=-1} is that @code{--switch-generation} will
not make a zeroth generation, so if a specified generation does not
exist, the current generation will not be changed.

@item --search-paths[=@var{kind}]
@cindex search paths
Report environment variable definitions, in Bash syntax, that may be
needed in order to use the set of installed packages.  These environment
variables are used to specify @dfn{search paths} for files used by some
of the installed packages.

For example, GCC needs the @code{CPATH} and @code{LIBRARY_PATH}
environment variables to be defined so it can look for headers and
libraries in the user's profile (@pxref{Environment Variables,,, gcc,
Using the GNU Compiler Collection (GCC)}).  If GCC and, say, the C
library are installed in the profile, then @code{--search-paths} will
suggest setting these variables to @code{@var{profile}/include} and
@code{@var{profile}/lib}, respectively.

The typical use case is to define these environment variables in the
shell:

@example
$ eval `guix package --search-paths`
@end example

@var{kind} may be one of @code{exact}, @code{prefix}, or @code{suffix},
meaning that the returned environment variable definitions will either
be exact settings, or prefixes or suffixes of the current value of these
variables.  When omitted, @var{kind} defaults to @code{exact}.

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

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

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


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

@item --verbose
Produce verbose output.  In particular, emit the build log of the
environment on the standard error port.

@item --bootstrap
Use the bootstrap Guile to build the profile.  This option is only
useful to distribution developers.

@end table

In addition to these actions, @command{guix package} supports the
following options to query the current state of a profile, or the
availability of packages:

@table @option

@item --search=@var{regexp}
@itemx -s @var{regexp}
@cindex searching for packages
List the available packages whose name, synopsis, or description matches
@var{regexp}.  Print all the metadata of matching packages in
@code{recutils} format (@pxref{Top, GNU recutils databases,, recutils,
GNU recutils manual}).

This allows specific fields to be extracted using the @command{recsel}
command, for instance:

@example
$ guix package -s malloc | recsel -p name,version
name: glibc
version: 2.17

name: libgc
version: 7.2alpha6
@end example

Similarly, to show the name of all the packages available under the
terms of the GNU@tie{}LGPL version 3:

@example
$ guix package -s "" | recsel -p name -e 'license ~ "LGPL 3"'
name: elfutils

name: gmp
@dots{}
@end example

It is also possible to refine search results using several @code{-s}
flags.  For example, the following command returns a list of board
games:

@example
$ guix package -s '\<board\>' -s game | recsel -p name
name: gnubg
@dots{}
@end example

If we were to omit @code{-s game}, we would also get software packages
that deal with printed circuit boards; removing the angle brackets
around @code{board} would further add packages that have to do with
keyboards.

And now for a more elaborate example.  The following command searches
for cryptographic libraries, filters out Haskell, Perl, Python, and Ruby
libraries, and prints the name and synopsis of the matching packages:

@example
$ guix package -s crypto -s library | \
    recsel -e '! (name ~ "^(ghc|perl|python|ruby)")' -p name,synopsis
@end example

@noindent
@xref{Selection Expressions,,, recutils, GNU recutils manual}, for more
information on @dfn{selection expressions} for @code{recsel -e}.

@item --show=@var{package}
Show details about @var{package}, taken from the list of available packages, in
@code{recutils} format (@pxref{Top, GNU recutils databases,, recutils, GNU
recutils manual}).

@example
$ guix package --show=python | recsel -p name,version
name: python
version: 2.7.6

name: python
version: 3.3.5
@end example

You may also specify the full name of a package to only get details about a
specific version of it:
@example
$ guix package --show=python-3.3.5 | recsel -p name,version
name: python
version: 3.3.5
@end example



@item --list-installed[=@var{regexp}]
@itemx -I [@var{regexp}]
List the currently installed packages in the specified profile, with the
most recently installed packages shown last.  When @var{regexp} is
specified, list only installed packages whose name matches @var{regexp}.

For each installed package, print the following items, separated by
tabs: the package name, its version string, the part of the package that
is installed (for instance, @code{out} for the default output,
@code{include} for its headers, etc.), and the path of this package in
the store.

@item --list-available[=@var{regexp}]
@itemx -A [@var{regexp}]
List packages currently available in the distribution for this system
(@pxref{GNU Distribution}).  When @var{regexp} is specified, list only
installed packages whose name matches @var{regexp}.

For each package, print the following items separated by tabs: its name,
its version string, the parts of the package (@pxref{Packages with
Multiple Outputs}), and the source location of its definition.

@item --list-generations[=@var{pattern}]
@itemx -l [@var{pattern}]
Return a list of generations along with their creation dates; for each
generation, show the installed packages, with the most recently
installed packages shown last.  Note that the zeroth generation is never
shown.

For each installed package, print the following items, separated by
tabs: the name of a package, its version string, the part of the package
that is installed (@pxref{Packages with Multiple Outputs}), and the
location of this package in the store.

