path: root/doc/guix-cookbook.texi

\input texinfo
@c -*-texinfo-*-

@c %**start of header
@setfilename guix-cookbook.info
@documentencoding UTF-8
@settitle GNU Guix Cookbook
@c %**end of header

@copying
Copyright @copyright{} 2019 Ricardo Wurmus@*
Copyright @copyright{} 2019 Efraim Flashner@*
Copyright @copyright{} 2019 Pierre Neidhardt@*

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.
@end copying

@dircategory System administration
@direntry
* Guix cookbook: (guix-cookbook).    Tutorials and examples for GNU Guix.
@end direntry

@titlepage
@title GNU Guix Cookbook
@subtitle Tutorials and examples for using the GNU Guix Functional Package Manager
@author The GNU Guix Developers

@page
@vskip 0pt plus 1filll

@insertcopying
@end titlepage

@contents

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

This document presents tutorials and detailed examples for GNU@tie{}Guix, a
functional package management tool written for the GNU system.  Please
@pxref{Top,,, guix, GNU Guix reference manual} for details about the system,
its API, and related concepts.

@c TRANSLATORS: You can replace the following paragraph with information on
@c how to join your own translation team and how to report issues with the
@c translation.
If you would like to translate this document in your native language, consider
joining the @uref{https://translationproject.org/domain/guix-cookbook.html,
Translation Project}.

@menu
* Scheme tutorials::            Meet your new favorite language!
* Packaging::                   Packaging tutorials
* System Configuration::        Customizing the GNU System
* Advanced package management:: Power to the users!

* Acknowledgments::             Thanks!
* GNU Free Documentation License::  The license of this document.
* Concept Index::               Concepts.

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

Scheme tutorials

* A Scheme Crash Course::       Learn the basics of Scheme

Packaging

* Packaging Tutorial::          Let's add a package to Guix!

System Configuration

* Customizing the Kernel::      Creating and using a custom Linux kernel


@end detailmenu
@end menu

@c *********************************************************************
@node Scheme tutorials
@chapter Scheme tutorials

GNU@tie{}Guix is written in the general purpose programming language Scheme,
and many of its features can be accessed and manipulated programmatically.
You can use Scheme to generate package definitions, to modify them, to build
them, to deploy whole operating systems, etc.

Knowing the basics of how to program in Scheme will unlock many of the
advanced features Guix provides --- and you don't even need to be an
experienced programmer to use them!

Let's get started!

@node A Scheme Crash Course
@section A Scheme Crash Course

@cindex Scheme, crash course

Guix uses the Guile implementation of Scheme.  To start playing with the
language, install it with @code{guix install guile} and start a
@uref{https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop,
REPL} by running @code{guile} from the command line.

Alternatively you can also run @code{guix environment --ad-hoc guile -- guile}
if you'd rather not have Guile installed in your user profile.

In the following examples we use the @code{>} symbol to denote the REPL
prompt, that is, the line reserved for user input.  @xref{Using Guile
Interactively,,, guile, GNU Guile Reference Manual}) for more details on the
REPL.

@itemize
@item
Scheme syntax boils down to a tree of expressions (or @emph{s-expression} in
Lisp lingo).  An expression can be a literal such as numbers and strings, or a
compound which is a parenthesized list of compounds and literals.  @code{#t}
and @code{#f} stand for the Booleans ``true'' and ``false'', respectively.

Examples of valid expressions:

@lisp
> "Hello World!"
"Hello World!"
> 17
17
> (display (string-append "Hello " "Guix" "\n"))
"Hello Guix!"
@end lisp

@item
This last example is a function call nested in another function call.  When a
parenthesized expression is evaluated, the first term is the function and the
rest are the arguments passed to the function.  Every function returns the
last evaluated expression as its return value.

@item
Anonymous functions are declared with the @code{lambda} term:

@lisp
> (lambda (x) (* x x))
#<procedure 120e348 at <unknown port>:24:0 (x)>
@end lisp

The above procedure returns the square of its argument.  Since everything is
an expression, the @code{lambda} expression returns an anonymous procedure,
which can in turn be applied to an argument:

@lisp
> ((lambda (x) (* x x)) 3)
9
@end lisp

@item
Anything can be assigned a global name with @code{define}:

@lisp
> (define a 3)
> (define square (lambda (x) (* x x)))
> (square a)
9
@end lisp

@item
Procedures can be defined more concisely with the following syntax:

@lisp
(define (square x) (* x x))
@end lisp

@item
A list structure can be created with the @code{list} procedure:

@lisp
> (list 2 a 5 7)
(2 3 5 7)
@end lisp

@item
The @emph{quote} disables evaluation of a parenthesized expression: the first
term is not called over the other terms.  Thus it effectively returns a list
of terms.

@lisp
> '(display (string-append "Hello " "Guix" "\n"))
(display (string-append "Hello " "Guix" "\n"))
> '(2 a 5 7)
(2 a 5 7)
@end lisp

@item
The @emph{quasiquote} disables evaluation of a parenthesized expression until
a comma re-enables it.  Thus it provides us with fine-grained control over
what is evaluated and what is not.

@lisp
> `(2 a 5 7 (2 ,a 5 ,(+ a 4)))
(2 a 5 7 (2 3 5 7))
@end lisp

Note that the above result is a list of mixed elements: numbers, symbols (here
@code{a}) and the last element is a list itself.

@item
Multiple variables can be named locally with @code{let}:

@lisp
> (define x 10)
> (let ((x 2)
        (y 3))
    (list x y))
(2 3)
> x
10
> y
ERROR: In procedure module-lookup: Unbound variable: y
@end lisp

Use @code{let*} to allow later variable declarations to refer to earlier
definitions.

@lisp
> (let* ((x 2)
         (y (* x 3)))
    (list x y))
(2 6)
@end lisp

@item
The keyword syntax is @code{#:}; it is used to create unique identifiers.
@pxref{Keywords,,, guile, GNU Guile Reference Manual}.

@item
The percentage @code{%} is typically used for read-only global variables in
the build stage.  Note that it is merely a convention, like @code{_} in C.
Scheme treats @code{%} exactly the same as any other letter.

@item
Modules are created with @code{define-module}.  For instance

@lisp
(define-module (guix build-system ruby)
  #:use-module (guix store)
  #:export (ruby-build
            ruby-build-system))
@end lisp

defines the module @code{guix build-system ruby} which must be located in
@file{guix/build-system/ruby.scm} somewhere in the Guile load path.  It
depends on the @code{(guix store)} module and it exports two variables,
@code{ruby-build} and @code{ruby-build-system}.
@end itemize

For a more detailed introduction, check out
@uref{http://www.troubleshooters.com/codecorn/scheme_guile/hello.htm, Scheme
at a Glance}, by Steve Litt.

One of the reference Scheme books is the seminal ``Structure and
Interpretation of Computer Programs'', by Harold Abelson and Gerald Jay
Sussman, with Julie Sussman.  You'll find a
@uref{https://mitpress.mit.edu/sites/default/files/sicp/index.html, free copy
online}, together with
@uref{https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-001-structure-and-interpretation-of-computer-programs-spring-2005/video-lectures/,
videos of the lectures by the authors}.  The book is available in Texinfo
format as the @code{sicp} Guix package.  Go ahead, run @code{guix install
sicp} and start reading with @code{info sicp} (@pxref{,,, sicp, Structure and Interpretation of Computer Programs}).
An @uref{https://sarabander.github.io/sicp/, unofficial ebook is also
available}.

You'll find more books, tutorials and other resources at
@url{https://schemers.org/}.


@c *********************************************************************
@node Packaging
@chapter Packaging

@cindex packaging

This chapter is dedicated to teaching you how to add packages to the
collection of packages that come with GNU Guix.  This involves writing package
definitions in Guile Scheme, organizing them in package modules, and building
them.

@menu
* Packaging Tutorial::         A tutorial on how to add packages to Guix.
@end menu

@node Packaging Tutorial
@section Packaging Tutorial

GNU Guix stands out as the @emph{hackable} package manager, mostly because it
uses @uref{https://www.gnu.org/software/guile/, GNU Guile}, a powerful
high-level programming language, one of the
@uref{https://en.wikipedia.org/wiki/Scheme_%28programming_language%29, Scheme}
dialects from the
@uref{https://en.wikipedia.org/wiki/Lisp_%28programming_language%29, Lisp family}.

Package definitions are also written in Scheme, which empowers Guix in some
very unique ways, unlike most other package managers that use shell scripts or
simple languages.

@itemize
@item
Use functions, structures, macros and all of Scheme expressiveness for your
package definitions.

@item
Inheritance makes it easy to customize a package by inheriting from it and
modifying only what is needed.
 
@item
Batch processing: the whole package collection can be parsed, filtered and
processed.  Building a headless server with all graphical interfaces stripped
out?  It's possible.  Want to rebuild everything from source using specific
compiler optimization flags?  Pass the @code{#:make-flags "..."} argument to
the list of packages.  It wouldn't be a stretch to think
@uref{https://wiki.gentoo.org/wiki/USE_flag, Gentoo USE flags} here, but this
goes even further: the changes don't have to be thought out beforehand by the
packager, they can be @emph{programmed} by the user!
@end itemize

The following tutorial covers all the basics around package creation with Guix.
It does not assume much knowledge of the Guix system nor of the Lisp language.
The reader is only expected to be familiar with the command line and to have some
basic programming knowledge.

@node A ``Hello World'' package
@subsection A ``Hello World'' package

The ``Defining Packages'' section of the manual introduces the basics of Guix
packaging (@pxref{Defining Packages,,, guix, GNU Guix Reference Manual}).  In
the following section, we will partly go over those basics again.

GNU@tie{}Hello is a dummy project that serves as an idiomatic example for
packaging.  It uses the GNU build system (@code{./configure && make && make
install}).  Guix already provides a package definition which is a perfect
example to start with.  You can look up its declaration with @code{guix edit
hello} from the command line.  Let's see how it looks:

@lisp
(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)
    (synopsis "Hello, GNU world: An example GNU package")
    (description
     "GNU Hello prints the message \"Hello, world!\" and then exits.  It
serves as an example of standard GNU coding practices.  As such, it supports
command-line arguments, multiple languages, and so on.")
    (home-page "https://www.gnu.org/software/hello/")
    (license gpl3+)))
@end lisp

As you can see, most of it is rather straightforward.  But let's review the
fields together:

@table @samp
@item name
The project name.  Using Scheme conventions, we prefer to keep it
lower case, without underscore and using dash-separated words.

@item source
This field contains a description of the source code origin.  The
@code{origin} record contains these fields:

@enumerate
@item  The method, here @code{url-fetch} to download via HTTP/FTP, but other methods
    exist, such as @code{git-fetch} for Git repositories.
@item  The URI, which is typically some @code{https://} location for @code{url-fetch}.  Here
    the special `mirror://gnu` refers to a set of well known locations, all of
    which can be used by Guix to fetch the source, should some of them fail.
@item  The @code{sha256} checksum of the requested file.  This is essential to ensure
    the source is not corrupted.  Note that Guix works with base32 strings,
    hence the call to the @code{base32} function.
@end enumerate

@item build-system

This is where the power of abstraction provided by the Scheme language really
shines: in this case, the @code{gnu-build-system} abstracts away the famous
@code{./configure && make && make install} shell invocations.  Other build
systems include the @code{trivial-build-system} which does not do anything and
requires from the packager to program all the build steps, the
@code{python-build-system}, the @code{emacs-build-system}, and many more
(@pxref{Build Systems,,, guix, GNU Guix Reference Manual}).

@item synopsis
It should be a concise summary of what the package does.  For many packages a
tagline from the project's home page can be used as the synopsis.

@item description
Same as for the synopsis, it's fine to re-use the project description from the
homepage.  Note that Guix uses Texinfo syntax.

@item home-page
Use HTTPS if available.

@item license
See @code{guix/licenses.scm} in the project source for a full list of
available licenses.
@end table

Time to build our first package!  Nothing fancy here for now: we will stick to a
dummy @code{my-hello}, a copy of the above declaration.

As with the ritualistic ``Hello World'' taught with most programming languages,
this will possibly be the most ``manual'' approach.  We will work out an ideal
setup later; for now we will go the simplest route.

Save the following to a file @file{my-hello.scm}.

@lisp
(use-modules (guix packages)
             (guix download)
             (guix build-system gnu)
             (guix licenses))

(package
  (name "my-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)
  (synopsis "Hello, Guix world: An example custom Guix package")
  (description
   "GNU Hello prints the message \"Hello, world!\" and then exits.  It
serves as an example of standard GNU coding practices.  As such, it supports
command-line arguments, multiple languages, and so on.")
  (home-page "https://www.gnu.org/software/hello/")
  (license gpl3+))
@end lisp

We will explain the extra code in a moment.

Feel free to play with the different values of the various fields.  If you
change the source, you'll need to update the checksum.  Indeed, Guix refuses to
build anything if the given checksum does not match the computed checksum of the
source code.  To obtain the correct checksum of the package declaration, we
need to download the source, compute the sha256 checksum and convert it to
base32.

Thankfully, Guix can automate this task for us; all we need is to provide the
URI:

@c TRANSLATORS: This is example shell output.
@example sh
$ guix download mirror://gnu/hello/hello-2.10.tar.gz

Starting download of /tmp/guix-file.JLYgL7
From https://ftpmirror.gnu.org/gnu/hello/hello-2.10.tar.gz...
following redirection to `https://mirror.ibcp.fr/pub/gnu/hello/hello-2.10.tar.gz'...
 …10.tar.gz  709KiB                                 2.5MiB/s 00:00 [##################] 100.0%
/gnu/store/hbdalsf5lpf01x4dcknwx6xbn6n5km6k-hello-2.10.tar.gz
0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i
@end example

In this specific case the output tells us which mirror was chosen.
If the result of the above command is not the same as in the above snippet,
update your @code{my-hello} declaration accordingly.

Note that GNU package tarballs come with an OpenPGP signature, so you
should definitely check the signature of this tarball with `gpg` to
authenticate it before going further:

@c TRANSLATORS: This is example shell output.
@example sh
$ guix download mirror://gnu/hello/hello-2.10.tar.gz.sig

Starting download of /tmp/guix-file.03tFfb
From https://ftpmirror.gnu.org/gnu/hello/hello-2.10.tar.gz.sig...
following redirection to `https://ftp.igh.cnrs.fr/pub/gnu/hello/hello-2.10.tar.gz.sig'...
 ….tar.gz.sig  819B                                                                                                                       1.2MiB/s 00:00 [##################] 100.0%
/gnu/store/rzs8wba9ka7grrmgcpfyxvs58mly0sx6-hello-2.10.tar.gz.sig
0q0v86n3y38z17rl146gdakw9xc4mcscpk8dscs412j22glrv9jf
$ gpg --verify /gnu/store/rzs8wba9ka7grrmgcpfyxvs58mly0sx6-hello-2.10.tar.gz.sig /gnu/store/hbdalsf5lpf01x4dcknwx6xbn6n5km6k-hello-2.10.tar.gz
gpg: Signature made Sun 16 Nov 2014 01:08:37 PM CET
gpg:                using RSA key A9553245FDE9B739
gpg: Good signature from "Sami Kerola <kerolasa@@iki.fi>" [unknown]
gpg:                 aka "Sami Kerola (http://www.iki.fi/kerolasa/) <kerolasa@@iki.fi>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 8ED3 96E3 7E38 D471 A005  30D3 A955 3245 FDE9 B739
@end example

You can then happily run

@c TRANSLATORS: Do not translate this command
@example sh
$ guix package --install-from-file=my-hello.scm
@end example

You should now have @code{my-hello} in your profile!

@c TRANSLATORS: Do not translate this command
@example sh
$ guix package --list-installed=my-hello
my-hello	2.10	out
/gnu/store/f1db2mfm8syb8qvc357c53slbvf1g9m9-my-hello-2.10
@end example

We've gone as far as we could without any knowledge of Scheme.  Before moving
on to more complex packages, now is the right time to brush up on your Scheme
knowledge.  @pxref{A Scheme Crash Course} to get up to speed.

@node Setup
@subsection Setup

In the rest of this chapter we will rely on some basic Scheme
programming knowledge.  Now let's detail the different possible setups
for working on Guix packages.

There are several ways to set up a Guix packaging environment.

We recommend you work directly on the Guix source checkout since it makes it
easier for everyone to contribute to the project.

But first, let's look at other possibilities.

@node Local file
@subsubsection Local file

This is what we previously did with @samp{my-hello}.  With the Scheme basics we've
covered, we are now able to explain the leading chunks.  As stated in @code{guix
package --help}:

@example
  -f, --install-from-file=FILE
                         install the package that the code within FILE
                         evaluates to
@end example

Thus the last expression @emph{must} return a package, which is the case in our
earlier example.

The @code{use-modules} expression tells which of the modules we need in the file.
Modules are a collection of values and procedures.  They are commonly called
``libraries'' or ``packages'' in other programming languages.

@node @samp{GUIX_PACKAGE_PATH}
@subsubsection @samp{GUIX_PACKAGE_PATH}

@emph{Note: Starting from Guix 0.16, the more flexible Guix @dfn{channels} are the
preferred way and supersede @samp{GUIX_PACKAGE_PATH}.  See next section.}

It can be tedious to specify the file from the command line instead of simply
calling @code{guix package --install my-hello} as you would do with the official
packages.

Guix makes it possible to streamline the process by adding as many ``package
declaration directories'' as you want.

Create a directory, say @samp{~./guix-packages} and add it to the @samp{GUIX_PACKAGE_PATH}
environment variable:

@example
$ mkdir ~/guix-packages
$ export GUIX_PACKAGE_PATH=~/guix-packages
@end example

To add several directories, separate them with a colon (@code{:}).

Our previous @samp{my-hello} needs some adjustments though:

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

(define-public my-hello
  (package
    (name "my-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)
    (synopsis "Hello, Guix world: An example custom Guix package")
    (description
     "GNU Hello prints the message \"Hello, world!\" and then exits.  It
serves as an example of standard GNU coding practices.  As such, it supports
command-line arguments, multiple languages, and so on.")
    (home-page "https://www.gnu.org/software/hello/")
    (license gpl3+)))
@end lisp

Note that we have assigned the package value to an exported variable name with
@code{define-public}.  This is effectively assigning the package to the @code{my-hello}
variable so that it can be referenced, among other as dependency of other
packages.

If you use @code{guix package --install-from-file=my-hello.scm} on the above file, it
will fail because the last expression, @code{define-public}, does not return a
package.  If you want to use @code{define-public} in this use-case nonetheless, make
sure the file ends with an evaluation of @code{my-hello}:

@lisp
; ...
(define-public my-hello
  ; ...
  )

my-hello
@end lisp

This last example is not very typical.

Now @samp{my-hello} should be part of the package collection like all other official
packages.  You can verify this with:

@example
$ guix package --show=my-hello
@end example

@node Guix channels
@subsubsection Guix channels

Guix 0.16 features channels, which is very similar to @samp{GUIX_PACKAGE_PATH} but
provides better integration and provenance tracking.  Channels are not
necessarily local, they can be maintained as a public Git repository for
instance.  Of course, several channels can be used at the same time.

@xref{Channels,,, guix, GNU Guix Reference Manual} for setup details.

@node Direct checkout hacking
@subsubsection Direct checkout hacking

Working directly on the Guix project is recommended: it reduces the friction
when the time comes to submit your changes upstream to let the community benefit
from your hard work!

Unlike most software distributions, the Guix repository holds in one place both
the tooling (including the package manager) and the package definitions.  This
choice was made so that it would give developers the flexibility to modify the
API without breakage by updating all packages at the same time.  This reduces
development inertia.

Check out the official @uref{https://git-scm.com/, Git} repository:

@example
$ git clone https://git.savannah.gnu.org/git/guix.git
@end example

In the rest of this article, we use @samp{$GUIX_CHECKOUT} to refer to the location of
the checkout.


Follow the instructions in the manual (@pxref{Contributing,,, guix, GNU Guix
Reference Manual}) to set up the repository environment.

Once ready, you should be able to use the package definitions from the
repository environment.

Feel free to edit package definitions found in @samp{$GUIX_CHECKOUT/gnu/packages}.

The @samp{$GUIX_CHECKOUT/pre-inst-env} script lets you use @samp{guix} over the package
collection of the repository (@pxref{Running Guix Before It Is
Installed,,, guix, GNU Guix Reference Manual}).

@itemize
@item
Search packages, such as Ruby:

@example
  $ cd $GUIX_CHECKOUT
  $ ./pre-inst-env guix package --list-available=ruby
      ruby    1.8.7-p374      out     gnu/packages/ruby.scm:119:2
      ruby    2.1.6   out     gnu/packages/ruby.scm:91:2
      ruby    2.2.2   out     gnu/packages/ruby.scm:39:2
@end example

@item
Build a package, here Ruby version 2.1:

@example
  $ ./pre-inst-env guix build --keep-failed ruby@@2.1
  /gnu/store/c13v73jxmj2nir2xjqaz5259zywsa9zi-ruby-2.1.6
@end example

@item
Install it to your user profile:

@example
  $ ./pre-inst-env guix package --install ruby@@2.1
@end example

@item
Check for common mistakes:

@example
  $ ./pre-inst-env guix lint ruby@@2.1
@end example
@end itemize

Guix strives at maintaining a high packaging standard; when contributing to the
Guix project, remember to

@itemize
@item
follow the coding style (@pxref{Coding Style,,, guix, GNU Guix Reference Manual}),
@item
and review the check list from the manual (@pxref{Submitting Patches,,, guix, GNU Guix Reference Manual}).
@end itemize

Once you are happy with the result, you are welcome to send your contribution to
make it part of Guix.  This process is also detailed in the manual.  (@pxref{Contributing,,, guix, GNU Guix Reference Manual})


It's a community effort so the more join in, the better Guix becomes!

@node Extended example
@subsection Extended example

The above ``Hello World'' example is as simple as it goes.  Packages can be more
complex than that and Guix can handle more advanced scenarios.  Let's look at
another, more sophisticated package (slightly modified from the source):

@lisp
(define-module (gnu packages version-control)
  #:use-module ((guix licenses) #:prefix license:)
  #:use-module (guix utils)
  #:use-module (guix packages)
  #:use-module (guix git-download)
  #:use-module (guix build-system cmake)
  #:use-module (gnu packages ssh)
  #:use-module (gnu packages web)
  #:use-module (gnu packages pkg-config)
  #:use-module (gnu packages python)
  #:use-module (gnu packages compression)
  #:use-module (gnu packages tls))

(define-public my-libgit2
  (let ((commit "e98d0a37c93574d2c6107bf7f31140b548c6a7bf")
        (revision "1"))
    (package
      (name "my-libgit2")
      (version (git-version "0.26.6" revision commit))
      (source (origin
                (method git-fetch)
                (uri (git-reference
                      (url "https://github.com/libgit2/libgit2/")
                      (commit commit)))
                (file-name (git-file-name name version))
                (sha256
                 (base32
                  "17pjvprmdrx4h6bb1hhc98w9qi6ki7yl57f090n9kbhswxqfs7s3"))
                (patches (search-patches "libgit2-mtime-0.patch"))
                (modules '((guix build utils)))
                (snippet '(begin
                            ;; Remove bundled software.
                            (delete-file-recursively "deps")
                            #t))))
      (build-system cmake-build-system)
      (outputs '("out" "debug"))
      (arguments
       `(#:tests? #t                            ; Run the test suite (this is the default)
         #:configure-flags '("-DUSE_SHA1DC=ON") ; SHA-1 collision detection
         #:phases
         (modify-phases %standard-phases
           (add-after 'unpack 'fix-hardcoded-paths
             (lambda _
               (substitute* "tests/repo/init.c"
                 (("#!/bin/sh") (string-append "#!" (which "sh"))))
               (substitute* "tests/clar/fs.h"
                 (("/bin/cp") (which "cp"))
                 (("/bin/rm") (which "rm")))
               #t))
           ;; Run checks more verbosely.
           (replace 'check
             (lambda _ (invoke "./libgit2_clar" "-v" "-Q")))
           (add-after 'unpack 'make-files-writable-for-tests
               (lambda _ (for-each make-file-writable (find-files "." ".*")))))))
      (inputs
       `(("libssh2" ,libssh2)
         ("http-parser" ,http-parser)
         ("python" ,python-wrapper)))
      (native-inputs
       `(("pkg-config" ,pkg-config)))
      (propagated-inputs
       ;; These two libraries are in 'Requires.private' in libgit2.pc.
       `(("openssl" ,openssl)
         ("zlib" ,zlib)))
      (home-page "https://libgit2.github.com/")
      (synopsis "Library providing Git core methods")
      (description
       "Libgit2 is a portable, pure C implementation of the Git core methods
provided as a re-entrant linkable library with a solid API, allowing you to
write native speed custom Git applications in any language with bindings.")
      ;; GPLv2 with linking exception
      (license license:gpl2))))
@end lisp

(In those cases were you only want to tweak a few fields from a package
definition, you should rely on inheritance instead of copy-pasting everything.
See below.)

Let's discuss those fields in depth.

@subsubsection @code{git-fetch} method

Unlike the @code{url-fetch} method, @code{git-fetch} expects a @code{git-reference} which takes
a Git repository and a commit.  The commit can be any Git reference such as
tags, so if the @code{version} is tagged, then it can be used directly.  Sometimes
the tag is prefixed with a @code{v}, in which case you'd use @code{(commit (string-append
"v" version))}.

To ensure that the source code from the Git repository is stored in a unique
directory with a readable name we use @code{(file-name (git-file-name name
version))}.

Note that there is also a @code{git-version} procedure that can be used to derive the
version when packaging programs for a specific commit.

@subsubsection Snippets

Snippets are quoted (i.e. non-evaluated) Scheme code that are a means of patching
the source.  They are a Guix-y alternative to the traditional @samp{.patch} files.
Because of the quote, the code in only evaluated when passed to the Guix daemon
for building.  There can be as many snippets as needed.

Snippets might need additional Guile modules which can be imported from the
@code{modules} field.

@subsubsection Inputs

First, a syntactic comment: See the quasi-quote / comma syntax?

@lisp
    (native-inputs
     `(("pkg-config" ,pkg-config)))
@end lisp

is equivalent to

@lisp
    (native-inputs
     (list (list "pkg-config" pkg-config)))
@end lisp

You'll mostly see the former because it's shorter.

There are 3 different input types.  In short:

@table @asis
@item native-inputs
Required for building but not runtime -- installing a package
through a substitute won't install these inputs.
@item inputs
Installed in the store but not in the profile, as well as being
present at build time.
@item propagated-inputs
Installed in the store and in the profile, as well as
being present at build time.
@end table

@xref{Package Reference,,, guix, GNU Guix Reference Manual} for more details.

The distinction between the various inputs is important: if a dependency can be
handled as an @emph{input} instead of a @emph{propagated input}, it should be done so, or
else it ``pollutes'' the user profile for no good reason.

For instance, a user installing a graphical program that depends on a
command line tool might only be interested in the graphical part, so there is no
need to force the command line tool into the user profile.  The dependency is a
concern to the package, not to the user.  @emph{Inputs} make it possible to handle
dependencies without bugging the user by adding undesired executable files (or
libraries) to their profile.

Same goes for @emph{native-inputs}: once the program is installed, build-time
dependencies can be safely garbage-collected.
It also matters when a substitute is available, in which case only the @emph{inputs}
and @emph{propagated inputs} will be fetched: the @emph{native inputs} are not required to
install a package from a substitute.

@subsubsection Outputs

Just like how a package can have multiple inputs, it can also produce multiple
outputs.

Each output corresponds to a separate directory in the store.

The user can choose which output to install; this is useful to save space or
to avoid polluting the user profile with unwanted executables or libraries.

Output separation is optional.  When the @code{outputs} field is left out, the
default and only output (the complete package) is referred to as @code{"out"}.

Typical separate output names include @code{debug} and @code{doc}.

It's advised to separate outputs only when you've shown it's worth it: if the
output size is significant (compare with @code{guix size}) or in case the package is
modular.

@subsubsection Build system arguments

The @code{arguments} is a keyword-value list used to configure the build process.

The simplest argument @code{#:tests?} can be used to disable the test suite when
building the package.  This is mostly useful when the package does not feature
any test suite.  It's strongly recommended to keep the test suite on if there is
one.

Another  common argument is @code{:make-flags}, which specifies a list of flags to
append when running make, as you would from the command line.  For instance, the
following flags

@lisp
#:make-flags (list (string-append "prefix=" (assoc-ref %outputs "out"))
                   "CC=gcc")
@end lisp

translate into

@example
$ make CC=gcc prefix=/gnu/store/...-<out>
@end example

This sets the C compiler to @code{gcc} and the @code{prefix} variable (the installation
directory in Make parlance) to @code{(assoc-ref %outputs "out")}, which is a build-stage
global variable pointing to the destination directory in the store (something like
@samp{/gnu/store/...-my-libgit2-20180408}).

Similarly, it's possible to set the configure flags:

@lisp
#:configure-flags '("-DUSE_SHA1DC=ON")
@end lisp

The @code{%build-inputs} variable is also generated in scope.  It's an association
table that maps the input names to their store directories.

The @code{phases} keyword lists the sequential steps of the build system.  Typically
phases include @code{unpack}, @code{configure}, @code{build}, @code{install} and @code{check}.  To know
more about those phases, you need to work out the appropriate build system
definition in @samp{$GUIX_CHECKOUT/guix/build/gnu-build-system.scm}:

@lisp
(define %standard-phases
  ;; Standard build phases, as a list of symbol/procedure pairs.
  (let-syntax ((phases (syntax-rules ()
                         ((_ p ...) `((p . ,p) ...)))))
    (phases set-SOURCE-DATE-EPOCH set-paths install-locale unpack
            bootstrap
            patch-usr-bin-file
            patch-source-shebangs configure patch-generated-file-shebangs
            build check install
            patch-shebangs strip
            validate-runpath
            validate-documentation-location
            delete-info-dir-file
            patch-dot-desktop-files
            install-license-files
            reset-gzip-timestamps
            compress-documentation)))
@end lisp

Or from the REPL:

@lisp
> (add-to-load-path "/path/to/guix/checkout")
> ,module (guix build gnu-build-system)
> (map first %standard-phases)
(set-SOURCE-DATE-EPOCH set-paths install-locale unpack bootstrap patch-usr-bin-file patch-source-shebangs configure patch-generated-file-shebangs build check install patch-shebangs strip validate-runpath validate-documentation-location delete-info-dir-file patch-dot-desktop-files install-license-files reset-gzip-timestamps compress-documentation)
@end lisp

If you want to know more about what happens during those phases, consult the
associated procedures.

For instance, as of this writing the definition of @code{unpack} for the GNU build
system is

@lisp
(define* (unpack #:key source #:allow-other-keys)
  "Unpack SOURCE in the working directory, and change directory within the
source.  When SOURCE is a directory, copy it in a sub-directory of the current
working directory."
  (if (file-is-directory? source)
      (begin
        (mkdir "source")
        (chdir "source")

        ;; Preserve timestamps (set to the Epoch) on the copied tree so that
        ;; things work deterministically.
        (copy-recursively source "."
                          #:keep-mtime? #t))
      (begin
        (if (string-suffix? ".zip" source)
            (invoke "unzip" source)
            (invoke "tar" "xvf" source))
        (chdir (first-subdirectory "."))))
  #t)
@end lisp

Note the @code{chdir} call: it changes the working directory to where the source was
unpacked.
Thus every phase following the @code{unpack} will use the source as a working
directory, which is why we can directly work on the source files.
That is to say, unless a later phase changes the working directory to something
else.

We modify the list of @code{%standard-phases} of the build system with the
@code{modify-phases} macro as per the list of specified modifications, which may have
the following forms:

@itemize
@item
@code{(add-before PHASE NEW-PHASE PROCEDURE)}: Run @code{PROCEDURE} named @code{NEW-PHASE} before @code{PHASE}.
@item
@code{(add-after PHASE NEW-PHASE PROCEDURE)}: Same, but afterwards.
@item
@code{(replace PHASE PROCEDURE)}.
@item
@code{(delete PHASE)}.
@end itemize

The @code{PROCEDURE} supports the keyword arguments @code{inputs} and @code{outputs}.  Each
input (whether @emph{native}, @emph{propagated} or not) and output directory is referenced
by their name in those variables.  Thus @code{(assoc-ref outputs "out")} is the store
directory of the main output of the package.  A phase procedure may look like
this:

@lisp
(lambda* (#:key inputs outputs #:allow-other-keys)
  (let (((bash-directory (assoc-ref inputs "bash"))
         (output-directory (assoc-ref outputs "out"))
         (doc-directory (assoc-ref outputs "doc"))
  ; ...
  #t)
@end lisp

The procedure must return @code{#t} on success.  It's brittle to rely on the return
value of the last expression used to tweak the phase because there is no
guarantee it would be a @code{#t}.  Hence the trailing @code{#t} to ensure the right value
is returned on success.

@subsubsection Code staging

The astute reader may have noticed the quasi-quote and comma syntax in the
argument field.  Indeed, the build code in the package declaration should not be
evaluated on the client side, but only when passed to the Guix daemon.  This
mechanism of passing code around two running processes is called @uref{https://arxiv.org/abs/1709.00833, code staging}.

@subsubsection Utility functions

When customizing @code{phases}, we often need to write code that mimics the
equivalent system invocations (@code{make}, @code{mkdir}, @code{cp}, etc.) commonly used during
regular ``Unix-style'' installations.

Some like @code{chmod} are native to Guile.
@xref{,,, guile, Guile reference manual} for a complete list.

Guix provides additional helper functions which prove especially handy in the
context of package management.

Some of those functions can be found in
@samp{$GUIX_CHECKOUT/guix/guix/build/utils.scm}.  Most of them mirror the behaviour
of the traditional Unix system commands:

@table @asis
@item which
Like the @samp{which} system command.
@item find-files
Akin to the @samp{find} system command.
@item mkdir-p
Like @samp{mkdir -p}, which creates all parents as needed.
@item install-file
Similar to @samp{install} when installing a file to a (possibly
non-existing) directory.  Guile has @code{copy-file} which works
like @samp{cp}.
@item copy-recursively
Like @samp{cp -r}.
@item delete-file-recursively
Like @samp{rm -rf}.
@item invoke
Run an executable.  This should be used instead of @code{system*}.
@item with-directory-excursion
Run the body in a different working directory,
then restore the previous working directory.
@item substitute*
A ``@command{sed}-like'' function.
@end table

@subsubsection Module prefix

The license in our last example needs a prefix: this is because of how the
@code{license} module was imported in the package, as @code{#:use-module ((guix licenses)
#:prefix license:)}.  The Guile module import mechanism
(@pxref{Using Guile Modules,,, guile, Guile reference manual})
gives the user full control over namespacing: this is needed to avoid
clashes between, say, the
@samp{zlib} variable from @samp{licenses.scm} (a @emph{license} value) and the @samp{zlib} variable
from @samp{compression.scm} (a @emph{package} value).

@node Other build systems
@subsection Other build systems

What we've seen so far covers the majority of packages using a build system
other than the @code{trivial-build-system}.  The latter does not automate anything
and leaves you to build everything manually.  This can be more demanding and we
won't cover it here for now, but thankfully it is rarely necessary to fall back
on this system.

For the other build systems, such as ASDF, Emacs, Perl, Ruby and many more, the
process is very similar to the GNU build system except for a few specialized
arguments.

Learn more about build systems in
@itemize
@item
@uref{https://www.gnu.org/software/guix/manual/en/html_node/Build-Systems.html#Build-Systems, the manual, section 4.2 Build systems},
@item
the source code in the @samp{$GUIX_CHECKOUT/guix/build} and
@samp{$GUIX_CHECKOUT/guix/build-system} directories.
@end itemize

@node Programmable and automated package definition
@subsection Programmable and automated package definition

We can't repeat it enough: having a full-fledged programming language at hand
empowers us in ways that reach far beyond traditional package management.

Let's illustrate this with some awesome features of Guix!

@node Recursive importers
@subsubsection Recursive importers

You might find some build systems good enough that there is little to do at all
to write a package, to the point that it becomes repetitive and tedious after a
while.  A @emph{raison d'être} of computers is to replace human beings at those
boring tasks.  So let's tell Guix to do this for us and create the package
definition of an R package from CRAN (the output is trimmed for conciseness):

@example
$ guix import cran --recursive walrus

(define-public r-mc2d
    ; ...
    (license gpl2+)))

(define-public r-jmvcore
    ; ...
    (license gpl2+)))

(define-public r-wrs2
    ; ...
    (license gpl3)))

(define-public r-walrus
  (package
    (name "r-walrus")
    (version "1.0.3")
    (source
      (origin
        (method url-fetch)
        (uri (cran-uri "walrus" version))
        (sha256
          (base32
            "1nk2glcvy4hyksl5ipq2mz8jy4fss90hx6cq98m3w96kzjni6jjj"))))
    (build-system r-build-system)
    (propagated-inputs
      `(("r-ggplot2" ,r-ggplot2)
        ("r-jmvcore" ,r-jmvcore)
        ("r-r6" ,r-r6)
        ("r-wrs2" ,r-wrs2)))
    (home-page "https://github.com/jamovi/walrus")
    (synopsis "Robust Statistical Methods")
    (description
      "This package provides a toolbox of common robust statistical
tests, including robust descriptives, robust t-tests, and robust ANOVA.
It is also available as a module for 'jamovi' (see
<https://www.jamovi.org> for more information).  Walrus is based on the
WRS2 package by Patrick Mair, which is in turn based on the scripts and
work of Rand Wilcox.  These analyses are described in depth in the book
'Introduction to Robust Estimation & Hypothesis Testing'.")
    (license gpl3)))
@end example

The recursive importer won't import packages for which Guix already has package
definitions, except for the very first.

Not all applications can be packaged this way, only those relying on a select
number of supported systems.  Read about the full list of importers in
the guix import section of the manual
(@pxref{Invoking guix import,,, guix, GNU Guix Reference Manual}).

@node Automatic update
@subsubsection Automatic update

Guix can be smart enough to check for updates on systems it knows.  It can
report outdated package definitions with

@example
$ guix refresh hello
@end example

In most cases, updating a package to a newer version requires little more than
changing the version number and the checksum.  Guix can do that automatically as
well:

@example
$ guix refresh hello --update
@end example

@node Inheritance
@subsubsection Inheritance

If you've started browsing the existing package definitions, you might have
noticed that a significant number of them have a @code{inherit} field:

@lisp
(define-public adwaita-icon-theme
  (package (inherit gnome-icon-theme)
    (name "adwaita-icon-theme")
    (version "3.26.1")
    (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnome/sources/" name "/"
                                  (version-major+minor version) "/"
                                  name "-" version ".tar.xz"))
              (sha256
               (base32
                "17fpahgh5dyckgz7rwqvzgnhx53cx9kr2xw0szprc6bnqy977fi8"))))
    (native-inputs
     `(("gtk-encode-symbolic-svg" ,gtk+ "bin")))))
@end lisp

All unspecified fields are inherited from the parent package.  This is very
convenient to create alternative packages, for instance with different source,
version or compilation options.

@node Getting help
@subsection Getting help

Sadly, some applications can be tough to package.  Sometimes they need a patch to
work with the non-standard filesystem hierarchy enforced by the store.
Sometimes the tests won't run properly.  (They can be skipped but this is not
recommended.)  Other times the resulting package won't be reproducible.

Should you be stuck, unable to figure out how to fix any sort of packaging
issue, don't hesitate to ask the community for help.

See the @uref{https://www.gnu.org/software/guix/contact/, Guix homepage} for information on the mailing lists, IRC, etc.

@node Conclusion
@subsection Conclusion

This tutorial was a showcase of the sophisticated package management that Guix
boasts.  At this point we have mostly restricted this introduction to the
@code{gnu-build-system} which is a core abstraction layer on which more advanced
abstractions are based.

Where do we go from here?  Next we ought to dissect the innards of the build
system by removing all abstractions, using the @code{trivial-build-system}: this
should give us a thorough understanding of the process before investigating some
more advanced packaging techniques and edge cases.

Other features worth exploring are the interactive editing and debugging
capabilities of Guix provided by the Guile REPL@.

Those fancy features are completely optional and can wait; now is a good time
to take a well-deserved break.  With what we've introduced here you should be
well armed to package lots of programs.  You can get started right away and
hopefully we will see your contributions soon!

@node References
@subsection References

@itemize
@item
The @uref{https://www.gnu.org/software/guix/manual/en/html_node/Defining-Packages.html, package reference in the manual}

@item
@uref{https://gitlab.com/pjotrp/guix-notes/blob/master/HACKING.org, Pjotr’s hacking guide to GNU Guix}

@item
@uref{https://www.gnu.org/software/guix/guix-ghm-andreas-20130823.pdf, ``GNU Guix: Package without a scheme!''}, by Andreas Enge
@end itemize

@c *********************************************************************
@node System Configuration
@chapter System Configuration

Guix offers a flexible language for declaratively configuring your Guix
System.  This flexibility can at times be overwhelming.  The purpose of this
chapter is to demonstrate some advanced configuration concepts.

@pxref{System Configuration,,, guix, GNU Guix Reference Manual} for a complete
reference.

@menu
* Customizing the Kernel::     Creating and using a custom Linux kernel on Guix System.
@end menu

@node Customizing the Kernel
@section Customizing the Kernel

Guix is, at its core, a source based distribution with substitutes
(@pxref{Substitutes,,, guix, GNU Guix Reference Manual}), and as such building
packages from their source code is an expected part of regular package
installations and upgrades.  Given this starting point, it makes sense that
efforts are made to reduce the amount of time spent compiling packages, and
recent changes and upgrades to the building and distribution of substitutes
continues to be a topic of discussion within Guix.

The kernel, while not requiring an overabundance of RAM to build, does take a
rather long time on an average machine.  The official kernel configuration, as
is the case with many GNU/Linux distributions, errs on the side of
inclusiveness, and this is really what causes the build to take such a long
time when the kernel is built from source.

The Linux kernel, however, can also just be described as a regular old
package, and as such can be customized just like any other package.  The
procedure is a little bit different, although this is primarily due to the
nature of how the package definition is written.

The @code{linux-libre} kernel package definition is actually a procedure which
creates a package.

@lisp
(define* (make-linux-libre version hash supported-systems
                           #:key
                           ;; A function that takes an arch and a variant.
                           ;; See kernel-config for an example.
                           (extra-version #f)
                           (configuration-file #f)
                           (defconfig "defconfig")
                           (extra-options %default-extra-linux-options)
                           (patches (list %boot-logo-patch)))
  ...)
@end lisp

The current @code{linux-libre} package is for the 5.1.x series, and is
declared like this:

@lisp
(define-public linux-libre
  (make-linux-libre %linux-libre-version
                    %linux-libre-hash
                    '("x86_64-linux" "i686-linux" "armhf-linux" "aarch64-linux")
                    #:patches %linux-libre-5.1-patches
                    #:configuration-file kernel-config))
@end lisp

Any keys which are not assigned values inherit their default value from the
@code{make-linux-libre} definition.  When comparing the two snippets above,
you may notice that the code comment in the first doesn't actually refer to
the @code{#:extra-version} keyword; it is actually for
@code{#:configuration-file}.  Because of this, it is not actually easy to
include a custom kernel configuration from the definition, but don't worry,
there are other ways to work with what we do have.

There are two ways to create a kernel with a custom kernel configuration.  The
first is to provide a standard @file{.config} file during the build process by
including an actual @file{.config} file as a native input to our custom
kernel.  The following is a snippet from the custom @code{'configure} phase of
the @code{make-linux-libre} package definition:

@lisp
(let ((build  (assoc-ref %standard-phases 'build))
      (config (assoc-ref (or native-inputs inputs) "kconfig")))

  ;; Use a custom kernel configuration file or a default
  ;; configuration file.
  (if config
      (begin
        (copy-file config ".config")
        (chmod ".config" #o666))
      (invoke "make" ,defconfig))
@end lisp

Below is a sample kernel package.  The @code{linux-libre} package is nothing
special and can be inherited from and have its fields overridden like any
other package:

@lisp
(define-public linux-libre/E2140
  (package
    (inherit linux-libre)
    (native-inputs
     `(("kconfig" ,(local-file "E2140.config"))
      ,@@(alist-delete "kconfig"
                      (package-native-inputs linux-libre))))))
@end lisp

In the same directory as the file defining @code{linux-libre-E2140} is a file
named @file{E2140.config}, which is an actual kernel configuration file.  The
@code{defconfig} keyword of @code{make-linux-libre} is left blank here, so the
only kernel configuration in the package is the one which was included in the
@code{native-inputs} field.

The second way to create a custom kernel is to pass a new value to the
@code{extra-options} keyword of the @code{make-linux-libre} procedure.  The
@code{extra-options} keyword works with another function defined right below
it:

@lisp
(define %default-extra-linux-options
  `(;; https://lists.gnu.org/archive/html/guix-devel/2014-04/msg00039.html
   ("CONFIG_DEVPTS_MULTIPLE_INSTANCES" . #t)
   ;; Modules required for initrd:
   ("CONFIG_NET_9P" . m)
   ("CONFIG_NET_9P_VIRTIO" . m)
   ("CONFIG_VIRTIO_BLK" . m)
   ("CONFIG_VIRTIO_NET" . m)
   ("CONFIG_VIRTIO_PCI" . m)
   ("CONFIG_VIRTIO_BALLOON" . m)
   ("CONFIG_VIRTIO_MMIO" . m)
   ("CONFIG_FUSE_FS" . m)
   ("CONFIG_CIFS" . m)
   ("CONFIG_9P_FS" . m)))

(define (config->string options)
  (string-join (map (match-lambda
                      ((option . 'm)
                       (string-append option "=m"))
                      ((option . #t)
                       (string-append option "=y"))
                      ((option . #f)
                       (string-append option "=n")))
                    options)
               "\n"))
@end lisp

And in the custom configure script from the `make-linux-libre` package:

@lisp
;; Appending works even when the option wasn't in the
;; file.  The last one prevails if duplicated.
(let ((port (open-file ".config" "a"))
      (extra-configuration ,(config->string extra-options)))
  (display extra-configuration port)
  (close-port port))

(invoke "make" "oldconfig"))))
@end lisp

So by not providing a configuration-file the @file{.config} starts blank, and
then we write into it the collection of flags that we want.  Here's another
custom kernel:

@lisp
(define %macbook41-full-config
  (append %macbook41-config-options
          %filesystems
          %efi-support
          %emulation
          (@@@@ (gnu packages linux) %default-extra-linux-options)))

(define-public linux-libre-macbook41
  ;; XXX: Access the internal 'make-linux-libre' procedure, which is
  ;; private and unexported, and is liable to change in the future.
  ((@@@@ (gnu packages linux) make-linux-libre) (@@@@ (gnu packages linux) %linux-libre-version)
                      (@@@@ (gnu packages linux) %linux-libre-hash)
                      '("x86_64-linux")
                      #:extra-version "macbook41"
                      #:patches (@@@@ (gnu packages linux) %linux-libre-5.1-patches)
                      #:extra-options %macbook41-config-options))
@end lisp

In the above example @code{%filesystems} is a collection of flags enabling
different filesystem support, @code{%efi-support} enables EFI support and
@code{%emulation} enables a x86_64-linux machine to act in 32-bit mode also.
@code{%default-extra-linux-options} are the ones quoted above, which had to be
added in since they were replaced in the @code{extra-options} keyword.

This all sounds like it should be doable, but how does one even know which
modules are required for a particular system?  Two places that can be helpful
in trying to answer this question is the
@uref{https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Kernel, Gentoo
Handbook} and the
@uref{https://www.kernel.org/doc/html/latest/admin-guide/README.html?highlight=localmodconfig,
documentation from the kernel itself}.  From the kernel documentation, it
seems that @code{make localmodconfig} is the command we want.

In order to actually run @code{make localmodconfig} we first need to get and
unpack the kernel source code:

@example shell
tar xf $(guix build linux-libre --source)
@end example

Once inside the directory containing the source code run @code{touch .config}
to create an initial, empty @file{.config} to start with.  @code{make
localmodconfig} works by seeing what you already have in @file{.config} and
letting you know what you're missing.  If the file is blank then you're
missing everything.  The next step is to run:

@example shell
guix environment linux-libre -- make localmodconfig
@end example

and note the output.  Do note that the @file{.config} file is still empty.
The output generally contains two types of warnings.  The first start with
"WARNING" and can actually be ignored in our case.  The second read:

@example shell
module pcspkr did not have configs CONFIG_INPUT_PCSPKR
@end example

For each of these lines, copy the @code{CONFIG_XXXX_XXXX} portion into the
@file{.config} in the directory, and append @code{=m}, so in the end it looks
like this:

@example shell
CONFIG_INPUT_PCSPKR=m
CONFIG_VIRTIO=m
@end example

After copying all the configuration options, run @code{make localmodconfig}
again to make sure that you don't have any output starting with ``module''.
After all of these machine specific modules there are a couple more left that
are also needed.  @code{CONFIG_MODULES} is necessary so that you can build and
load modules separately and not have everything built into the kernel.
@code{CONFIG_BLK_DEV_SD} is required for reading from hard drives.  It is
possible that there are other modules which you will need.

This post does not aim to be a guide to configuring your own kernel however,
so if you do decide to build a custom kernel you'll have to seek out other
guides to create a kernel which is just right for your needs.

The second way to setup the kernel configuration makes more use of Guix's
features and allows you to share configuration segments between different
kernels.  For example, all machines using EFI to boot have a number of EFI
configuration flags that they need.  It is likely that all the kernels will
share a list of filesystems to support.  By using variables it is easier to
see at a glance what features are enabled and to make sure you don't have
features in one kernel but missing in another.

Left undiscussed however, is Guix's initrd and its customization.  It is
likely that you'll need to modify the initrd on a machine using a custom
kernel, since certain modules which are expected to be built may not be
available for inclusion into the initrd.

@c *********************************************************************
@node Advanced package management
@chapter Advanced package management

Guix is a functional package manager that offers many features beyond
what more traditional package managers can do.  To the uninitiated,
those features might not have obvious use cases at first.  The purpose
of this chapter is to demonstrate some advanced package management
concepts.

@pxref{Package Management,,, guix, GNU Guix Reference Manual} for a complete
reference.

@menu
* Guix Profiles in Practice::     Strategies for multiple profiles and manifests.
@end menu

@node Guix Profiles in Practice
@section Guix Profiles in Practice

Guix provides a very useful feature that may be quite foreign to newcomers:
@emph{profiles}.  They are a way to group package installations together and all users
on the same system are free to use as many profiles as they want.

Whether you're a developer or not, you may find that multiple profiles bring you
great power and flexibility.  While they shift the paradigm somewhat compared to
@emph{traditional package managers}, they are very convenient to use once you've
understood how to set them up.

If you are familiar with Python's @samp{virtualenv}, you can think of a profile as a
kind of universal @samp{virtualenv} that can hold any kind of software whatsoever, not
just Python software.  Furthermore, profiles are self-sufficient: they capture
all the runtime dependencies which guarantees that all programs within a profile
will always work at any point in time.

Multiple profiles have many benefits:

@itemize
@item
Clean semantic separation of the various packages a user needs for different contexts.

@item
Multiple profiles can be made available into the environment either on login
or within a dedicated shell.

@item
Profiles can be loaded on demand.  For instance, the user can use multiple
shells, each of them running different profiles.

@item
Isolation: Programs from one profile will not use programs from the other, and
the user can even install different versions of the same programs to the two
profiles without conflict.

@item
Deduplication: Profiles share dependencies that happens to be the exact same.
This makes multiple profiles storage-efficient.

@item
Reproducible: when used with declarative manifests, a profile can be fully
specified by the Guix commit that was active when it was set up.  This means
that the exact same profile can be
@uref{https://guix.gnu.org/blog/2018/multi-dimensional-transactions-and-rollbacks-oh-my/,
set up anywhere and anytime}, with just the commit information.  See the
section on @ref{Reproducible profiles}.

@item
Easier upgrades and maintenance: Multiple profiles make it easy to keep
package listings at hand and make upgrades completely friction-less.
@end itemize

Concretely, here follows some typical profiles:

@itemize
@item
The dependencies of a project you are working on.

@item
Your favourite programming language libraries.

@item
Laptop-specific programs (like @samp{powertop}) that you don't need on a desktop.

@item
@TeX{}live (this one can be really useful when you need to install just one
package for this one document you've just received over email).

@item
Games.
@end itemize

Let's dive in the set up!

@node Basic setup with manifests
@subsection Basic setup with manifests

A Guix profile can be set up @emph{via} a so-called @emph{manifest specification} that looks like
this:

@lisp
(specifications->manifest
  '("package-1"
    ;; Version 1.3 of package-2.
    "package-2@@1.3"
    ;; The "lib" output of package-3.
    "package-3:lib"
    ; ...
    "package-N"))
@end lisp

@pxref{Invoking guix package,,, guix, GNU Guix Reference Manual}, for
the syntax details.

We can create a manifest specification per profile and install them this way:

@example
GUIX_EXTRA_PROFILES=$HOME/.guix-extra-profiles
mkdir -p "$GUIX_EXTRA_PROFILES"/my-project # if it does not exist yet
guix package --manifest=/path/to/guix-my-project-manifest.scm --profile="$GUIX_EXTRA_PROFILES"/my-project/my-project
@end example

Here we set an arbitrary variable @samp{GUIX_EXTRA_PROFILES} to point to the directory
where we will store our profiles in the rest of this article.

Placing all your profiles in a single directory, with each profile getting its
own sub-directory, is somewhat cleaner.  This way, each sub-directory will
contain all the symlinks for precisely one profile.  Besides, "looping over
profiles" becomes obvious from any programming language (e.g. a shell script) by
simply looping over the sub-directories of @samp{$GUIX_EXTRA_PROFILES}.

Note that it's also possible to loop over the output of

@example
guix package --list-profiles
@end example

although you'll probably have to filter out @samp{~/.config/guix/current}.

To enable all profiles on login, add this to your @samp{~/.bash_profile} (or similar):

@example
for i in $GUIX_EXTRA_PROFILES/*; do
  profile=$i/$(basename "$i")
  if [ -f "$profile"/etc/profile ]; then
    GUIX_PROFILE="$profile"
    . "$GUIX_PROFILE"/etc/profile
  fi
  unset profile
done
@end example

Note to Guix System users: the above reflects how your default profile
@samp{~/.guix-profile} is activated from @samp{/etc/profile}, that latter being loaded by
@samp{~/.bashrc} by default.

You can obviously choose to only enable a subset of them:

@example
for i in "$GUIX_EXTRA_PROFILES"/my-project-1 "$GUIX_EXTRA_PROFILES"/my-project-2; do
  profile=$i/$(basename "$i")
  if [ -f "$profile"/etc/profile ]; then
    GUIX_PROFILE="$profile"
    . "$GUIX_PROFILE"/etc/profile
  fi
  unset profile
done
@end example

When a profile is off, it's straightforward to enable it for an individual shell
without "polluting" the rest of the user session:

@example
GUIX_PROFILE="path/to/my-project" ; . "$GUIX_PROFILE"/etc/profile
@end example

The key to enabling a profile is to @emph{source} its @samp{etc/profile} file.  This file
contains shell code that exports the right environment variables necessary to
activate the software contained in the profile.  It is built automatically by
Guix and meant to be sourced.
It contains the same variables you would get if you ran:

@example
guix package --search-paths=prefix --profile=$my_profile"
@end example

Once again, see (@pxref{Invoking guix package,,, guix, GNU Guix Reference Manual})
for the command line options.

To upgrade a profile, simply install the manifest again:

@example
guix package -m /path/to/guix-my-project-manifest.scm -p "$GUIX_EXTRA_PROFILES"/my-project/my-project
@end example

To upgrade all profiles, it's easy enough to loop over them.  For instance,
assuming your manifest specifications are stored in
@samp{~/.guix-manifests/guix-$profile-manifest.scm}, with @samp{$profile} being the name
of the profile (e.g. "project1"), you could do the following in Bourne shell:

@example
for profile in "$GUIX_EXTRA_PROFILES"/*; do
  guix package --profile="$profile" --manifest="$HOME/.guix-manifests/guix-$profile-manifest.scm"
done
@end example

Each profile has its own generations:

@example
guix package -p "$GUIX_EXTRA_PROFILES"/my-project/my-project --list-generations
@end example

You can roll-back to any generation of a given profile:

@example
guix package -p "$GUIX_EXTRA_PROFILES"/my-project/my-project --switch-generations=17
@end example

Finally, if you want to switch to a profile without inheriting from the
current environment, you can activate it from an empty shell:

@example
env -i $(which bash) --login --noprofile --norc
. my-project/etc/profile
@end example

@node Required packages
@subsection Required packages

Activating a profile essentially boils down to exporting a bunch of
environmental variables.  This is the role of the @samp{etc/profile} within the
profile.

@emph{Note: Only the environmental variables of the packages that consume them will
be set.}

For instance, @samp{MANPATH} won't be set if there is no consumer application for man
pages within the profile.  So if you need to transparently access man pages once
the profile is loaded, you've got two options:

@itemize
@item
Either export the variable manually, e.g.
@example
export MANPATH=/path/to/profile$@{MANPATH:+:@}$MANPATH
@end example

@item
Or include @samp{man-db} to the profile manifest.
@end itemize

The same is true for @samp{INFOPATH} (you can install @samp{info-reader}),
@samp{PKG_CONFIG_PATH} (install @samp{pkg-config}), etc.

@node Default profile
@subsection Default profile

What about the default profile that Guix keeps in @samp{~/.guix-profile}?

You can assign it the role you want.  Typically you would install the manifest
of the packages you want to use all the time.

Alternatively, you could keep it "manifest-less" for throw-away packages
that you would just use for a couple of days.
This way makes it convenient to run

@example
guix install package-foo
guix upgrade package-bar
@end example

without having to specify the path to a profile.

@node The benefits of manifests
@subsection The benefits of manifests

Manifests are a convenient way to keep your package lists around and, say,
to synchronize them across multiple machines using a version control system.

A common complaint about manifests is that they can be slow to install when they
contain large number of packages.  This is especially cumbersome when you just
want get an upgrade for one package within a big manifest.

This is one more reason to use multiple profiles, which happen to be just
perfect to break down manifests into multiple sets of semantically connected
packages.  Using multiple, small profiles provides more flexibility and
usability.

Manifests come with multiple benefits.  In particular, they ease maintenance:

@itemize
@item
When a profile is set up from a manifest, the manifest itself is
self-sufficient to keep a "package listing" around and reinstall the profile
later or on a different system.  For ad-hoc profiles, we would need to
generate a manifest specification manually and maintain the package versions
for the packages that don't use the default version.

@item
@code{guix package --upgrade} always tries to update the packages that have
propagated inputs, even if there is nothing to do.  Guix manifests remove this
problem.

@item
When partially upgrading a profile, conflicts may arise (due to diverging
dependencies between the updated and the non-updated packages) and they can be
annoying to resolve manually.  Manifests remove this problem altogether since
all packages are always upgraded at once.

@item
As mentioned above, manifests allow for reproducible profiles, while the
imperative @code{guix install}, @code{guix upgrade}, etc. do not, since they produce
different profiles every time even when they hold the same packages.  See
@uref{https://issues.guix.gnu.org/issue/33285, the related discussion on the matter}.

@item
Manifest specifications are usable by other @samp{guix} commands.  For example, you
can run @code{guix weather -m manifest.scm} to see how many substitutes are
available, which can help you decide whether you want to try upgrading today
or wait a while.  Another example: you can run @code{guix pack -m manifest.scm} to
create a pack containing all the packages in the manifest (and their
transitive references).

@item
Finally, manifests have a Scheme representation, the @samp{<manifest>} record type.
They can be manipulated in Scheme and passed to the various Guix @uref{https://en.wikipedia.org/wiki/Api, APIs}.
@end itemize

It's important to understand that while manifests can be used to declare
profiles, they are not strictly equivalent: profiles have the side effect that
they "pin" packages in the store, which prevents them from being
garbage-collected (@pxref{Invoking guix gc,,, guix, GNU Guix Reference Manual})
and ensures that they will still be available at any point in
the future.

Let's take an example:

@enumerate
@item
We have an environment for hacking on a project for which there isn't a Guix
package yet.  We build the environment using a manifest, and then run @code{guix
   environment -m manifest.scm}.  So far so good.

@item
Many weeks pass and we have run a couple of @code{guix pull} in the mean time.
Maybe a dependency from our manifest has been updated; or we may have run
@code{guix gc} and some packages needed by our manifest have been
garbage-collected.

@item
Eventually, we set to work on that project again, so we run @code{guix environment
   -m manifest.scm}.  But now we have to wait for Guix to build and install
stuff!
@end enumerate

Ideally, we could spare the rebuild time.  And indeed we can, all we need is to
install the manifest to a profile and use @code{GUIX_PROFILE=/the/profile;
. "$GUIX_PROFILE"/etc/profile} as explained above: this guarantees that our
hacking environment will be available at all times.

@emph{Security warning:} While keeping old profiles around can be convenient, keep in
mind that outdated packages may not have received the latest security fixes.

@node Reproducible profiles
@subsection Reproducible profiles

To reproduce a profile bit-for-bit, we need two pieces of information:

@itemize
@item
a manifest,
@item
a Guix channel specification.
@end itemize

Indeed, manifests alone might not be enough: different Guix versions (or
different channels) can produce different outputs for a given manifest.

You can output the Guix channel specification with @samp{guix describe
--format=channels}.
Save this to a file, say @samp{channel-specs.scm}.

On another computer, you can use the channel specification file and the manifest
to reproduce the exact same profile:

@example
GUIX_EXTRA_PROFILES=$HOME/.guix-extra-profiles
GUIX_EXTRA=$HOME/.guix-extra

mkdir "$GUIX_EXTRA"/my-project
guix pull --channels=channel-specs.scm --profile "$GUIX_EXTRA/my-project/guix"

mkdir -p "$GUIX_EXTRA_PROFILES/my-project"
"$GUIX_EXTRA"/my-project/guix/bin/guix package --manifest=/path/to/guix-my-project-manifest.scm --profile="$GUIX_EXTRA_PROFILES"/my-project/my-project
@end example

It's safe to delete the Guix channel profile you've just installed with the
channel specification, the project profile does not depend on it.

@c *********************************************************************
@node Acknowledgments
@chapter Acknowledgments

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

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

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

This document includes adapted sections from articles that have previously
been published on the Guix blog at @uref{https://guix.gnu.org/blog}.


@c *********************************************************************
@node GNU Free Documentation License
@appendix GNU Free Documentation License
@cindex license, GNU Free Documentation License
@include fdl-1.3.texi

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

@bye

@c Local Variables:
@c ispell-local-dictionary: "american";
@c End: