guix.texi

@menu
* Defining Packages::           Defining new packages.
* The Store::                   Manipulating the package store.
* Derivations::                 Low-level interface to package derivations.
* The Store Monad::             Purely functional interface to the store.
@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
(use-modules (guix packages)
             (guix download)
             (guix build-system gnu)
             (guix licenses))

(define hello
  (package
    (name "hello")
    (version "2.8")
    (source (origin
             (method url-fetch)
             (uri (string-append "mirror://gnu/hello/hello-" version
                                 ".tar.gz"))
             (sha256
              (base32 "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))))
    (build-system gnu-build-system)
    (inputs `(("gawk" ,gawk)))
    (synopsis "GNU Hello")
    (description "Yeah...")
    (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 @var{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"}.

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.
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 is set to @var{gnu-build-system}.  The
@var{gnu-build-system} variable is defined in the @code{(guix
build-system gnu)} module, and is bound to a @code{<build-system>}
object.

Naturally, @var{gnu-build-system} represents the familiar GNU Build
System, and variants thereof (@pxref{Configuration, configuration and
makefile conventions,, standards, GNU Coding Standards}).  In a
nutshell, packages using the GNU Build System may be configured, built,
and installed with the usual @code{./configure && make && make check &&
make install} command sequence.  This is what @var{gnu-build-system}
does.

In addition, @var{gnu-build-system} ensures that the ``standard''
environment for GNU packages is available.  This includes tools such as
GCC, Coreutils, Bash, Make, Diffutils, and Patch.

@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.

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

There are other fields that package definitions may provide.  Of
particular interest is the @code{arguments} field.  When specified, it
must be bound to a list of additional arguments to be passed to the
build system.  For instance, the above definition could be augmented
with the following field initializer:

@example
    (arguments `(#:tests? #f
                 #:configure-flags '("--enable-silent-rules")))
@end example

@noindent
These are keyword arguments (@pxref{Optional Arguments, keyword
arguments in Guile,, guile, GNU Guile Reference Manual}).  They are
passed to @var{gnu-build-system}, which interprets them as meaning ``do
not run @code{make check}'', and ``run @file{configure} with the
@code{--enable-silent-rules} flag''.  The value of these keyword
parameters is actually evaluated in the @dfn{build stratum}---i.e., by a
Guile process launched by the daemon (@pxref{Derivations}).

Once a package definition is in place@footnote{Simple package
definitions like the one above may be automatically converted from the
Nixpkgs distribution using the @command{guix import} command.}, the
package may actually be built using the @code{guix build} command-line
tool (@pxref{Invoking guix build}).  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{/nix/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


@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{/nix/store}.
Sub-directories in the store are referred to as @dfn{store paths}.  The
store has an associated database that contains information such has 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] [#:hash-mode #f] [#:inputs '()] [#:env-vars '()] [#:system (%current-system)] [#:references-graphs #f]
Build a derivation with the given arguments, and return the resulting
@code{<derivation>} object.

When @var{hash}, @var{hash-algo}, and @var{hash-mode} are given, a
@dfn{fixed-output derivation} is created---i.e., one whose result is
known in advance, such as a file download.

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.
@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)
              #:env-vars '(("HOME" . "/homeless"))))
@result{} #<derivation /nix/store/@dots{}-foo.drv => /nix/store/@dots{}-foo>
@end lisp

As can be guessed, this primitive is cumbersome to use directly.  An
improved variant is @code{build-expression->derivation}, which allows
the caller to directly pass a Guile expression as the build script:

@deffn {Scheme Procedure} build-expression->derivation @var{store} @
       @var{name} @var{exp} @
       [#:system (%current-system)] [#:inputs '()] @
       [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
       [#:env-vars '()] [#:modules '()] @
       [#:references-graphs #f] [#: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}.
@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 /nix/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 /nix/store/@dots{}-goo.drv => @dots{}>
@end lisp

@cindex strata of code
Remember that the build expression passed to
@code{build-expression->derivation} is run by a separate Guile process
than the one that calls @code{build-expression->derivation}: it is run
by a Guile process launched by the daemon, typically in a chroot.  So,
while there is a single language for both the @dfn{host} and the build
side, there are really two @dfn{strata} of code: the host-side, and the
build-side code@footnote{The term @dfn{stratum} in this context was
coined by Manuel Serrano et al. in the context of their work on Hop.}.
This distinction is important to keep in mind, notably when using
higher-level constructs such as @var{gnu-build-system} (@pxref{Defining
Packages}).  For this reason, Guix modules that are meant to be used in
the build stratum are kept in the @code{(guix build @dots{})} name
space.

@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 includes 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 (profile.sh store)
  ;; Return the name of a shell script in the store that
  ;; initializes the 'PATH' environment variable.
  (let* ((drv (package-derivation store coreutils))
         (out (derivation->output-path drv)))
    (add-text-to-store store "profile.sh"
                       (format #f "export PATH=~a/bin" out))))
@end example

Using @code{(guix monads)}, it may be rewritten as a monadic function:

@example
(define (profile.sh)
  ;; Same, but return a monadic value.
  (mlet %store-monad ((bin (package-file coreutils "bin")))
    (text-file "profile.sh"
               (string-append "export PATH=" bin))))
@end example

There are two things to note in the second version: the @code{store}
parameter is now implicit, and the monadic value returned by
@code{package-file}---a wrapper around @code{package-derivation} and
@code{derivation->output-path}---is @dfn{bound} using @code{mlet}
instead of plain @code{let}.

Calling the monadic @code{profile.sh} has no effect.  To get the desired
effect, one must use @code{run-with-store}:

@example
(run-with-store (open-connection) (profile.sh))
@result{} /nix/store/...-profile.sh
@end example

The main syntactic forms to deal with monads in general 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
procedure @var{mproc}@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.}.
@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

The interface to the store monad provided by @code{(guix monads)} is as
follows.

@defvr {Scheme Variable} %store-monad
The store 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}
Return as a monadic value the absolute file name in the store of the file
containing @var{text}.
@end deffn

@deffn {Monadic Procedure} package-file @var{package} [@var{file}] @
       [#:system (%current-system)] [#: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}.
@end deffn

@deffn {Monadic Procedure} derivation-expression @var{name} @var{exp} @
       [#:system (%current-system)] [#:inputs '()] @
       [#:outputs '("out")] [#:hash #f] @
       [#:hash-algo #f] [#:env-vars '()] [#:modules '()] @
       [#:references-graphs #f] [#:guile-for-build #f]
Monadic version of @code{build-expression->derivation}
(@pxref{Derivations}).
@end deffn

@deffn {Monadic Procedure} package->derivation @var{package} [@var{system}]
Monadic version of @code{package-derivation} (@pxref{Defining
Packages}).
@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 download::      Downloading a file and printing its hash.
* Invoking guix hash::          Computing the cryptographic hash of a file.
* Invoking guix refresh::       Updating package definitions.
@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{/nix/store/@dots{}-coreutils-8.19.drv}.  Alternatively, the
@code{--expression} option may be used to specify a Scheme expression
that evaluates to a package; this is useful when disambiguation among
several same-named packages or package variants is needed.

The @var{options} may be zero or more of the following:

@table @code

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

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

Alternately, @var{expr} may 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{/nix/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 --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 --derivations
@itemx -d
Return the derivation paths, not the output paths, of the given
packages.

@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 --no-substitutes
Do not use substitutes for build products.  That is, always build things
locally instead of allowing downloads of pre-built binaries.

@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 --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 --root=@var{file}
@itemx -r @var{file}
Make @var{file} a symlink to the result, and register it as a garbage
collector root.

@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 --log-file
Return the build log file names for the given
@var{package-or-derivation}s, or raise an error if build logs are
missing.

This works regardless of how packages or derivations are specified.  For
instance, the following invocations are equivalent:

@example
guix build --log-file `guix build -d guile`
guix build --log-file `guix build guile`
guix build --log-file guile
guix build --log-file -e '(@@ (gnu packages guile) guile-2.0)'
@end example


@end table

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
store)} module.

@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.

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}, @ref{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.

@end table

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

The primary audience of the @command{guix refresh} command is developers
of the GNU software distribution.  By default, it reports any packages
provided by the distribution that are outdated compared to the latest
upstream version, like this:

@example
$ guix refresh
gnu/packages/gettext.scm:29:13: gettext would be upgraded from 0.18.1.1 to 0.18.2.1
gnu/packages/glib.scm:77:12: glib would be upgraded from 2.34.3 to 2.37.0
@end example

It does so by browsing each package's FTP directory and determining the
highest version number of the source tarballs
therein@footnote{Currently, this only works for GNU packages.}.

When passed @code{--update}, it modifies distribution source files to
update the version numbers and source tarball hashes of those packages'
recipes (@pxref{Defining Packages}).  This is achieved by downloading
each package's latest source tarball and its associated OpenPGP
signature, authenticating the downloaded tarball against its signature
using @command{gpg}, and finally computing its hash.  When the public
key used to sign the tarball is missing from the user's keyring, an
attempt is made to automatically retrieve it from a public key server;
when it's successful, the key is added to the user's keyring; otherwise,
@command{guix refresh} reports an error.

The following options are supported:

@table @code

@item --update
@itemx -u
Update distribution source files (package recipes) in place.
@ref{Defining Packages}, for more information on package definitions.

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

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

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

@end table

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

@example
guix refresh -u emacs idutils
@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.

The following options can be used to customize GnuPG operation:

@table @code

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

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

@end table


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

Guix comes with a distribution 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}.} that forms the basis of the GNU system.  This
includes 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/package-list.html,on-line} or by
running @command{guix package} (@pxref{Invoking guix package}):

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

Our goal is to build 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.

The GNU distribution is 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 mips64el-linux
little-endian 64-bit MIPS processors, specifically the Loongson series,
n32 application binary interface (ABI), and Linux-Libre kernel.

@end table

@noindent
For information on porting to other architectures or kernels,
@xref{Porting}.

@menu
* Installing Debugging Files::  Feeding the debugger.
* 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::        Configuring a GNU system.
@end menu

Building this distribution is a cooperative effort, and you are invited
to join!  @ref{Contributing}, for information about how you can help.


@node Installing Debugging Files
@section Installing 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 -i 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}.

@c XXX: keep me up-to-date
The @code{debug} output mechanism in Guix is implemented by the
@code{gnu-build-system} (@pxref{Defining Packages}).  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 Package Modules
@section Package Modules

From a programming viewpoint, the package definitions of the
distribution are provided by Guile modules in the @code{(gnu packages
...)} name space (@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)} module provides facilities for searching for packages.

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,
@ref{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,
@ref{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:

@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.

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/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 (except when using
@code{--no-substitutes}).  The only place where human intervention is
needed is to review and apply the patch.


@menu
* Software Freedom::     What may go into the distribution.
* Package Naming::       What's in a name?
* Version Numbers::      When the name is not enough.
* Python Modules::       Taming the snake.
@end menu

@node Software Freedom
@subsection Software Freedom