When @var{pattern} is used, the command returns only matching
generations.  Valid patterns include:

@itemize
@item @emph{Integers and comma-separated integers}.  Both patterns denote
generation numbers.  For instance, @code{--list-generations=1} returns
the first one.

And @code{--list-generations=1,8,2} outputs three generations in the
specified order.  Neither spaces nor trailing commas are allowed.

@item @emph{Ranges}.  @code{--list-generations=2..9} prints the
specified generations and everything in between.  Note that the start of
a range must be smaller than its end.

It is also possible to omit the endpoint.  For example,
@code{--list-generations=2..}, returns all generations starting from the
second one.

@item @emph{Durations}.  You can also get the last @emph{N}@tie{}days, weeks,
or months by passing an integer along with the first letter of the
duration.  For example, @code{--list-generations=20d} lists generations
that are up to 20 days old.
@end itemize

@item --delete-generations[=@var{pattern}]
@itemx -d [@var{pattern}]
When @var{pattern} is omitted, delete all generations except the current
one.

This command accepts the same patterns as @option{--list-generations}.
When @var{pattern} is specified, delete the matching generations.  When
@var{pattern} specifies a duration, generations @emph{older} than the
specified duration match.  For instance, @code{--delete-generations=1m}
deletes generations that are more than one month old.

If the current generation matches, it is @emph{not} deleted.  Also, the
zeroth generation is never deleted.

Note that deleting generations prevents rolling back to them.
Consequently, this command must be used with care.

@end table

Finally, since @command{guix package} may actually start build
processes, it supports all the common build options (@pxref{Common Build
Options}).  It also supports package transformation options, such as
@option{--with-source} (@pxref{Package Transformation Options}).
However, note that package transformations are lost when upgrading; to
preserve transformations across upgrades, you should define your own
package variant in a Guile module and add it to @code{GUIX_PACKAGE_PATH}
(@pxref{Defining Packages}).


@node Substitutes
@section Substitutes

@cindex substitutes
@cindex pre-built binaries
Guix supports transparent source/binary deployment, which means that it
can either build things locally, or download pre-built items from a
server.  We call these pre-built items @dfn{substitutes}---they are
substitutes for local build results.  In many cases, downloading a
substitute is much faster than building things locally.

Substitutes can be anything resulting from a derivation build
(@pxref{Derivations}).  Of course, in the common case, they are
pre-built package binaries, but source tarballs, for instance, which
also result from derivation builds, can be available as substitutes.

The @code{hydra.gnu.org} server is a front-end to a build farm that
builds packages from the GNU distribution continuously for some
architectures, and makes them available as substitutes (@pxref{Emacs
Hydra}, for information on how to query the continuous integration
server).  This is the
default source of substitutes; it can be overridden by passing the
@option{--substitute-urls} option either to @command{guix-daemon}
(@pxref{daemon-substitute-urls,, @code{guix-daemon --substitute-urls}})
or to client tools such as @command{guix package}
(@pxref{client-substitute-urls,, client @option{--substitute-urls}
option}).

Substitute URLs can be either HTTP or HTTPS@footnote{For HTTPS access,
the Guile bindings of GnuTLS must be installed.  @xref{Requirements}.}
HTTPS is recommended because communications are encrypted; conversely,
using HTTP makes all communications visible to an eavesdropper, who
could use the information gathered to determine, for instance, whether
your system has unpatched security vulnerabilities.

@cindex security
@cindex digital signatures
To allow Guix to download substitutes from @code{hydra.gnu.org} or a
mirror thereof, you
must add its public key to the access control list (ACL) of archive
imports, using the @command{guix archive} command (@pxref{Invoking guix
archive}).  Doing so implies that you trust @code{hydra.gnu.org} to not
be compromised and to serve genuine substitutes.

This public key is installed along with Guix, in
@code{@var{prefix}/share/guix/hydra.gnu.org.pub}, where @var{prefix} is
the installation prefix of Guix.  If you installed Guix from source,
make sure you checked the GPG signature of
@file{guix-@value{VERSION}.tar.gz}, which contains this public key file.
Then, you can run something like this:

@example
# guix archive --authorize < hydra.gnu.org.pub
@end example

Once this is in place, the output of a command like @code{guix build}
should change from something like:

@example
$ guix build emacs --dry-run
The following derivations would be built:
   /gnu/store/yr7bnx8xwcayd6j95r2clmkdl1qh688w-emacs-24.3.drv
   /gnu/store/x8qsh1hlhgjx6cwsjyvybnfv2i37z23w-dbus-1.6.4.tar.gz.drv
   /gnu/store/1ixwp12fl950d15h2cj11c73733jay0z-alsa-lib-1.0.27.1.tar.bz2.drv
   /gnu/store/nlma1pw0p603fpfiqy7kn4zm105r5dmw-util-linux-2.21.drv
@dots{}
@end example

@noindent
to something like:

@example
$ guix build emacs --dry-run
The following files would be downloaded:
   /gnu/store/pk3n22lbq6ydamyymqkkz7i69wiwjiwi-emacs-24.3
   /gnu/store/2ygn4ncnhrpr61rssa6z0d9x22si0va3-libjpeg-8d
   /gnu/store/71yz6lgx4dazma9dwn2mcjxaah9w77jq-cairo-1.12.16
   /gnu/store/7zdhgp0n1518lvfn8mb96sxqfmvqrl7v-libxrender-0.9.7
@dots{}
@end example

@noindent
This indicates that substitutes from @code{hydra.gnu.org} are usable and
will be downloaded, when possible, for future builds.

Guix ignores substitutes that are not signed, or that are not signed by
one of the keys listed in the ACL.  It also detects and raises an error
when attempting to use a substitute that has been tampered with.

@vindex http_proxy
Substitutes are downloaded over HTTP or HTTPS.
The @code{http_proxy} environment
variable can be set in the environment of @command{guix-daemon} and is
honored for downloads of substitutes.  Note that the value of
@code{http_proxy} in the environment where @command{guix build},
@command{guix package}, and other client commands are run has
@emph{absolutely no effect}.

When using HTTPS, the server's X.509 certificate is @emph{not} validated
(in other words, the server is not authenticated), contrary to what
HTTPS clients such as Web browsers usually do.  This is because Guix
authenticates substitute information itself, as explained above, which
is what we care about (whereas X.509 certificates are about
authenticating bindings between domain names and public keys.)

The substitute mechanism can be disabled globally by running
@code{guix-daemon} with @code{--no-substitutes} (@pxref{Invoking
guix-daemon}).  It can also be disabled temporarily by passing the
@code{--no-substitutes} option to @command{guix package}, @command{guix
build}, and other command-line tools.


@unnumberedsubsec On Trusting Binaries

Today, each individual's control over their own computing is at the
mercy of institutions, corporations, and groups with enough power and
determination to subvert the computing infrastructure and exploit its
weaknesses.  While using @code{hydra.gnu.org} substitutes can be
convenient, we encourage users to also build on their own, or even run
their own build farm, such that @code{hydra.gnu.org} is less of an
interesting target.  One way to help is by publishing the software you
build using @command{guix publish} so that others have one more choice
of server to download substitutes from (@pxref{Invoking guix publish}).

Guix has the foundations to maximize build reproducibility
(@pxref{Features}).  In most cases, independent builds of a given
package or derivation should yield bit-identical results.  Thus, through
a diverse set of independent package builds, we can strengthen the
integrity of our systems.  The @command{guix challenge} command aims to
help users assess substitute servers, and to assist developers in
finding out about non-deterministic package builds (@pxref{Invoking guix
challenge}).  Similarly, the @option{--check} option of @command{guix
build} allows users to check whether previously-installed substitutes
are genuine by rebuilding them locally (@pxref{build-check,
@command{guix build --check}}).

In the future, we want Guix to have support to publish and retrieve
binaries to/from other users, in a peer-to-peer fashion.  If you would
like to discuss this project, join us on @email{guix-devel@@gnu.org}.


@node Packages with Multiple Outputs
@section Packages with Multiple Outputs

@cindex multiple-output packages
@cindex package outputs

Often, packages defined in Guix have a single @dfn{output}---i.e., the
source package leads to exactly one directory in the store.  When running
@command{guix package -i glibc}, one installs the default output of the
GNU libc package; the default output is called @code{out}, but its name
can be omitted as shown in this command.  In this particular case, the
default output of @code{glibc} contains all the C header files, shared
libraries, static libraries, Info documentation, and other supporting
files.

Sometimes it is more appropriate to separate the various types of files
produced from a single source package into separate outputs.  For
instance, the GLib C library (used by GTK+ and related packages)
installs more than 20 MiB of reference documentation as HTML pages.
To save space for users who do not need it, the documentation goes to a
separate output, called @code{doc}.  To install the main GLib output,
which contains everything but the documentation, one would run:

@example
guix package -i glib
@end example

The command to install its documentation is:

@example
guix package -i glib:doc
@end example

Some packages install programs with different ``dependency footprints''.
For instance, the WordNet package installs both command-line tools and
graphical user interfaces (GUIs).  The former depend solely on the C
library, whereas the latter depend on Tcl/Tk and the underlying X
libraries.  In this case, we leave the command-line tools in the default
output, whereas the GUIs are in a separate output.  This allows users
who do not need the GUIs to save space.  The @command{guix size} command
can help find out about such situations (@pxref{Invoking guix size}).
@command{guix graph} can also be helpful (@pxref{Invoking guix graph}).

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


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

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

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

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

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

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

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

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

@item --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: