guix.texi

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.  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.
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
@cindex substitutes, authorization thereof
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
@cindex 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

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

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

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

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

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

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

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

@item --free-space=@var{free}
@itemx -F @var{free}
Collect garbage until @var{free} space is available under
@file{/gnu/store}, if possible; @var{free} denotes storage space, such
as @code{500MiB}, as described above.

When @var{free} or more is already available in @file{/gnu/store}, do
nothing and exit immediately.

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

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

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

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

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

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

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

@end table

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

@table @code

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

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

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

@end table

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

@table @option

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

By default, make sure that all the store items marked as valid in the
database of the daemon actually exist in @file{/gnu/store}.

When provided, @var{options} must be a comma-separated list containing one
or more of @code{contents} and @code{repair}.

When passing @option{--verify=contents}, the daemon computes the
content hash of each store item and compares it against its hash in the
database.  Hash mismatches are reported as data corruptions.  Because it
traverses @emph{all the files in the store}, this command can take a
long time, especially on systems with a slow disk drive.

@cindex repairing the store
@cindex corruption, recovering from
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.  A lightweight alternative, when you know exactly
which items in the store are corrupt, is @command{guix build --repair}
(@pxref{Invoking guix build}).

@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}

@cindex upgrading Guix
@cindex updating Guix
@cindex @command{guix pull}
@cindex pull
Packages are installed or upgraded to the latest version available in
the distribution currently available on your local machine.  To update
that distribution, along with the Guix tools, you must run @command{guix
pull}: the command downloads the latest Guix source code and package
descriptions, and deploys it.

On completion, @command{guix package} will use packages and package
versions from this just-retrieved copy of Guix.  Not only that, but all
the Guix commands and Scheme modules will also be taken from that latest
version.  New @command{guix} sub-commands added by the update also
become available.

Any user can update their Guix copy using @command{guix pull}, and the
effect is limited to the user who run @command{guix pull}.  For
instance, when user @code{root} runs @command{guix pull}, this has no
effect on the version of Guix that user @code{alice} sees, and vice
versa@footnote{Under the hood, @command{guix pull} updates the
@file{~/.config/guix/latest} symbolic link to point to the latest Guix,
and the @command{guix} command loads code from there.  Currently, the
only way to roll back an invocation of @command{guix pull} is to
manually update this symlink to point to the previous Guix.}.

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.

With some Git servers, this can be used to deploy any version of Guix.
For example, to download and deploy version 0.12.0 of Guix from the
canonical Git repo:

@example
guix pull --url=https://git.savannah.gnu.org/cgit/guix.git/snapshot/v0.12.0.tar.gz
@end example

It can also be used to deploy arbitrary Git revisions:

@example
guix pull --url=https://git.savannah.gnu.org/cgit/guix.git/snapshot/74d862e8a.tar.gz
@end example

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

In addition, @command{guix pull} supports all the common build options
(@pxref{Common Build Options}).

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

Occasionally you want to pass software to people who are not (yet!)
lucky enough to be using Guix.  You'd tell them to run @command{guix
package -i @var{something}}, but that's not possible in this case.  This
is where @command{guix pack} comes in.

@cindex pack
@cindex bundle
@cindex application bundle
@cindex software bundle
The @command{guix pack} command creates a shrink-wrapped @dfn{pack} or
@dfn{software bundle}: it creates a tarball or some other archive
containing the binaries of the software you're interested in, and all
its dependencies.  The resulting archive can be used on any machine that
does not have Guix, and people can run the exact same binaries as those
you have with Guix.  The pack itself is created in a bit-reproducible
fashion, so anyone can verify that it really contains the build results
that you pretend to be shipping.

For example, to create a bundle containing Guile, Emacs, Geiser, and all
their dependencies, you can run:

@example
$ guix pack guile emacs geiser
@dots{}
/gnu/store/@dots{}-pack.tar.gz
@end example

The result here is a tarball containing a @file{/gnu/store} directory
with all the relevant packages.  The resulting tarball contains a
@dfn{profile} with the three packages of interest; the profile is the
same as would be created by @command{guix package -i}.  It is this
mechanism that is used to create Guix's own standalone binary tarball
(@pxref{Binary Installation}).

Users of this pack would have to run
@file{/gnu/store/@dots{}-profile/bin/guile} to run Guile, which you may
find inconvenient.  To work around it, you can create, say, a
@file{/opt/gnu/bin} symlink to the profile:

@example
guix pack -S /opt/gnu/bin=bin guile emacs geiser
@end example

@noindent
That way, users can happily type @file{/opt/gnu/bin/guile} and enjoy.

Alternatively, you can produce a pack in the Docker image format using
the following command:

@example
guix pack -f docker guile emacs geiser
@end example

@noindent
The result is a tarball that can be passed to the @command{docker load}
command.  See the
@uref{https://docs.docker.com/engine/reference/commandline/load/, Docker
documentation} for more information.

Several command-line options allow you to customize your pack:

@table @code
@item --format=@var{format}
@itemx -f @var{format}
Produce a pack in the given @var{format}.

The available formats are:

@table @code
@item tarball
This is the default format.  It produces a tarball containing all the
specifies binaries and symlinks.

@item docker
This produces a tarball that follows the
@uref{https://github.com/docker/docker/blob/master/image/spec/v1.2.md,
Docker Image Specification}.
@end table

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

This has the same purpose as the same-named option in @command{guix
build} (@pxref{Additional Build Options, @code{--expression} in
@command{guix build}}).

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

@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{Specifying target triplets, GNU
configuration triplets,, autoconf, Autoconf}).

@item --compression=@var{tool}
@itemx -C @var{tool}
Compress the resulting tarball using @var{tool}---one of @code{gzip},
@code{bzip2}, @code{xz}, or @code{lzip}.

@item --symlink=@var{spec}
@itemx -S @var{spec}
Add the symlinks specified by @var{spec} to the pack.  This option can
appear several times.

@var{spec} has the form @code{@var{source}=@var{target}}, where
@var{source} is the symlink that will be created and @var{target} is the
symlink target.

For instance, @code{-S /opt/gnu/bin=bin} creates a @file{/opt/gnu/bin}
symlink pointing to the @file{bin} sub-directory of the profile.

@item --localstatedir
Include the ``local state directory'', @file{/var/guix}, in the
resulting pack.

@file{/var/guix} contains the store database (@pxref{The Store}) as well
as garbage-collector roots (@pxref{Invoking guix gc}).  Providing it in
the pack means that the store is ``complete'' and manageable by Guix;
not providing it pack means that the store is ``dead'': items cannot be
added to it or removed from it after extraction of the pack.

One use case for this is the Guix self-contained binary tarball
(@pxref{Binary Installation}).
@end table

In addition, @command{guix pack} supports all the common build options
(@pxref{Common Build Options}) and all the package transformation
options (@pxref{Package Transformation Options}).


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

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

@cindex exporting store items
To export store files as an archive to standard output, run:

@example
guix archive --export @var{options} @var{specifications}...
@end example

@var{specifications} may be either store file names or package
specifications, as for @command{guix package} (@pxref{Invoking guix
package}).  For instance, the following command creates an archive
containing the @code{gui} output of the @code{git} package and the main
output of @code{emacs}:

@example
guix archive --export git:gui /gnu/store/...-emacs-24.3 > great.nar
@end example

If the specified packages are not built yet, @command{guix archive}
automatically builds them.  The build process may be controlled with the
common build options (@pxref{Common Build Options}).

To transfer the @code{emacs} package to a machine connected over SSH,
one would run:

@example
guix archive --export -r emacs | ssh the-machine guix archive --import
@end example

@noindent
Similarly, a complete user profile may be transferred from one machine
to another like this:

@example
guix archive --export -r $(readlink -f ~/.guix-profile) | \
  ssh the-machine guix-archive --import
@end example

@noindent
However, note that, in both examples, all of @code{emacs} and the
profile as well as all of their dependencies are transferred (due to
@code{-r}), regardless of what is already available in the store on the
target machine.  The @code{--missing} option can help figure out which
items are missing from the target store.  The @command{guix copy}
command simplifies and optimizes this whole process, so this is probably
what you should use in this case (@pxref{Invoking guix copy}).

@cindex nar, archive format
@cindex normalized archive (nar)
Archives are stored in the ``normalized archive'' or ``nar'' format, which is
comparable in spirit to `tar', but with differences
that make it more appropriate for our purposes.  First, rather than
recording all Unix metadata for each file, the nar format only mentions
the file type (regular, directory, or symbolic link); Unix permissions
and owner/group are dismissed.  Second, the order in which directory
entries are stored always follows the order of file names according to
the C locale collation order.  This makes archive production fully
deterministic.

When exporting, the daemon digitally signs the contents of the archive,
and that digital signature is appended.  When importing, the daemon
verifies the signature and rejects the import in case of an invalid
signature or if the signing key is not authorized.
@c FIXME: Add xref to daemon doc about signatures.

The main options are:

@table @code
@item --export
Export the specified store files or packages (see below.)  Write the
resulting archive to the standard output.

Dependencies are @emph{not} included in the output, unless
@code{--recursive} is passed.

@item -r
@itemx --recursive
When combined with @code{--export}, this instructs @command{guix
archive} to include dependencies of the given items in the archive.
Thus, the resulting archive is self-contained: it contains the closure
of the exported store items.

@item --import
Read an archive from the standard input, and import the files listed
therein into the store.  Abort if the archive has an invalid digital
signature, or if it is signed by a public key not among the authorized
keys (see @code{--authorize} below.)

@item --missing
Read a list of store file names from the standard input, one per line,
and write on the standard output the subset of these files missing from
the store.

@item --generate-key[=@var{parameters}]
@cindex signing, archives
Generate a new key pair for the daemon.  This is a prerequisite before
archives can be exported with @code{--export}.  Note that this operation
usually takes time, because it needs to gather enough entropy to
generate the key pair.

The generated key pair is typically stored under @file{/etc/guix}, in
@file{signing-key.pub} (public key) and @file{signing-key.sec} (private
key, which must be kept secret.)  When @var{parameters} is omitted,
an ECDSA key using the Ed25519 curve is generated, or, for Libgcrypt
versions before 1.6.0, it is a 4096-bit RSA key.
Alternatively, @var{parameters} can specify
@code{genkey} parameters suitable for Libgcrypt (@pxref{General
public-key related Functions, @code{gcry_pk_genkey},, gcrypt, The
Libgcrypt Reference Manual}).

@item --authorize
@cindex authorizing, archives
Authorize imports signed by the public key passed on standard input.
The public key must be in ``s-expression advanced format''---i.e., the
same format as the @file{signing-key.pub} file.

The list of authorized keys is kept in the human-editable file
@file{/etc/guix/acl}.  The file contains
@url{http://people.csail.mit.edu/rivest/Sexp.txt, ``advanced-format
s-expressions''} and is structured as an access-control list in the
@url{http://theworld.com/~cme/spki.txt, Simple Public-Key Infrastructure
(SPKI)}.

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

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

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

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

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

@end table

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

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

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

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

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

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

@node Defining Packages
@section Defining Packages

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

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

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

@noindent
Without being a Scheme expert, the reader may have guessed the meaning
of the various fields here.  This expression binds the variable
@code{hello} to a @code{<package>} object, which is essentially a record
(@pxref{SRFI-9, Scheme records,, guile, GNU Guile Reference Manual}).
This package object can be inspected using procedures found in the
@code{(guix packages)} module; for instance, @code{(package-name hello)}
returns---surprise!---@code{"hello"}.

With luck, you may be able to import part or all of the definition of
the package you are interested in from another repository, using the
@code{guix import} command (@pxref{Invoking guix import}).

In the example above, @var{hello} is defined in a module of its own,
@code{(gnu packages hello)}.  Technically, this is not strictly
necessary, but it is convenient to do so: all the packages defined in
modules under @code{(gnu packages @dots{})} are automatically known to
the command-line tools (@pxref{Package Modules}).

There are a few points worth noting in the above package definition:

@itemize
@item
The @code{source} field of the package is an @code{<origin>} object
(@pxref{origin Reference}, for the complete reference).
Here, the @code{url-fetch} method from @code{(guix download)} is used,
meaning that the source is a file to be downloaded over FTP or HTTP.

The @code{mirror://gnu} prefix instructs @code{url-fetch} to use one of
the GNU mirrors defined in @code{(guix download)}.

The @code{sha256} field specifies the expected SHA256 hash of the file
being downloaded.  It is mandatory, and allows Guix to check the
integrity of the file.  The @code{(base32 @dots{})} form introduces the
base32 representation of the hash.  You can obtain this information with
@code{guix download} (@pxref{Invoking guix download}) and @code{guix
hash} (@pxref{Invoking guix hash}).

@cindex patches
When needed, the @code{origin} form can also have a @code{patches} field
listing patches to be applied, and a @code{snippet} field giving a
Scheme expression to modify the source code.

@item
@cindex GNU Build System
The @code{build-system} field specifies the procedure to build the
package (@pxref{Build Systems}).  Here, @var{gnu-build-system}
represents the familiar GNU Build System, where packages may be
configured, built, and installed with the usual @code{./configure &&
make && make check && make install} command sequence.

@item
The @code{arguments} field specifies options for the build system
(@pxref{Build Systems}).  Here it is interpreted by
@var{gnu-build-system} as a request run @file{configure} with the
@code{--enable-silent-rules} flag.

@cindex quote
@cindex quoting
@findex '
@findex quote
What about these quote (@code{'}) characters?  They are Scheme syntax to
introduce a literal list; @code{'} is synonymous with @code{quote}.
@xref{Expression Syntax, quoting,, guile, GNU Guile Reference Manual},
for details.  Here the value of the @code{arguments} field is a list of
arguments passed to the build system down the road, as with @code{apply}
(@pxref{Fly Evaluation, @code{apply},, guile, GNU Guile Reference
Manual}).

The hash-colon (@code{#:}) sequence defines a Scheme @dfn{keyword}
(@pxref{Keywords,,, guile, GNU Guile Reference Manual}), and
@code{#:configure-flags} is a keyword used to pass a keyword argument
to the build system (@pxref{Coding With Keywords,,, guile, GNU Guile
Reference Manual}).

@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.

@cindex backquote (quasiquote)
@findex `
@findex quasiquote
@cindex comma (unquote)
@findex ,
@findex unquote
@findex ,@@
@findex unquote-splicing
Again, @code{`} (a backquote, synonymous with @code{quasiquote}) allows
us to introduce a literal list in the @code{inputs} field, while
@code{,} (a comma, synonymous with @code{unquote}) allows us to insert a
value in that list (@pxref{Expression Syntax, unquote,, guile, GNU Guile
Reference Manual}).

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}), troubleshooting any build failures
you encounter (@pxref{Debugging Build Failures}).  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.
@vindex GUIX_PACKAGE_PATH
Lastly, @pxref{Package Modules}, for information
on how to extend the distribution by adding your own package definitions
to @code{GUIX_PACKAGE_PATH}.

Finally, updating the package definition to a new upstream version
can be partly automated by the @command{guix refresh} command
(@pxref{Invoking guix refresh}).

Behind the scenes, a derivation corresponding to the @code{<package>}
object is first computed by the @code{package-derivation} procedure.
That derivation is stored in a @code{.drv} file under @file{/gnu/store}.
The build actions it prescribes may then be realized by using the
@code{build-derivations} procedure (@pxref{The Store}).

@deffn {Scheme Procedure} package-derivation @var{store} @var{package} [@var{system}]
Return the @code{<derivation>} object of @var{package} for @var{system}
(@pxref{Derivations}).

@var{package} must be a valid @code{<package>} object, and @var{system}
must be a string denoting the target system type---e.g.,
@code{"x86_64-linux"} for an x86_64 Linux-based GNU system.  @var{store}
must be a connection to the daemon, which operates on the store
(@pxref{The Store}).
@end deffn

@noindent
@cindex cross-compilation
Similarly, it is possible to compute a derivation that cross-builds a
package for some other system:

@deffn {Scheme Procedure} package-cross-derivation @var{store} @
            @var{package} @var{target} [@var{system}]
Return the @code{<derivation>} object of @var{package} cross-built from
@var{system} to @var{target}.

@var{target} must be a valid GNU triplet denoting the target hardware
and operating system, such as @code{"mips64el-linux-gnu"}
(@pxref{Configuration Names, GNU configuration triplets,, configure, GNU
Configure and Build System}).
@end deffn

@cindex package transformations
@cindex input rewriting
@cindex dependency tree rewriting
Packages can be manipulated in arbitrary ways.  An example of a useful
transformation is @dfn{input rewriting}, whereby the dependency tree of
a package is rewritten by replacing specific inputs by others:

@deffn {Scheme Procedure} package-input-rewriting @var{replacements} @
           [@var{rewrite-name}]
Return a procedure that, when passed a package, replaces its direct and
indirect dependencies (but not its implicit inputs) according to
@var{replacements}.  @var{replacements} is a list of package pairs; the