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

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@copying
Copyright @copyright{} 2019, 2022 Ricardo Wurmus@*
Copyright @copyright{} 2019 Efraim Flashner@*
Copyright @copyright{} 2019 Pierre Neidhardt@*
Copyright @copyright{} 2020 Oleg Pykhalov@*
Copyright @copyright{} 2020 Matthew Brooks@*
Copyright @copyright{} 2020 Marcin Karpezo@*
Copyright @copyright{} 2020 Brice Waegeneire@*
Copyright @copyright{} 2020 André Batista@*
Copyright @copyright{} 2020 Christine Lemmer-Webber@*
Copyright @copyright{} 2021 Joshua Branson@*
Copyright @copyright{} 2022, 2023 Maxim Cournoyer@*
Copyright @copyright{} 2023-2024 Ludovic Courtès@*
Copyright @copyright{} 2023 Thomas Ieong@*
Copyright @copyright{} 2024 Florian Pelz@*

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
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@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.
This manual is also available in French (@pxref{Top,,, guix-cookbook.fr,
Livre de recettes de GNU Guix}), German (@pxref{Top,,, guix-cookbook.de,
GNU-Guix-Kochbuch}), Korean (@pxref{Top,,, guix-cookbook.ko, GNU Guix 쿡북}),
Brazilian Portuguese (@pxref{Top,,, guix-cookbook.pt_BR,
Livro de Receitas do GNU Guix}), Slovak (@pxref{Top,,, guix-cookbook.sk,
Receptár GNU Guix}), and Swedish (@pxref{Top,,, guix-cookbook.sv,
Kokbok för GNU Guix}).  If you would like to translate
this document in your native language, consider joining
@uref{https://translate.fedoraproject.org/projects/guix/documentation-cookbook,
Weblate} (@pxref{Translating Guix,,, guix, GNU Guix reference
manual}).

@menu
* Scheme tutorials::            Meet your new favorite language!
* Packaging::                   Packaging tutorials
* System Configuration::        Customizing the GNU System
* Containers::                  Isolated environments and nested systems
* Virtual Machines::            Virtual machines usage and configuration
* Advanced package management::  Power to the users!
* Software Development::        Environments, continuous integration, etc.
* Environment management::      Control environment
* Installing Guix on a Cluster::  High-performance computing.

* 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::

Packaging

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

Packaging Tutorial

* A ``Hello World'' package::
* Setup::
* Extended example::
* Other build systems::
* Programmable and automated package definition::
* Getting help::
* Conclusion::
* References::

Setup

* Local file::
* Channels::
* Direct checkout hacking::

Programmable and automated package definition

* Recursive importers::
* Automatic update::
* Inheritance::

System Configuration

* Auto-Login to a Specific TTY::  Automatically Login a User to a Specific TTY
* Customizing the Kernel::      Creating and using a custom Linux kernel on Guix System.
* Guix System Image API::       Customizing images to target specific platforms.
* Using security keys::         How to use security keys with Guix System.
* Dynamic DNS mcron job::       Job to update the IP address behind a DuckDNS host name.
* Connecting to Wireguard VPN::  Connecting to a Wireguard VPN.
* Customizing a Window Manager::  Handle customization of a Window manager on Guix System.
* Running Guix on a Linode Server::  Running Guix on a Linode Server.
* Running Guix on a Kimsufi Server::  Running Guix on a Kimsufi Server.
* Setting up a bind mount::     Setting up a bind mount in the file-systems definition.
* Getting substitutes from Tor::  Configuring Guix daemon to get substitutes through Tor.
* Setting up NGINX with Lua::   Configuring NGINX web-server to load Lua modules.
* Music Server with Bluetooth Audio::  Headless music player with Bluetooth output.

Customizing a Window Manager

* StumpWM::
* Session lock::

Session lock

* Xorg::

Containers

* Guix Containers::             Perfectly isolated environments
* Guix System Containers::      A system inside your system

Guix System Containers

* A Database Container::
* Container Networking::

Virtual Machines

* Network bridge for QEMU::
* Routed network for libvirt::

Advanced package management

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

Guix Profiles in Practice

* Basic setup with manifests::
* Required packages::
* Default profile::
* The benefits of manifests::
* Reproducible profiles::

Software Development

* Getting Started::             Step 0: using `guix shell'.
* Building with Guix::          Step 1: building your code.
* The Repository as a Channel::  Step 2: turning the repo in a channel.
* Package Variants::            Bonus: Defining variants.
* Setting Up Continuous Integration::  Step 3: continuous integration.
* Build Manifest::              Bonus: Manifest.
* Wrapping Up::                 Recap.

Environment management

* Guix environment via direnv::  Setup Guix environment with direnv

Installing Guix on a Cluster

* Setting Up a Head Node::      The node that runs the daemon.
* Setting Up Compute Nodes::    Client nodes.
* Cluster Network Access::      Dealing with network access restrictions.
* Cluster Disk Usage::          Disk usage considerations.
* Cluster Security Considerations::  Keeping the cluster secure.

@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!

@menu
* A Scheme Crash Course::
@end menu

@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
@dfn{REPL}---short for @uref{https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop,
@dfn{read-eval-print loop}}---by running @code{guile} from the command line.

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

In the following examples, lines show what you would type at the REPL;
lines starting with ``@result{}'' show evaluation results, while lines
starting with ``@print{}'' show things that get printed.  @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{#true}
and @code{#false} (abbreviated @code{#t} and @code{#f}) stand for the
Booleans ``true'' and ``false'', respectively.

Examples of valid expressions:

@lisp
"Hello World!"
@result{} "Hello World!"

17
@result{} 17

(display (string-append "Hello " "Guix" "\n"))
@print{} Hello Guix!
@result{} #<unspecified>
@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---@dfn{procedures} in Scheme parlance---are declared
with the @code{lambda} term:

@lisp
(lambda (x) (* x x))
@result{} #<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)
@result{} 9
@end lisp

Procedures are regular values just like numbers, strings, Booleans, and
so on.

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

@lisp
(define a 3)
(define square (lambda (x) (* x x)))
(square a)
@result{} 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)
@result{} (2 3 5 7)
@end lisp

@item
Standard procedures are provided by the @code{(srfi srfi-1)} module to
create and process lists (@pxref{SRFI-1, list processing,, guile, GNU
Guile Reference Manual}).  Here are some of the most useful ones in
action:

@lisp
(use-modules (srfi srfi-1))  ;import list processing procedures

(append (list 1 2) (list 3 4))
@result{} (1 2 3 4)

(map (lambda (x) (* x x)) (list 1 2 3 4))
@result{} (1 4 9 16)

(delete 3 (list 1 2 3 4))        @result{} (1 2 4)
(filter odd? (list 1 2 3 4))     @result{} (1 3)
(remove even? (list 1 2 3 4))    @result{} (1 3)
(find number? (list "a" 42 "b")) @result{} 42
@end lisp

Notice how the first argument to @code{map}, @code{filter},
@code{remove}, and @code{find} is a procedure!

@item
@cindex S-expression
The @dfn{quote} disables evaluation of a parenthesized expression, also
called an S-expression or ``s-exp'': the first term is not called over
the other terms (@pxref{Expression Syntax, quote,, guile, GNU Guile
Reference Manual}).  Thus it effectively returns a list of terms.

@lisp
'(display (string-append "Hello " "Guix" "\n"))
@result{} (display (string-append "Hello " "Guix" "\n"))

'(2 a 5 7)
@result{} (2 a 5 7)
@end lisp

@item
The @code{quasiquote} (@code{`}, a backquote) disables evaluation of a
parenthesized expression until @code{unquote} (@code{,}, 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)))
@result{} (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
@cindex G-expressions, syntax
@cindex gexps, syntax
@findex #~
@findex #$
@findex gexp
@findex ungexp
Guix defines a variant of S-expressions on steroids called
@dfn{G-expressions} or ``gexps'', which come with a variant of
@code{quasiquote} and @code{unquote}: @code{#~} (or @code{gexp}) and
@code{#$} (or @code{ungexp}).  They let you @emph{stage code for later
execution}.

For example, you'll encounter gexps in some package definitions where
they provide code to be executed during the package build process.  They
look like this:

@lisp
(use-modules (guix gexp)           ;so we can write gexps
             (gnu packages base))  ;for 'coreutils'

;; Below is a G-expression representing staged code.
#~(begin
    ;; Invoke 'ls' from the package defined by the 'coreutils'
    ;; variable.
    (system* #$(file-append coreutils "/bin/ls") "-l")

    ;; Create this package's output directory.
    (mkdir #$output))
@end lisp

@xref{G-Expressions,,, guix, GNU Guix Reference Manual}, for more on
gexps.

@item
Multiple variables can be named locally with @code{let} (@pxref{Local
Bindings,,, guile, GNU Guile Reference Manual}):

@lisp
(define x 10)
(let ((x 2)
      (y 3))
  (list x y))
@result{} (2 3)

x
@result{} 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))
@result{} (2 6)
@end lisp

@item
@dfn{Keywords} are typically used to identify the named parameters of a
procedure.  They are prefixed by @code{#:} (hash, colon) followed by
alphanumeric characters: @code{#:like-this}.
@xref{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} (@pxref{Creating Guile
Modules,,, guile, GNU Guile Reference Manual}).  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}.

@xref{Package Modules,,, guix, GNU Guix Reference Manual}, for info on
modules that define packages.
@end itemize

@quotation Going further
Scheme is a language that has been widely used to teach programming and
you'll find plenty of material using it as a vehicle.  Here's a
selection of documents to learn more about Scheme:

@itemize
@item
@uref{https://spritely.institute/static/papers/scheme-primer.html, @i{A
Scheme Primer}}, by Christine Lemmer-Webber and the Spritely Institute.

@item
@uref{http://www.troubleshooters.com/codecorn/scheme_guile/hello.htm,
@i{Scheme at a Glance}}, by Steve Litt.

@item
@c There used to be a copy at mitpress.mit.edu but it vanished.
@uref{https://sarabander.github.io/sicp/,
@i{Structure and Interpretation of Computer Programs}}, by Harold
Abelson and Gerald Jay Sussman, with Julie Sussman.  Colloquially known
as ``SICP'', this book is a reference.

You can also install it and read it from your computer:

@example
guix install sicp info-reader
info sicp
@end example

@end itemize

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


@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.

@menu
* A ``Hello World'' package::
* Setup::
* Extended example::
* Other build systems::
* Programmable and automated package definition::
* Getting help::
* Conclusion::
* References::
@end menu

@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.

@menu
* Local file::
* Channels::
* Direct checkout hacking::
@end menu

@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 Channels
@subsubsection Channels

@cindex channel
Guix and its package collection can be extended through @dfn{channels}.
A channel is a Git repository, public or not, containing @file{.scm}
files that provide packages (@pxref{Defining Packages,,, guix, GNU Guix
Reference Manual}) or services (@pxref{Defining Services,,, guix, GNU
Guix Reference Manual}).

How would you go about creating a channel?  First, create a directory
that will contain your @file{.scm} files, say @file{~/my-channel}:

@example
mkdir ~/my-channel
@end example

Suppose you want to add the @samp{my-hello} package we saw previously;
it first needs some adjustments:

@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 how do you make that package visible to @command{guix} commands so
you can test your packages?  You need to add the directory to the search
path using the @option{-L} command-line option, as in these examples:

@example
guix show -L ~/my-channel my-hello
guix build -L ~/my-channel my-hello
@end example

The final step is to turn @file{~/my-channel} into an actual channel,
making your package collection seamlessly available @i{via} any
@command{guix} command.  To do that, you first need to make it a Git
repository:

@example
cd ~/my-channel
git init
git add my-hello.scm
git commit -m "First commit of my channel."
@end example

And that's it, you have a channel!  From there on, you can add this
channel to your channel configuration in
@file{~/.config/guix/channels.scm} (@pxref{Specifying Additional
Channels,,, guix, GNU Guix Reference Manual}); assuming you keep your
channel local for now, the @file{channels.scm} would look something like
this:

@lisp
(append (list (channel
                (name 'my-channel)
                (url (string-append "file://" (getenv "HOME")
                                    "/my-channel"))))
        %default-channels)
@end lisp

Next time you run @command{guix pull}, your channel will be picked up
and the packages it defines will be readily available to all the
@command{guix} commands, even if you do not pass @option{-L}.  The
@command{guix describe} command will show that Guix is, indeed, using
both the @code{my-channel} and the @code{guix} channels.

@xref{Creating a Channel,,, guix, GNU Guix Reference Manual}, for
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 compression)
  #:use-module (gnu packages pkg-config)
  #:use-module (gnu packages python)
  #:use-module (gnu packages ssh)
  #:use-module (gnu packages tls)
  #:use-module (gnu packages web))

(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)))
                ;; Remove bundled software.
                (snippet '(delete-file-recursively "deps"))))
      (build-system cmake-build-system)
      (outputs '("out" "debug"))
      (arguments
       `(#:tests? #true                         ; 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")))))
           ;; Run checks more verbosely.
           (replace 'check
             (lambda* (#:key tests? #:allow-other-keys)
               (when tests?
                 (invoke "./libgit2_clar" "-v" "-Q"))))
           (add-after 'unpack 'make-files-writable-for-tests
             (lambda _ (for-each make-file-writable (find-files ".")))))))
      (inputs
       (list libssh2 http-parser python-wrapper))
      (native-inputs
       (list pkg-config))
      (propagated-inputs
       ;; These two libraries are in 'Requires.private' in libgit2.pc.
       (list openssl 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
directory with a descriptive name, we use @code{(file-name (git-file-name name
version))}.

The @code{git-version} procedure can be used to derive the
version when packaging programs for a specific commit, following the
Guix contributor guidelines (@pxref{Version Numbers,,, guix, GNU Guix
Reference Manual}).

How does one obtain the @code{sha256} hash that's in there, you ask?  By
invoking @command{guix hash} on a checkout of the desired commit, along
these lines:

@example
git clone https://github.com/libgit2/libgit2/
cd libgit2
git checkout v0.26.6
guix hash -rx .
@end example

@command{guix hash -rx} computes a SHA256 hash over the whole directory,
excluding the @file{.git} sub-directory (@pxref{Invoking guix hash,,,
guix, GNU Guix Reference Manual}).

In the future, @command{guix download} will hopefully be able to do
these steps for you, just like it does for regular downloads.

@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 @file{.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

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.

@quotation Note
You may see here and there snippets where package inputs are written
quite differently, like so:

@lisp
;; The "old style" for inputs.
(inputs
 `(("libssh2" ,libssh2)
   ("http-parser" ,http-parser)
   ("python" ,python-wrapper)))
@end lisp

This is the ``old style'', where each input in the list is explicitly
given a label (a string).  It is still supported but we recommend using
the style above instead.  @xref{package Reference,,, guix, GNU Guix
Reference Manual}, for more info.
@end quotation

@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
@file{/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")
,use (guix build gnu-build-system)
(map first %standard-phases)
@result{} (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? #true))
      (begin
        (if (string-suffix? ".zip" source)
            (invoke "unzip" source)
            (invoke "tar" "xvf" source))
        (chdir (first-subdirectory "."))))
  #true)
@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 @var{phase} @var{new-phase} @var{procedure})}: Run @var{procedure} named @var{new-phase} before @var{phase}.
@item
@code{(add-after @var{phase} @var{new-phase} @var{procedure})}: Same, but afterwards.
@item
@code{(replace @var{phase} @var{procedure})}.
@item
@code{(delete @var{phase})}.
@end itemize

The @var{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")))
    ;; ...
    #true))
@end lisp

The procedure must return @code{#true} 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{#true}.  Hence the trailing @code{#true} 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 @code
@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

@xref{Build Utilities,,, guix, GNU Guix Reference Manual}, for more
information on these utilities.

@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.

@xref{Build Systems,,, guix, GNU Guix Reference Manual}, for more
information on build systems, or check the source code in the
@samp{$GUIX_CHECKOUT/guix/build} and
@samp{$GUIX_CHECKOUT/guix/build-system} directories.

@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!

@menu
* Recursive importers::
* Automatic update::
* Inheritance::
@end menu

@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
      (list r-ggplot2 r-jmvcore r-r6 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 (list `(,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 file system 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
* Auto-Login to a Specific TTY::  Automatically Login a User to a Specific TTY
* Customizing the Kernel::      Creating and using a custom Linux kernel on Guix System.
* Guix System Image API::       Customizing images to target specific platforms.
* Using security keys::         How to use security keys with Guix System.
* Dynamic DNS mcron job::       Job to update the IP address behind a DuckDNS host name.
* Connecting to Wireguard VPN::  Connecting to a Wireguard VPN.
* Customizing a Window Manager::  Handle customization of a Window manager on Guix System.
* Running Guix on a Linode Server::  Running Guix on a Linode Server.
* Running Guix on a Kimsufi Server::  Running Guix on a Kimsufi Server.
* Setting up a bind mount::     Setting up a bind mount in the file-systems definition.
* Getting substitutes from Tor::  Configuring Guix daemon to get substitutes through Tor.
* Setting up NGINX with Lua::   Configuring NGINX web-server to load Lua modules.
* Music Server with Bluetooth Audio::  Headless music player with Bluetooth output.
@end menu

@node Auto-Login to a Specific TTY
@section Auto-Login to a Specific TTY

While the Guix manual explains auto-login one user to @emph{all} TTYs (
@pxref{auto-login to TTY,,, guix, GNU Guix Reference Manual}), some
might prefer a situation, in which one user is logged into one TTY with
the other TTYs either configured to login different users or no one at
all.  Note that one can auto-login one user to any TTY, but it is
usually advisable to avoid @code{tty1}, which, by default, is used to
log warnings and errors.

Here is how one might set up auto login for one user to one tty:

@lisp
(define (auto-login-to-tty config tty user)
  (if (string=? tty (mingetty-configuration-tty config))
        (mingetty-configuration
         (inherit config)
         (auto-login user))
        config))

(define %my-services
  (modify-services %base-services
    ;; @dots{}
    (mingetty-service-type config =>
                           (auto-login-to-tty
                            config "tty3" "alice"))))

(operating-system
  ;; @dots{}
  (services %my-services))
@end lisp

One could also @code{compose} (@pxref{Higher-Order Functions,,, guile,
The Guile Reference Manual}) @code{auto-login-to-tty} to login multiple
users to multiple ttys.

Finally, here is a note of caution.  Setting up auto login to a TTY,
means that anyone can turn on your computer and run commands as your
regular user.
However, if you have an encrypted root partition, and thus already need
to enter a passphrase when the system boots, auto-login might be a
convenient option.


@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 gnu-revision source supported-systems
                            #:key
                            (extra-version #f)
                            ;; A function that takes an arch and a variant.
                            ;; See kernel-config for an example.
                            (configuration-file #f)
                            (defconfig "defconfig")
                            (extra-options (default-extra-linux-options version)))
  ...)
@end lisp

The current @code{linux-libre} package is for the 5.15.x series, and is
declared like this:

@lisp
(define-public linux-libre-5.15
  (make-linux-libre* linux-libre-5.15-version
                     linux-libre-5.15-gnu-revision
                     linux-libre-5.15-source
                     '("x86_64-linux" "i686-linux" "armhf-linux"
                       "aarch64-linux" "riscv64-linux")
                     #: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,
notice the code comment that refers to @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 version)
  `(;; https://lists.gnu.org/archive/html/guix-devel/2014-04/msg00039.html
   ("CONFIG_DEVPTS_MULTIPLE_INSTANCES" . #true)
   ;; 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 . #true)
                       (string-append option "=y"))
                      ((option . #false)
                       (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
          %file-systems
          %efi-support
          %emulation
          ((@@@@ (gnu packages linux) default-extra-linux-options) version)))

(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-gnu-revision)
   (@@@@ (gnu packages linux) linux-libre-source)
   '("x86_64-linux")
   #:extra-version "macbook41"
   #:extra-options %macbook41-config-options))
@end lisp

In the above example @code{%file-systems} is a collection of flags
enabling different file system support, @code{%efi-support} enables EFI
support and @code{%emulation} enables a x86_64-linux machine to act in
32-bit mode also.  The @code{default-extra-linux-options} procedure is
the one defined above, which had to be used to avoid loosing the default
configuration options of 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 shell -D 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 file systems 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.

@node Guix System Image API
@section Guix System Image API

Historically, Guix System is centered around an @code{operating-system}
structure.  This structure contains various fields ranging from the
bootloader and kernel declaration to the services to install.

Depending on the target machine, that can go from a standard
@code{x86_64} machine to a small ARM single board computer such as the
Pine64, the image constraints can vary a lot.  The hardware
manufacturers will impose different image formats with various partition
sizes and offsets.

To create images suitable for all those machines, a new abstraction is
necessary: that's the goal of the @code{image} record.  This record
contains all the required information to be transformed into a
standalone image, that can be directly booted on any target machine.

@lisp
(define-record-type* <image>
  image make-image
  image?
  (name               image-name ;symbol
                      (default #f))
  (format             image-format) ;symbol
  (target             image-target
                      (default #f))
  (size               image-size  ;size in bytes as integer
                      (default 'guess))
  (operating-system   image-operating-system  ;<operating-system>
                      (default #f))
  (partitions         image-partitions ;list of <partition>
                      (default '()))
  (compression?       image-compression? ;boolean
                      (default #t))
  (volatile-root?     image-volatile-root? ;boolean
                      (default #t))
  (substitutable?     image-substitutable? ;boolean
                      (default #t)))
@end lisp

This record contains the operating-system to instantiate. The
@code{format} field defines the image type and can be @code{efi-raw},
@code{qcow2} or @code{iso9660} for instance. In the future, it could be
extended to @code{docker} or other image types.

A new directory in the Guix sources is dedicated to images definition. For now
there are four files:

@itemize @bullet
@item @file{gnu/system/images/hurd.scm}
@item @file{gnu/system/images/pine64.scm}
@item @file{gnu/system/images/novena.scm}
@item @file{gnu/system/images/pinebook-pro.scm}
@end itemize

Let's have a look to @file{pine64.scm}. It contains the
@code{pine64-barebones-os} variable which is a minimal definition of an
operating-system dedicated to the @b{Pine A64 LTS} board.

@lisp
(define pine64-barebones-os
  (operating-system
   (host-name "vignemale")
   (timezone "Europe/Paris")
   (locale "en_US.utf8")
   (bootloader (bootloader-configuration
                (bootloader u-boot-pine64-lts-bootloader)
                (targets '("/dev/vda"))))
   (initrd-modules '())
   (kernel linux-libre-arm64-generic)
   (file-systems (cons (file-system
                        (device (file-system-label "my-root"))
                        (mount-point "/")
                        (type "ext4"))
                       %base-file-systems))
   (services (cons (service agetty-service-type
                            (agetty-configuration
                             (extra-options '("-L")) ; no carrier detect
                             (baud-rate "115200")
                             (term "vt100")
                             (tty "ttyS0")))
                   %base-services))))
@end lisp

The @code{kernel} and @code{bootloader} fields are pointing to packages
dedicated to this board.

Right below, the @code{pine64-image-type} variable is also defined.

@lisp
(define pine64-image-type
  (image-type
   (name 'pine64-raw)
   (constructor (cut image-with-os arm64-disk-image <>))))
@end lisp

It's using a record we haven't talked about yet, the @code{image-type} record,
defined this way:

@lisp
(define-record-type* <image-type>
  image-type make-image-type
  image-type?
  (name           image-type-name) ;symbol
  (constructor    image-type-constructor)) ;<operating-system> -> <image>
@end lisp

The main purpose of this record is to associate a name to a procedure
transforming an @code{operating-system} to an image.  To understand why
it is necessary, let's have a look to the command producing an image
from an @code{operating-system} configuration file:

@example
guix system image my-os.scm
@end example

This command expects an @code{operating-system} configuration but how
should we indicate that we want an image targeting a Pine64 board?  We
need to provide an extra information, the @code{image-type}, by passing
the @code{--image-type} or @code{-t} flag, this way:

@example
guix system image --image-type=pine64-raw my-os.scm
@end example

This @code{image-type} parameter points to the @code{pine64-image-type}
defined above. Hence, the @code{operating-system} declared in
@code{my-os.scm} will be applied the @code{(cut image-with-os
arm64-disk-image <>)} procedure to turn it into an image.

The resulting image looks like:

@lisp
(image
 (format 'disk-image)
 (target "aarch64-linux-gnu")
 (operating-system my-os)
 (partitions
  (list (partition
         (inherit root-partition)
         (offset root-offset)))))
@end lisp

which is the aggregation of the @code{operating-system} defined in
 @code{my-os.scm} to the @code{arm64-disk-image} record.

But enough Scheme madness. What does this image API bring to the Guix user?

One can run:

@example
mathieu@@cervin:~$ guix system --list-image-types
The available image types are:

   - unmatched-raw
   - rock64-raw
   - pinebook-pro-raw
   - pine64-raw
   - novena-raw
   - hurd-raw
   - hurd-qcow2
   - qcow2
   - iso9660
   - uncompressed-iso9660
   - tarball
   - efi-raw
   - mbr-raw
   - docker
   - wsl2
   - raw-with-offset
   - efi32-raw
@end example

and by writing an @code{operating-system} file based on
@code{pine64-barebones-os}, you can customize your image to your
preferences in a file (@file{my-pine-os.scm}) like this:

@lisp
(use-modules (gnu services linux)
             (gnu system images pine64))

(let ((base-os pine64-barebones-os))
  (operating-system
    (inherit base-os)
    (timezone "America/Indiana/Indianapolis")
    (services
     (cons
      (service earlyoom-service-type
               (earlyoom-configuration
                (prefer-regexp "icecat|chromium")))
      (operating-system-user-services base-os)))))
@end lisp

run:

@example
guix system image --image-type=pine64-raw my-pine-os.scm
@end example

or,

@example
guix system image --image-type=hurd-raw my-hurd-os.scm
@end example

to get an image that can be written directly to a hard drive and booted
from.

Without changing anything to @code{my-hurd-os.scm}, calling:

@example
guix system image --image-type=hurd-qcow2 my-hurd-os.scm
@end example

will instead produce a Hurd QEMU image.

@node Using security keys
@section Using security keys
@cindex 2FA, two-factor authentication
@cindex U2F, Universal 2nd Factor
@cindex security key, configuration

The use of security keys can improve your security by providing a second
authentication source that cannot be easily stolen or copied, at least
for a remote adversary (something that you have), to the main secret (a
passphrase -- something that you know), reducing the risk of
impersonation.

The example configuration detailed below showcases what minimal
configuration needs to be made on your Guix System to allow the use of a
Yubico security key.  It is hoped the configuration can be useful for
other security keys as well, with minor adjustments.

@subsection Configuration for use as a two-factor authenticator (2FA)

To be usable, the udev rules of the system should be extended with
key-specific rules.  The following shows how to extend your udev rules
with the @file{lib/udev/rules.d/70-u2f.rules} udev rule file provided by
the @code{libfido2} package from the @code{(gnu packages
security-token)} module and add your user to the @samp{"plugdev"} group
it uses:

@lisp
(use-package-modules ... security-token ...)
...
(operating-system
 ...
 (users (cons* (user-account
               (name "your-user")
               (group "users")
               (supplementary-groups
		'("wheel" "netdev" "audio" "video"
                  "plugdev"))           ;<- added system group
               (home-directory "/home/your-user"))
              %base-user-accounts))
 ...
 (services
  (cons*
   ...
   (udev-rules-service 'fido2 libfido2 #:groups '("plugdev")))))
@end lisp

After re-configuring your system and re-logging in your graphical
session so that the new group is in effect for your user, you can verify
that your key is usable by launching:

@example
guix shell ungoogled-chromium -- chromium chrome://settings/securityKeys
@end example

and validating that the security key can be reset via the ``Reset your
security key'' menu.  If it works, congratulations, your security key is
ready to be used with applications supporting two-factor authentication
(2FA).

@subsection Disabling OTP code generation for a Yubikey
@cindex disabling yubikey OTP
If you use a Yubikey security key and are irritated by the spurious OTP
codes it generates when inadvertently touching the key (e.g. causing you
to become a spammer in the @samp{#guix} channel when discussing from
your favorite IRC client!), you can disable it via the following
@command{ykman} command:

@example
guix shell python-yubikey-manager -- ykman config usb --force --disable OTP
@end example

Alternatively, you could use the @command{ykman-gui} command provided by
the @code{yubikey-manager-qt} package and either wholly disable the
@samp{OTP} application for the USB interface or, from the
@samp{Applications -> OTP} view, delete the slot 1 configuration, which
comes pre-configured with the Yubico OTP application.

@subsection Requiring a Yubikey to open a KeePassXC database
@cindex yubikey, keepassxc integration
The KeePassXC password manager application has support for Yubikeys, but
it requires installing a udev rules for your Guix System and some
configuration of the Yubico OTP application on the key.

The necessary udev rules file comes from the
@code{yubikey-personalization} package, and can be installed like:

@lisp
(use-package-modules ... security-token ...)
...
(operating-system
 ...
 (services
  (cons*
   ...
   (udev-rules-service 'yubikey yubikey-personalization))))
@end lisp

After reconfiguring your system (and reconnecting your Yubikey), you'll
then want to configure the OTP challenge/response application of your
Yubikey on its slot 2, which is what KeePassXC uses.  It's easy to do so
via the Yubikey Manager graphical configuration tool, which can be
invoked with:

@example
guix shell yubikey-manager-qt -- ykman-gui
@end example

First, ensure @samp{OTP} is enabled under the @samp{Interfaces} tab,
then navigate to @samp{Applications -> OTP}, and click the
@samp{Configure} button under the @samp{Long Touch (Slot 2)} section.
Select @samp{Challenge-response}, input or generate a secret key, and
click the @samp{Finish} button.  If you have a second Yubikey you'd like
to use as a backup, you should configure it the same way, using the
@emph{same} secret key.

Your Yubikey should now be detected by KeePassXC.  It can be added to a
database by navigating to KeePassXC's @samp{Database -> Database
Security...}  menu, then clicking the @samp{Add additional
protection...} button, then @samp{Add Challenge-Response}, selecting the
security key from the drop-down menu and clicking the @samp{OK} button
to complete the setup.

@node Dynamic DNS mcron job
@section Dynamic DNS mcron job

@cindex dynamic DNS, DDNS
If your @acronym{ISP, Internet Service Provider} only provides dynamic
IP addresses, it can be useful to setup a dynamic @acronym{DNS, Domain
Name System} (also known as @acronym{DDNS, Dynamic DNS}) service to
associate a static host name to a public but dynamic (often changing) IP
address.  There are multiple existing services that can be used for
this; in the following mcron job, @url{https://duckdns.org, DuckDNS} is
used.  It should also work with other dynamic DNS services that offer a
similar interface to update the IP address, such as
@url{https://freedns.afraid.org/}, with minor adjustments.

The mcron job is provided below, where @var{DOMAIN} should be
substituted for your own domain prefix, and the DuckDNS provided token
associated to @var{DOMAIN} added to the
@file{/etc/duckdns/@var{DOMAIN}.token} file.

@lisp
(define duckdns-job
  ;; Update personal domain IP every 5 minutes.
  #~(job '(next-minute (range 0 60 5))
	 #$(program-file
            "duckdns-update"
            (with-extensions (list guile-gnutls) ;required by (web client)
              #~(begin
                  (use-modules (ice-9 textual-ports)
                               (web client))
                  (let ((token (string-trim-both
                                (call-with-input-file "/etc/duckdns/@var{DOMAIN}.token"
                                  get-string-all)))
                        (query-template (string-append "https://www.duckdns.org/"
                                                       "update?domains=@var{DOMAIN}"
                                                       "&token=~a&ip=")))
                    (http-get (format #f query-template token))))))
         "duckdns-update"
         #:user "nobody"))
@end lisp

The job then needs to be added to the list of mcron jobs for your
system, using something like:

@lisp
(operating-system
 (services
  (cons* (service mcron-service-type
           (mcron-configuration
             (jobs (list duckdns-job ...))))
         ...
         %base-services)))
@end lisp

@node Connecting to Wireguard VPN
@section Connecting to Wireguard VPN

To connect to a Wireguard VPN server you need the kernel module to be
loaded in memory and a package providing networking tools that support
it (e.g.  @code{wireguard-tools} or @code{network-manager}).

Here is a configuration example for Linux-Libre < 5.6, where the module
is out of tree and need to be loaded manually---following revisions of
the kernel have it built-in and so don't need such configuration:

@lisp
(use-modules (gnu))
(use-service-modules desktop)
(use-package-modules vpn)

(operating-system
  ;; …
  (services (cons (simple-service 'wireguard-module
                                  kernel-module-loader-service-type
                                  '("wireguard"))
                  %desktop-services))
  (packages (cons wireguard-tools %base-packages))
  (kernel-loadable-modules (list wireguard-linux-compat)))
@end lisp

After reconfiguring and restarting your system you can either use
Wireguard tools or NetworkManager to connect to a VPN server.

@subsection Using Wireguard tools

To test your Wireguard setup it is convenient to use @command{wg-quick}.
Just give it a configuration file @command{wg-quick up ./wg0.conf}; or
put that file in @file{/etc/wireguard} and run @command{wg-quick up wg0}
instead.

@quotation Note
Be warned that the author described this command as a: “[…] very quick
and dirty bash script […]”.
@end quotation

@subsection Using NetworkManager

Thanks to NetworkManager support for Wireguard we can connect to our VPN
using @command{nmcli} command.  Up to this point this guide assumes that
you're using Network Manager service provided by
@code{%desktop-services}.  Ortherwise you need to adjust your services
list to load @code{network-manager-service-type} and reconfigure your
Guix system.

To import your VPN configuration execute nmcli import command:

@example shell
# nmcli connection import type wireguard file wg0.conf
Connection 'wg0' (edbee261-aa5a-42db-b032-6c7757c60fde) successfully added
@end example

This will create a configuration file in
@file{/etc/NetworkManager/wg0.nmconnection}.  Next connect to the
Wireguard server:

@example shell
$ nmcli connection up wg0
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/6)
@end example

By default NetworkManager will connect automatically on system boot.  To
change that behaviour you need to edit your config:

@example shell
# nmcli connection modify wg0 connection.autoconnect no
@end example

For more specific information about NetworkManager and wireguard
@uref{https://blogs.gnome.org/thaller/2019/03/15/wireguard-in-networkmanager/,see
this post by thaller}.

@node Customizing a Window Manager
@section Customizing a Window Manager
@cindex wm

@menu
* StumpWM::
* Session lock::
@end menu

@node StumpWM
@subsection StumpWM
@cindex stumpwm

You could install StumpWM with a Guix system by adding
@code{stumpwm} and optionally @code{`(,stumpwm "lib")}
packages to a system configuration file, e.g.@: @file{/etc/config.scm}.

An example configuration can look like this:

@lisp
(use-modules (gnu))
(use-package-modules wm)

(operating-system
  ;; …
  (packages (append (list sbcl stumpwm `(,stumpwm "lib"))
                    %base-packages)))
@end lisp

@cindex stumpwm fonts
By default StumpWM uses X11 fonts, which could be small or pixelated on
your system.  You could fix this by installing StumpWM contrib Lisp
module @code{sbcl-ttf-fonts}, adding it to Guix system packages:

@lisp
(use-modules (gnu))
(use-package-modules fonts wm)

(operating-system
  ;; …
  (packages (append (list sbcl stumpwm `(,stumpwm "lib"))
                    sbcl-ttf-fonts font-dejavu %base-packages)))
@end lisp

Then you need to add the following code to a StumpWM configuration file
@file{~/.stumpwm.d/init.lisp}:

@lisp
(require :ttf-fonts)
(setf xft:*font-dirs* '("/run/current-system/profile/share/fonts/"))
(setf clx-truetype:+font-cache-filename+ (concat (getenv "HOME")
                                                 "/.fonts/font-cache.sexp"))
(xft:cache-fonts)
(set-font (make-instance 'xft:font :family "DejaVu Sans Mono"
                                   :subfamily "Book" :size 11))
@end lisp

@node Session lock
@subsection Session lock
@cindex sessionlock

Depending on your environment, locking the screen of your session might come built in
or it might be something you have to set up yourself. If you use a desktop environment
like GNOME or KDE, it's usually built in. If you use a plain window manager like
StumpWM or EXWM, you might have to set it up yourself.

@menu
* Xorg::
@end menu

@node Xorg
@subsubsection Xorg

If you use Xorg, you can use the utility
@uref{https://www.mankier.com/1/xss-lock, xss-lock} to lock the screen of your session.
xss-lock is triggered by DPMS which since Xorg 1.8 is auto-detected and enabled if
ACPI is also enabled at kernel runtime.

To use xss-lock, you can simple execute it and put it into the background before
you start your window manager from e.g. your @file{~/.xsession}:

@example
xss-lock -- slock &
exec stumpwm
@end example

In this example, xss-lock uses @code{slock} to do the actual locking of the screen when
it determines it's appropriate, like when you suspend your device.

For slock to be allowed to be a screen locker for the graphical session, it needs to
be made setuid-root so it can authenticate users, and it needs a PAM service. This
can be achieved by adding the following service to your @file{config.scm}:

@lisp
(service screen-locker-services-type
         (screen-locker-configuration
          (name "slock")
          (program (file-append slock "/bin/slock"))))
@end lisp

If you manually lock your screen, e.g. by directly calling slock when you want to lock
your screen but not suspend it, it's a good idea to notify xss-lock about this so no
confusion occurs. This can be done by executing @code{xset s activate} immediately
before you execute slock.

@node Running Guix on a Linode Server
@section Running Guix on a Linode Server
@cindex linode, Linode

To run Guix on a server hosted by @uref{https://www.linode.com, Linode},
start with a recommended Debian server.  We recommend using the default
distro as a way to bootstrap Guix. Create your SSH keys.

@example
ssh-keygen
@end example

Be sure to add your SSH key for easy login to the remote server.
This is trivially done via Linode's graphical interface for adding
SSH keys.  Go to your profile and click add SSH Key.
Copy into it the output of:

@example
cat ~/.ssh/<username>_rsa.pub
@end example

Power the Linode down.

In the Linode's Storage tab, resize the Debian disk to be smaller.
30 GB free space is recommended.  Then click "Add a disk", and fill
out the form with the following:

@itemize @bullet
@item
Label: "Guix"

@item
Filesystem: ext4

@item
Set it to the remaining size
@end itemize

In the Configurations tab, press "Edit" on the default Debian profile.
Under "Block Device Assignment" click "Add a Device". It should be
@file{/dev/sdc} and you can select the "Guix" disk. Save Changes.

Now "Add a Configuration", with the following:
@itemize @bullet
@item
Label: Guix

@item
Kernel:GRUB 2 (it's at the bottom!  This step is @b{IMPORTANT!})

@item
Block device assignment:

@item
@file{/dev/sda}: Guix

@item
@file{/dev/sdb}: swap

@item
Root device: @file{/dev/sda}

@item
Turn off all the filesystem/boot helpers
@end itemize

Now power it back up, booting with the Debian configuration.  Once it's
running, ssh to your server via @code{ssh
root@@@var{<your-server-IP-here>}}. (You can find your server IP address in
your Linode Summary section.) Now you can run the "install guix from
@pxref{Binary Installation,,, guix, GNU Guix}" steps:

@example
sudo apt-get install gpg
wget https://sv.gnu.org/people/viewgpg.php?user_id=15145 -qO - | gpg --import -
wget https://git.savannah.gnu.org/cgit/guix.git/plain/etc/guix-install.sh
chmod +x guix-install.sh
./guix-install.sh
guix pull
@end example

Now it's time to write out a config for the server.  The key information
is below. Save the resulting file as @file{guix-config.scm}.

@lisp
(use-modules (gnu)
             (guix modules))
(use-service-modules networking
                     ssh)
(use-package-modules admin
                     package-management
                     ssh
                     tls)

(operating-system
  (host-name "my-server")
  (timezone "America/New_York")
  (locale "en_US.UTF-8")
  ;; This goofy code will generate the grub.cfg
  ;; without installing the grub bootloader on disk.
  (bootloader (bootloader-configuration
               (bootloader
                (bootloader
                 (inherit grub-bootloader)
                 (installer #~(const #true))))))
  (file-systems (cons (file-system
                        (device "/dev/sda")
                        (mount-point "/")
                        (type "ext4"))
                      %base-file-systems))


  (swap-devices (list "/dev/sdb"))


  (initrd-modules (cons "virtio_scsi"    ; Needed to find the disk
                        %base-initrd-modules))

  (users (cons (user-account
                (name "janedoe")
                (group "users")
                ;; Adding the account to the "wheel" group
                ;; makes it a sudoer.
                (supplementary-groups '("wheel"))
                (home-directory "/home/janedoe"))
               %base-user-accounts))

  (packages (cons* openssh-sans-x
                   %base-packages))

  (services (cons*
             (service dhcp-client-service-type)
             (service openssh-service-type
                      (openssh-configuration
                       (openssh openssh-sans-x)
                       (password-authentication? #false)
                       (authorized-keys
                        `(("janedoe" ,(local-file "janedoe_rsa.pub"))
                          ("root" ,(local-file "janedoe_rsa.pub"))))))
             %base-services)))
@end lisp

Replace the following fields in the above configuration:
@lisp
(host-name "my-server")       ; replace with your server name
; if you chose a linode server outside the U.S., then
; use tzselect to find a correct timezone string
(timezone "America/New_York") ; if needed replace timezone
(name "janedoe")              ; replace with your username
("janedoe" ,(local-file "janedoe_rsa.pub")) ; replace with your ssh key
("root" ,(local-file "janedoe_rsa.pub")) ; replace with your ssh key
@end lisp

The last line in the above example lets you log into the server as root
and set the initial root password (see the note at the end of this
recipe about root login).  After you have done this, you may
delete that line from your configuration and reconfigure to prevent root
login.

Copy your ssh public key (eg: @file{~/.ssh/id_rsa.pub}) as
@file{@var{<your-username-here>}_rsa.pub} and put
@file{guix-config.scm} in the same directory.  In a new terminal run
these commands.

@example
sftp root@@<remote server ip address>
put /path/to/files/<username>_rsa.pub .
put /path/to/files/guix-config.scm .
@end example

In your first terminal, mount the guix drive:

@example
mkdir /mnt/guix
mount /dev/sdc /mnt/guix
@end example

Due to the way we set up the bootloader section of the guix-config.scm,
only the grub configuration file will be installed.  So, we need to copy
over some of the other GRUB stuff already installed on the Debian system:

@example
mkdir -p /mnt/guix/boot/grub
cp -r /boot/grub/* /mnt/guix/boot/grub/
@end example

Now initialize the Guix installation:

@example
guix system init guix-config.scm /mnt/guix
@end example

Ok, power it down!
Now from the Linode console, select boot and select "Guix".

Once it boots, you should be able to log in via SSH!  (The server config
will have changed though.)  You may encounter an error like:

@example
$ ssh root@@<server ip address>
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:0B+wp33w57AnKQuHCvQP0+ZdKaqYrI/kyU7CfVbS7R4.
Please contact your system administrator.
Add correct host key in /home/joshua/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/joshua/.ssh/known_hosts:3
ECDSA host key for 198.58.98.76 has changed and you have requested strict checking.
Host key verification failed.
@end example

Either delete @file{~/.ssh/known_hosts} file, or delete the offending line
starting with your server IP address.

Be sure to set your password and root's password.

@example
ssh root@@<remote ip address>
passwd  ; for the root password
passwd <username> ; for the user password
@end example

You may not be able to run the above commands at this point.  If you
have issues remotely logging into your linode box via SSH, then you may
still need to set your root and user password initially by clicking on
the ``Launch Console'' option in your linode.  Choose the ``Glish''
instead of ``Weblish''.  Now you should be able to ssh into the machine.

Hooray!  At this point you can shut down the server, delete the
Debian disk, and resize the Guix to the rest of the size.
Congratulations!

By the way, if you save it as a disk image right at this point, you'll
have an easy time spinning up new Guix images!  You may need to
down-size the Guix image to 6144MB, to save it as an image.  Then you
can resize it again to the max size.

@node Running Guix on a Kimsufi Server
@section Running Guix on a Kimsufi Server
@cindex kimsufi, Kimsufi, OVH

To run Guix on a server hosted by @uref{https://www.kimsufi.com/,
Kimsufi}, click on the netboot tab then select rescue64-pro and restart.

OVH will email you the credentials required to ssh into a Debian system.

Now you can run the "install guix from @pxref{Binary Installation,,,
guix, GNU Guix}" steps:

@example
wget https://git.savannah.gnu.org/cgit/guix.git/plain/etc/guix-install.sh
chmod +x guix-install.sh
./guix-install.sh
guix pull
@end example

Partition the drives and format them, first stop the raid array:

@example
mdadm --stop /dev/md127
mdadm --zero-superblock /dev/sda2 /dev/sdb2
@end example

Then wipe the disks and set up the partitions, we will create
a RAID 1 array.

@example
wipefs -a /dev/sda
wipefs -a /dev/sdb

parted /dev/sda --align=opt -s -m -- mklabel gpt
parted /dev/sda --align=opt -s -m -- \
 mkpart bios_grub 1049kb 512MiB \
 set 1 bios_grub on
parted /dev/sda --align=opt -s -m -- \
 mkpart primary 512MiB -512MiB
 set 2 raid on
parted /dev/sda --align=opt -s -m -- mkpart primary linux-swap 512MiB 100%

parted /dev/sdb --align=opt -s -m -- mklabel gpt
parted /dev/sdb --align=opt -s -m -- \
     mkpart bios_grub 1049kb 512MiB \
     set 1 bios_grub on
parted /dev/sdb --align=opt -s -m -- \
     mkpart primary 512MiB -512MiB \
     set 2 raid on
parted /dev/sdb --align=opt -s -m -- mkpart primary linux-swap 512MiB 100%
@end example

Create the array:

@example
mdadm --create /dev/md127 --level=1 --raid-disks=2 \
  --metadata=0.90 /dev/sda2 /dev/sdb2
@end example

Now create file systems on the relevant partitions, first the boot
partitions:

@example
mkfs.ext4  /dev/sda1
mkfs.ext4  /dev/sdb1
@end example

Then the root partition:

@example
mkfs.ext4 /dev/md127
@end example

Initialize the swap partitions:

@example
mkswap /dev/sda3
swapon /dev/sda3
mkswap /dev/sdb3
swapon /dev/sdb3
@end example

Mount the guix drive:

@example
mkdir /mnt/guix
mount /dev/md127 /mnt/guix
@end example

Now is time to write an operating system declaration @file{os.scm} file;
here is a sample:

@lisp
(use-modules (gnu) (guix))
(use-service-modules networking ssh vpn virtualization sysctl admin mcron)
(use-package-modules ssh tls tmux vpn virtualization)

(operating-system
  (host-name "kimsufi")

  (bootloader (bootloader-configuration
	       (bootloader grub-bootloader)
	       (targets (list "/dev/sda" "/dev/sdb"))
	       (terminal-outputs '(console))))

  ;; Add a kernel module for RAID-1 (aka. "mirror").
  (initrd-modules (cons* "raid1"  %base-initrd-modules))

  (mapped-devices
   (list (mapped-device
          (source (list "/dev/sda2" "/dev/sdb2"))
          (target "/dev/md127")
          (type raid-device-mapping))))

  (swap-devices
   (list (swap-space
          (target "/dev/sda3"))
         (swap-space
          (target "/dev/sdb3"))))

  (issue
   ;; Default contents for /etc/issue.
   "\
This is the GNU system at Kimsufi.  Welcome.\n")

  (file-systems (cons* (file-system
		         (mount-point "/")
		         (device "/dev/md127")
		         (type "ext4")
		         (dependencies mapped-devices))
		       %base-file-systems))

  (users (cons (user-account
	        (name "guix")
	        (comment "guix")
	        (group "users")
	        (supplementary-groups '("wheel"))
	        (home-directory "/home/guix"))
	       %base-user-accounts))

  (sudoers-file
   (plain-file "sudoers" "\
root ALL=(ALL) ALL
%wheel ALL=(ALL) ALL
guix ALL=(ALL) NOPASSWD:ALL\n"))

  ;; Globally-installed packages.
  (packages (cons* tmux gnutls wireguard-tools %base-packages))
  (services
   (cons*
    (service static-networking-service-type
	     (list (static-networking
		    (addresses (list (network-address
				      (device "enp3s0")
				      (value "@var{server-ip-address}/24"))))
		    (routes (list (network-route
				   (destination "default")
				   (gateway "@var{server-gateway}"))))
		    (name-servers '("213.186.33.99")))))

    (service unattended-upgrade-service-type)

    (service openssh-service-type
	     (openssh-configuration
	      (openssh openssh-sans-x)
	      (permit-root-login #f)
	      (authorized-keys
	       `(("guix" ,(plain-file "@var{ssh-key-name.pub}"
                                      "@var{ssh-public-key-content}"))))))
    (modify-services %base-services
      (sysctl-service-type
       config =>
       (sysctl-configuration
	(settings (append '(("net.ipv6.conf.all.autoconf" . "0")
			    ("net.ipv6.conf.all.accept_ra" . "0"))
			  %default-sysctl-settings))))))))
@end lisp

Don't forget to substitute the @var{server-ip-address},
@var{server-gateway}, @var{ssh-key-name} and
@var{ssh-public-key-content} variables with your own values.

The gateway is the last usable IP in your block so if you have a server
with an IP of @samp{37.187.79.10} then its gateway will be
@samp{37.187.79.254}.

Transfer your operating system declaration @file{os.scm} file on the
server via the @command{scp} or @command{sftp} commands.

Now all that is left is to install Guix with a @code{guix system init}
and restart.

However we first need to set up a chroot, because the root partition of
the rescue system is mounted on an aufs partition and if you try to
install Guix it will fail at the GRUB install step complaining about the
canonical path of "aufs".

Install packages that will be used in the chroot:

@example
guix install bash-static parted util-linux-with-udev coreutils guix
@end example

Then run the following to create directories needed for the chroot:

@example
cd /mnt && \
mkdir -p bin etc gnu/store root/.guix-profile/ root/.config/guix/current \
  var/guix proc sys dev
@end example

Copy the host resolv.conf in the chroot:

@example
cp /etc/resolv.conf etc/
@end example

Mount block devices, the store and its database and the current guix config:

@example
mount --rbind /proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --rbind /dev /mnt/dev
mount --rbind /var/guix/ var/guix/
mount --rbind /gnu/store gnu/store/
mount --rbind /root/.config/ root/.config/
mount --rbind /root/.guix-profile/bin/ bin
mount --rbind /root/.guix-profile root/.guix-profile/
@end example

Chroot in /mnt and install the system:

@example
chroot /mnt/ /bin/bash

guix system init /root/os.scm /guix
@end example

Finally, from the web user interface (UI), change @samp{netboot} to
@samp{boot to disk} and restart (also from the web UI).

Wait a few minutes and try to ssh with @code{ssh
guix@@@var{server-ip-address>} -i @var{path-to-your-ssh-key}}

You should have a Guix system up and running on Kimsufi;
congratulations!

@node Setting up a bind mount
@section Setting up a bind mount

To bind mount a file system, one must first set up some definitions
before the @code{operating-system} section of the system definition.  In
this example we will bind mount a folder from a spinning disk drive to
@file{/tmp}, to save wear and tear on the primary SSD, without
dedicating an entire partition to be mounted as @file{/tmp}.

First, the source drive that hosts the folder we wish to bind mount
should be defined, so that the bind mount can depend on it.

@lisp
(define source-drive ;; "source-drive" can be named anything you want.
   (file-system
    (device (uuid "UUID goes here"))
    (mount-point "/path-to-spinning-disk-goes-here")
    (type "ext4"))) ;Make sure to set this to the appropriate type for your drive.
@end lisp

The source folder must also be defined, so that guix will know it's not
a regular block device, but a folder.
@lisp
;; "source-directory" can be named any valid variable name.
(define (%source-directory) "/path-to-spinning-disk-goes-here/tmp")
@end lisp

Finally, inside the @code{file-systems} definition, we must add the
mount itself.

@lisp
(file-systems (cons*

                ...<other drives omitted for clarity>...

                ;; Must match the name you gave the source drive in the earlier definition.
                source-drive

                (file-system
                 ;; Make sure "source-directory" matches your earlier definition.
                 (device (%source-directory))
                 (mount-point "/tmp")
                 ;; We are mounting a folder, not a partition, so this type needs to be "none"
                 (type "none")
                 (flags '(bind-mount))
                 ;; Ensure "source-drive" matches what you've named the variable for the drive.
                 (dependencies (list source-drive))
                 )

                 ...<other drives omitted for clarity>...

                ))
@end lisp

@node Getting substitutes from Tor
@section Getting substitutes from Tor

Guix daemon can use a HTTP proxy to get substitutes, here we are
configuring it to get them via Tor.

@quotation Warning
@emph{Not all} Guix daemon's traffic will go through Tor!  Only
HTTP/HTTPS will get proxied; FTP, Git protocol, SSH, etc connections
will still go through the clearnet.  Again, this configuration isn't
foolproof some of your traffic won't get routed by Tor at all.  Use it
at your own risk.

Also note that the procedure described here applies only to package
substitution. When you update your guix distribution with
@command{guix pull}, you still need to use @command{torsocks} if
you want to route the connection to guix's git repository servers
through Tor.
@end quotation

Guix's substitute server is available as a Onion service, if you want
to use it to get your substitutes through Tor configure your system as
follow:

@lisp
(use-modules (gnu))
(use-service-module base networking)

(operating-system
  …
  (services
    (cons
      (service tor-service-type
              (tor-configuration
                (config-file (plain-file "tor-config"
                                         "HTTPTunnelPort 127.0.0.1:9250"))))
      (modify-services %base-services
        (guix-service-type
          config => (guix-configuration
                      (inherit config)
                      ;; ci.guix.gnu.org's Onion service
                      (substitute-urls "\
@value{SUBSTITUTE-TOR-URL}")
                      (http-proxy "http://localhost:9250")))))))
@end lisp

This will keep a tor process running that provides a HTTP CONNECT tunnel
which will be used by @command{guix-daemon}.  The daemon can use other
protocols than HTTP(S) to get remote resources, request using those
protocols won't go through Tor since we are only setting a HTTP tunnel
here.  Note that @code{substitutes-urls} is using HTTPS and not HTTP or
it won't work, that's a limitation of Tor's tunnel; you may want to use
@command{privoxy} instead to avoid such limitations.

If you don't want to always get substitutes through Tor but using it just
some of the times, then skip the @code{guix-configuration}.  When you
want to get a substitute from the Tor tunnel run:

@example
sudo herd set-http-proxy guix-daemon http://localhost:9250
guix build \
  --substitute-urls=@value{SUBSTITUTE-TOR-URL} @dots{}
@end example

@node Setting up NGINX with Lua
@section Setting up NGINX with Lua
@cindex nginx, lua, openresty, resty

NGINX could be extended with Lua scripts.

Guix provides NGINX service with ability to load Lua module and specific
Lua packages, and reply to requests by evaluating Lua scripts.

The following example demonstrates system definition with configuration
to evaluate @file{index.lua} Lua script on HTTP request to
@uref{http://localhost/hello} endpoint:

@example
local shell = require "resty.shell"

local stdin = ""
local timeout = 1000  -- ms
local max_size = 4096  -- byte

local ok, stdout, stderr, reason, status =
   shell.run([[/run/current-system/profile/bin/ls /tmp]], stdin, timeout, max_size)

ngx.say(stdout)
@end example

@lisp
(use-modules (gnu))
(use-service-modules #;… web)
(use-package-modules #;… lua)
(operating-system
  ;; …
  (services
   ;; …
   (service nginx-service-type
            (nginx-configuration
             (modules
              (list
               (file-append nginx-lua-module "/etc/nginx/modules/ngx_http_lua_module.so")))
             (lua-package-path (list lua-resty-core
                                     lua-resty-lrucache
                                     lua-resty-signal
                                     lua-tablepool
                                     lua-resty-shell))
             (lua-package-cpath (list lua-resty-signal))
             (server-blocks
              (list (nginx-server-configuration
                     (server-name '("localhost"))
                     (listen '("80"))
                     (root "/etc")
                     (locations (list
                                 (nginx-location-configuration
                                  (uri "/hello")
                                  (body (list #~(format #f "content_by_lua_file ~s;"
                                                        #$(local-file "index.lua"))))))))))))))
@end lisp

@node Music Server with Bluetooth Audio
@section Music Server with Bluetooth Audio
@cindex mpd
@cindex music server, headless
@cindex bluetooth, ALSA configuration

MPD, the Music Player Daemon, is a flexible server-side application for
playing music.  Client programs on different machines on the network ---
a mobile phone, a laptop, a desktop workstation --- can connect to it to
control the playback of audio files from your local music collection.
MPD decodes the audio files and plays them back on one or many outputs.

By default MPD will play to the default audio device.  In the example
below we make things a little more interesting by setting up a headless
music server.  There will be no graphical user interface, no Pulseaudio
daemon, and no local audio output.  Instead we will configure MPD with
two outputs: a bluetooth speaker and a web server to serve audio streams
to any streaming media player.

Bluetooth is often rather frustrating to set up.  You will have to pair
your Bluetooth device and make sure that the device is automatically
connected as soon as it powers on.  The Bluetooth system service
returned by the @code{bluetooth-service} procedure provides the
infrastructure needed to set this up.

Reconfigure your system with at least the following services and
packages:

@lisp
(operating-system
  ;; …
  (packages (cons* bluez bluez-alsa
                   %base-packages))
  (services
   ;; …
   (dbus-service #:services (list bluez-alsa))
   (bluetooth-service #:auto-enable? #t)))
@end lisp

Start the @code{bluetooth} service and then use @command{bluetoothctl}
to scan for Bluetooth devices.  Try to identify your Bluetooth speaker
and pick out its device ID from the resulting list of devices that is
indubitably dominated by a baffling smorgasbord of your neighbors' home
automation gizmos.  This only needs to be done once:

@example
$ bluetoothctl 
[NEW] Controller 00:11:22:33:95:7F BlueZ 5.40 [default]

[bluetooth]# power on
[bluetooth]# Changing power on succeeded

[bluetooth]# agent on
[bluetooth]# Agent registered

[bluetooth]# default-agent
[bluetooth]# Default agent request successful

[bluetooth]# scan on
[bluetooth]# Discovery started
[CHG] Controller 00:11:22:33:95:7F Discovering: yes
[NEW] Device AA:BB:CC:A4:AA:CD My Bluetooth Speaker
[NEW] Device 44:44:FF:2A:20:DC My Neighbor's TV
@dots{}

[bluetooth]# pair AA:BB:CC:A4:AA:CD
Attempting to pair with AA:BB:CC:A4:AA:CD
[CHG] Device AA:BB:CC:A4:AA:CD Connected: yes

[My Bluetooth Speaker]# [CHG] Device AA:BB:CC:A4:AA:CD UUIDs: 0000110b-0000-1000-8000-00xxxxxxxxxx
[CHG] Device AA:BB:CC:A4:AA:CD UUIDs: 0000110c-0000-1000-8000-00xxxxxxxxxx
[CHG] Device AA:BB:CC:A4:AA:CD UUIDs: 0000110e-0000-1000-8000-00xxxxxxxxxx
[CHG] Device AA:BB:CC:A4:AA:CD Paired: yes
Pairing successful

[CHG] Device AA:BB:CC:A4:AA:CD Connected: no

[bluetooth]# 
[bluetooth]# trust AA:BB:CC:A4:AA:CD
[bluetooth]# [CHG] Device AA:BB:CC:A4:AA:CD Trusted: yes
Changing AA:BB:CC:A4:AA:CD trust succeeded

[bluetooth]# 
[bluetooth]# connect AA:BB:CC:A4:AA:CD
Attempting to connect to AA:BB:CC:A4:AA:CD
[bluetooth]# [CHG] Device AA:BB:CC:A4:AA:CD RSSI: -63
[CHG] Device AA:BB:CC:A4:AA:CD Connected: yes
Connection successful

[My Bluetooth Speaker]# scan off
[CHG] Device AA:BB:CC:A4:AA:CD RSSI is nil
Discovery stopped
[CHG] Controller 00:11:22:33:95:7F Discovering: no
@end example

Congratulations, you can now automatically connect to your Bluetooth
speaker!

It is now time to configure ALSA to use the @emph{bluealsa} Bluetooth
module, so that you can define an ALSA pcm device corresponding to your
Bluetooth speaker.  For a headless server using @emph{bluealsa} with a
fixed Bluetooth device is likely simpler than configuring Pulseaudio and
its stream switching behavior.  We configure ALSA by crafting a custom
@code{alsa-configuration} for the @code{alsa-service-type}.  The
configuration will declare a @code{pcm} type @code{bluealsa} from the
@code{bluealsa} module provided by the @code{bluez-alsa} package, and
then define a @code{pcm} device of that type for your Bluetooth speaker.

All that is left then is to make MPD send audio data to this ALSA
device.  We also add a secondary MPD output that makes the currently
played audio files available as a stream through a web server on port
8080.  When enabled a device on the network could listen to the audio
stream by connecting any capable media player to the HTTP server on port
8080, independent of the status of the Bluetooth speaker.

What follows is the outline of an @code{operating-system} declaration
that should accomplish the above-mentioned tasks:

@lisp
(use-modules (gnu))
(use-service-modules audio dbus sound #;… etc)
(use-package-modules audio linux #;… etc)
(operating-system
  ;; …
  (packages (cons* bluez bluez-alsa
                   %base-packages))
  (services
   ;; …
   (service mpd-service-type
            (mpd-configuration
             (user "your-username")
             (music-dir "/path/to/your/music")
             (address "192.168.178.20")
             (outputs (list (mpd-output
                             (type "alsa")
                             (name "MPD")
                             (extra-options
                              ;; Use the same name as in the ALSA
                              ;; configuration below.
                              '((device . "pcm.btspeaker"))))
                            (mpd-output
                             (type "httpd")
                             (name "streaming")
                             (enabled? #false)
                             (always-on? #true)
                             (tags? #true)
                             (mixer-type 'null)
                             (extra-options
                              '((encoder . "vorbis")
                                (port    . "8080")
                                (bind-to-address . "192.168.178.20")
                                (max-clients . "0") ;no limit
                                (quality . "5.0")
                                (format  . "44100:16:1"))))))))
   (dbus-service #:services (list bluez-alsa))
   (bluetooth-service #:auto-enable? #t)
   (service alsa-service-type
            (alsa-configuration
             (pulseaudio? #false) ;we don't need it
             (extra-options
              #~(string-append "\
# Declare Bluetooth audio device type \"bluealsa\" from bluealsa module
pcm_type.bluealsa @{
    lib \""
#$(file-append bluez-alsa "/lib/alsa-lib/libasound_module_pcm_bluealsa.so") "\"
@}

# Declare control device type \"bluealsa\" from the same module
ctl_type.bluealsa @{
    lib \""
#$(file-append bluez-alsa "/lib/alsa-lib/libasound_module_ctl_bluealsa.so") "\"
@}

# Define the actual Bluetooth audio device.
pcm.btspeaker @{
    type bluealsa
    device \"AA:BB:CC:A4:AA:CD\" # unique device identifier
    profile \"a2dp\"
@}

# Define an associated controller.
ctl.btspeaker @{
    type bluealsa
@}
"))))))
@end lisp

Enjoy the music with the MPD client of your choice or a media player
capable of streaming via HTTP!


@c *********************************************************************
@node Containers
@chapter Containers

The kernel Linux provides a number of shared facilities that are
available to processes in the system.  These facilities include a shared
view on the file system, other processes, network devices, user and
group identities, and a few others.  Since Linux 3.19 a user can choose
to @emph{unshare} some of these shared facilities for selected
processes, providing them (and their child processes) with a different
view on the system.

A process with an unshared @code{mount} namespace, for example, has its
own view on the file system --- it will only be able to see directories
that have been explicitly bound in its mount namespace.  A process with
its own @code{proc} namespace will consider itself to be the only
process running on the system, running as PID 1.

Guix uses these kernel features to provide fully isolated environments
and even complete Guix System containers, lightweight virtual machines
that share the host system's kernel.  This feature comes in especially
handy when using Guix on a foreign distribution to prevent interference
from foreign libraries or configuration files that are available
system-wide.

@menu
* Guix Containers::             Perfectly isolated environments
* Guix System Containers::      A system inside your system
@end menu

@node Guix Containers
@section Guix Containers

The easiest way to get started is to use @command{guix shell} with the
@option{--container} option.  @xref{Invoking guix shell,,, guix, GNU
Guix Reference Manual} for a reference of valid options.

The following snippet spawns a minimal shell process with most
namespaces unshared from the system.  The current working directory is
visible to the process, but anything else on the file system is
unavailable.  This extreme isolation can be very useful when you want to
rule out any sort of interference from environment variables, globally
installed libraries, or configuration files.

@example
guix shell --container
@end example

It is a bleak environment, barren, desolate.  You will find that not
even the GNU coreutils are available here, so to explore this deserted
wasteland you need to use built-in shell commands.  Even the usually
gigantic @file{/gnu/store} directory is reduced to a faint shadow of
itself.

@example sh
$ echo /gnu/store/*
/gnu/store/@dots{}-gcc-10.3.0-lib
/gnu/store/@dots{}-glibc-2.33
/gnu/store/@dots{}-bash-static-5.1.8
/gnu/store/@dots{}-ncurses-6.2.20210619
/gnu/store/@dots{}-bash-5.1.8
/gnu/store/@dots{}-profile
/gnu/store/@dots{}-readline-8.1.1
@end example

@cindex exiting a container
There isn't much you can do in an environment like this other than
exiting it.  You can use @key{^D} or @command{exit} to terminate this
limited shell environment.

@cindex exposing directories, container
@cindex sharing directories, container
@cindex mapping locations, container
You can make other directories available inside of the container
environment; use @option{--expose=DIRECTORY} to bind-mount the given
directory as a read-only location inside the container, or use
@option{--share=DIRECTORY} to make the location writable.  With an
additional mapping argument after the directory name you can control the
name of the directory inside the container.  In the following example we
map @file{/etc} on the host system to @file{/the/host/etc} inside a
container in which the GNU coreutils are installed.

@example sh
$ guix shell --container --share=/etc=/the/host/etc coreutils
$ ls /the/host/etc
@end example

Similarly, you can prevent the current working directory from being
mapped into the container with the @option{--no-cwd} option.  Another
good idea is to create a dedicated directory that will serve as the
container's home directory, and spawn the container shell from that
directory.

@cindex hide system libraries, container
@cindex avoid ABI mismatch, container
On a foreign system a container environment can be used to compile
software that cannot possibly be linked with system libraries or with
the system's compiler toolchain.  A common use-case in a research
context is to install packages from within an R session.  Outside of a
container environment there is a good chance that the foreign compiler
toolchain and incompatible system libraries are found first, resulting
in incompatible binaries that cannot be used by R.  In a container shell
this problem disappears, as system libraries and executables simply
aren't available due to the unshared @code{mount} namespace.

Let's take a comprehensive manifest providing a comfortable development
environment for use with R:

@lisp
(specifications->manifest
  (list "r-minimal"

        ;; base packages
        "bash-minimal"
        "glibc-locales"
        "nss-certs"

        ;; Common command line tools lest the container is too empty.
        "coreutils"
        "grep"
        "which"
        "wget"
        "sed"

        ;; R markdown tools
        "pandoc"

        ;; Toolchain and common libraries for "install.packages"
        "gcc-toolchain@@10"
        "gfortran-toolchain"
        "gawk"
        "tar"
        "gzip"
        "unzip"
        "make"
        "cmake"
        "pkg-config"
        "cairo"
        "libxt"
        "openssl"
        "curl"
        "zlib"))
@end lisp

Let's use this to run R inside a container environment.  For convenience
we share the @code{net} namespace to use the host system's network
interfaces.  Now we can build R packages from source the traditional way
without having to worry about ABI mismatch or incompatibilities.

@example sh
$ guix shell --container --network --manifest=manifest.scm -- R

R version 4.2.1 (2022-06-23) -- "Funny-Looking Kid"
Copyright (C) 2022 The R Foundation for Statistical Computing
@dots{}
> e <- Sys.getenv("GUIX_ENVIRONMENT")
> Sys.setenv(GIT_SSL_CAINFO=paste0(e, "/etc/ssl/certs/ca-certificates.crt"))
> Sys.setenv(SSL_CERT_FILE=paste0(e, "/etc/ssl/certs/ca-certificates.crt"))
> Sys.setenv(SSL_CERT_DIR=paste0(e, "/etc/ssl/certs"))
> install.packages("Cairo", lib=paste0(getwd()))
@dots{}
* installing *source* package 'Cairo' ...
@dots{}
* DONE (Cairo)

The downloaded source packages are in
	'/tmp/RtmpCuwdwM/downloaded_packages'
> library("Cairo", lib=getwd())
> # success!
@end example

Using container shells is fun, but they can become a little cumbersome
when you want to go beyond just a single interactive process.  Some
tasks become a lot easier when they sit on the rock solid foundation of
a proper Guix System and its rich set of system services.  The next
section shows you how to launch a complete Guix System inside of a
container.


@node Guix System Containers
@section Guix System Containers

The Guix System provides a wide array of interconnected system services
that are configured declaratively to form a dependable stateless GNU
System foundation for whatever tasks you throw at it.  Even when using
Guix on a foreign distribution you can benefit from the design of Guix
System by running a system instance as a container.  Using the same
kernel features of unshared namespaces mentioned in the previous
section, the resulting Guix System instance is isolated from the host
system and only shares file system locations that you explicitly
declare.

A Guix System container differs from the shell process created by
@command{guix shell --container} in a number of important ways.  While
in a container shell the containerized process is a Bash shell process,
a Guix System container runs the Shepherd as PID 1.  In a system
container all system services (@pxref{Services,,, guix, GNU Guix
Reference Manual}) are set up just as they would be on a Guix System in
a virtual machine or on bare metal---this includes daemons managed by
the GNU@tie{}Shepherd (@pxref{Shepherd Services,,, guix, GNU Guix
Reference Manual}) as well as other kinds of extensions to the operating
system (@pxref{Service Composition,,, guix, GNU Guix Reference Manual}).

The perceived increase in complexity of running a Guix System container
is easily justified when dealing with more complex applications that
have higher or just more rigid requirements on their execution
contexts---configuration files, dedicated user accounts, directories for
caches or log files, etc.  In Guix System the demands of this kind of
software are satisfied through the deployment of system services.


@menu
* A Database Container::
* Container Networking::
@end menu

@node A Database Container
@subsection A Database Container

A good example might be a PostgreSQL database server.  Much of the
complexity of setting up such a database server is encapsulated in this
deceptively short service declaration:

@lisp
(service postgresql-service-type
         (postgresql-configuration
          (postgresql postgresql-14)))
@end lisp

A complete operating system declaration for use with a Guix System
container would look something like this:

@lisp
(use-modules (gnu))
(use-package-modules databases)
(use-service-modules databases)

(operating-system
  (host-name "container")
  (timezone "Europe/Berlin")
  (file-systems (cons (file-system
                        (device (file-system-label "does-not-matter"))
                        (mount-point "/")
                        (type "ext4"))
                      %base-file-systems))
  (bootloader (bootloader-configuration
               (bootloader grub-bootloader)
               (targets '("/dev/sdX"))))
  (services
   (cons* (service postgresql-service-type
                   (postgresql-configuration
                    (postgresql postgresql-14)
                    (config-file
                     (postgresql-config-file
                      (log-destination "stderr")
                      (hba-file
                       (plain-file "pg_hba.conf"
                                   "\
local	all	all			trust
host	all	all	10.0.0.1/32 	trust"))
                      (extra-config
                       '(("listen_addresses" "*")
                         ("log_directory"    "/var/log/postgresql")))))))
          (service postgresql-role-service-type
                   (postgresql-role-configuration
                    (roles
                     (list (postgresql-role
                            (name "test")
                            (create-database? #t))))))
          %base-services)))
@end lisp

With @code{postgresql-role-service-type} we define a role ``test'' and
create a matching database, so that we can test right away without any
further manual setup.  The @code{postgresql-config-file} settings allow
a client from IP address 10.0.0.1 to connect without requiring
authentication---a bad idea in production systems, but convenient for
this example.

Let's build a script that will launch an instance of this Guix System as
a container.  Write the @code{operating-system} declaration above to a
file @file{os.scm} and then use @command{guix system container} to build
the launcher.  (@pxref{Invoking guix system,,, guix, GNU Guix Reference
Manual}).

@example
$ guix system container os.scm
The following derivations will be built:
  /gnu/store/@dots{}-run-container.drv
  @dots{}
building /gnu/store/@dots{}-run-container.drv...
/gnu/store/@dots{}-run-container
@end example

Now that we have a launcher script we can run it to spawn the new system
with a running PostgreSQL service.  Note that due to some as yet
unresolved limitations we need to run the launcher as the root user, for
example with @command{sudo}.

@example
$ sudo /gnu/store/@dots{}-run-container
system container is running as PID 5983
@dots{}
@end example

Background the process with @key{Ctrl-z} followed by @command{bg}.  Note
the process ID in the output; we will need it to connect to the
container later.  You know what?  Let's try attaching to the container
right now.  We will use @command{nsenter}, a tool provided by the
@code{util-linux} package:

@example
$ guix shell util-linux
$ sudo nsenter -a -t 5983
root@@container /# pgrep -a postgres
49 /gnu/store/@dots{}-postgresql-14.4/bin/postgres -D /var/lib/postgresql/data --config-file=/gnu/store/@dots{}-postgresql.conf -p 5432
51 postgres: checkpointer
52 postgres: background writer
53 postgres: walwriter
54 postgres: autovacuum launcher
55 postgres: stats collector
56 postgres: logical replication launcher
root@@container /# exit
@end example

The PostgreSQL service is running in the container!


@node Container Networking
@subsection Container Networking
@cindex container networking

What good is a Guix System running a PostgreSQL database service as a
container when we can only talk to it with processes originating in the
container?  It would be much better if we could talk to the database
over the network.

The easiest way to do this is to create a pair of connected virtual
Ethernet devices (known as @code{veth}).  We move one of the devices
(@code{ceth-test}) into the @code{net} namespace of the container and
leave the other end (@code{veth-test}) of the connection on the host
system.

@example
pid=5983
ns="guix-test"
host="veth-test"
client="ceth-test"

# Attach the new net namespace "guix-test" to the container PID.
sudo ip netns attach $ns $pid

# Create the pair of devices
sudo ip link add $host type veth peer name $client

# Move the client device into the container's net namespace
sudo ip link set $client netns $ns
@end example

Then we configure the host side:

@example
sudo ip link set $host up
sudo ip addr add 10.0.0.1/24 dev $host
@end example

@dots{}and then we configure the client side:

@example
sudo ip netns exec $ns  ip link set lo up
sudo ip netns exec $ns  ip link set $client up
sudo ip netns exec $ns  ip addr add 10.0.0.2/24 dev $client
@end example

At this point the host can reach the container at IP address 10.0.0.2,
and the container can reach the host at IP 10.0.0.1.  This is all we
need to talk to the database server inside the container from the host
system on the outside.

@example
$ psql -h 10.0.0.2 -U test
psql (14.4)
Type "help" for help.

test=> CREATE TABLE hello (who TEXT NOT NULL);
CREATE TABLE
test=> INSERT INTO hello (who) VALUES ('world');
INSERT 0 1
test=> SELECT * FROM hello;
  who
-------
 world
(1 row)
@end example

Now that we're done with this little demonstration let's clean up:

@example
sudo kill $pid
sudo ip netns del $ns
sudo ip link del $host
@end example

@c *********************************************************************
@node Virtual Machines
@chapter Virtual Machines

Guix can produce disk images (@pxref{Invoking guix system,,, guix, GNU
Guix Reference Manual}) that can be used with virtual machines solutions
such as virt-manager, GNOME Boxes or the more bare QEMU, among others.

This chapter aims to provide hands-on, practical examples that relates
to the usage and configuration of virtual machines on a Guix System.

@menu
* Network bridge for QEMU::
* Routed network for libvirt::
@end menu

@node Network bridge for QEMU
@section Network bridge for QEMU
@cindex Network bridge interface
@cindex networking, bridge
@cindex qemu, network bridge

By default, QEMU uses a so-called ``user mode'' host network back-end,
which is convenient as it does not require any configuration.
Unfortunately, it is also quite limited.  In this mode, the guest
@abbr{VM, virtual machine} can access the network the same way the host
would, but it cannot be reached from the host.  Additionally, since the
QEMU user networking mode relies on ICMP, ICMP-based networking tools
such as @command{ping} do @emph{not} work in this mode.  Thus, it is
often desirable to configure a network bridge, which enables the guest
to fully participate in the network.  This is necessary, for example,
when the guest is to be used as a server.

@subsection Creating a network bridge interface

There are many ways to create a network bridge.  The following command
shows how to use NetworkManager and its @command{nmcli} command line
interface (CLI) tool, which should already be available if your
operating system declaration is based on one of the desktop templates:

@example sh
# nmcli con add type bridge con-name br0 ifname br0
@end example

To have this bridge be part of your network, you must associate your
network bridge with the Ethernet interface used to connect with the
network.  Assuming your interface is named @samp{enp2s0}, the following
command can be used to do so:

@example sh
# nmcli con add type bridge-slave ifname enp2s0 master br0
@end example

@quotation Important
Only Ethernet interfaces can be added to a bridge.  For wireless
interfaces, consider the routed network approach detailed in
@xref{Routed network for libvirt}.
@end quotation

By default, the network bridge will allow your guests to obtain their IP
address via DHCP, if available on your local network.  For simplicity,
this is what we will use here.  To easily find the guests, they can be
configured to advertise their host names via mDNS.

@subsection Configuring the QEMU bridge helper script

QEMU comes with a helper program to conveniently make use of a network
bridge interface as an unprivileged user @pxref{Network options,,, QEMU,
QEMU Documentation}.  The binary must be made setuid root for proper
operation; this can be achieved by adding it to the
@code{setuid-programs} field of your (host) @code{operating-system}
definition, as shown below:

@example lisp
(setuid-programs
 (cons (file-append qemu "/libexec/qemu-bridge-helper")
       %setuid-programs))
@end example

The file @file{/etc/qemu/bridge.conf} must also be made to allow the
bridge interface, as the default is to deny all.  Add the following to
your list of services to do so:

@example lisp
(extra-special-file "/etc/qemu/host.conf" "allow br0\n")
@end example

@subsection Invoking QEMU with the right command line options

When invoking QEMU, the following options should be provided so that the
network bridge is used, after having selected a unique MAC address for
the guest.

@quotation Important
By default, a single MAC address is used for all guests, unless
provided.  Failing to provide different MAC addresses to each virtual
machine making use of the bridge would cause networking issues.
@end quotation

@example sh
$ qemu-system-x86_64 [...] \
    -device virtio-net-pci,netdev=user0,mac=XX:XX:XX:XX:XX:XX \
    -netdev bridge,id=user0,br=br0 \
    [...]
@end example

To generate MAC addresses that have the QEMU registered prefix, the
following snippet can be employed:

@example sh
mac_address="52:54:00:$(dd if=/dev/urandom bs=512 count=1 2>/dev/null \
                           | md5sum \
                           | sed -E 's/^(..)(..)(..).*$/\1:\2:\3/')"
echo $mac_address
@end example

@subsection Networking issues caused by Docker

If you use Docker on your machine, you may experience connectivity
issues when attempting to use a network bridge, which are caused by
Docker also relying on network bridges and configuring its own routing
rules.  The solution is add the following @code{iptables} snippet to
your @code{operating-system} declaration:

@example lisp
(service iptables-service-type
             (iptables-configuration
              (ipv4-rules (plain-file "iptables.rules" "\
*filter
:INPUT ACCEPT [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
-A FORWARD -i br0 -o br0 -j ACCEPT
COMMIT
"))
@end example

@node Routed network for libvirt
@section Routed network for libvirt
@cindex Virtual network bridge interface
@cindex networking, virtual bridge
@cindex libvirt, virtual network bridge

If the machine hosting your virtual machines is connected wirelessly to
the network, you won't be able to use a true network bridge as explained
in the preceding section (@pxref{Network bridge for QEMU}).  In this
case, the next best option is to use a @emph{virtual} bridge with static
routing and to configure a libvirt-powered virtual machine to use it
(via the @command{virt-manager} GUI for example).  This is similar to
the default mode of operation of QEMU/libvirt, except that instead of
using @abbr{NAT, Network Address Translation}, it relies on static
routes to join the @abbr{VM, virtual machine} IP address to the
@abbr{LAN, local area network}.  This provides two-way connectivity to
and from the virtual machine, which is needed for exposing services
hosted on the virtual machine.

@subsection Creating a virtual network bridge

A virtual network bridge consists of a few components/configurations,
such as a @abbr{TUN, network tunnel} interface, DHCP server (dnsmasq)
and firewall rules (iptables).  The @command{virsh} command, provided by
the @code{libvirt} package, makes it very easy to create a virtual
bridge.  You first need to choose a network subnet for your virtual
bridge; if your home LAN is in the @samp{192.168.1.0/24} network, you
could opt to use e.g.@: @samp{192.168.2.0/24}.  Define an XML file,
e.g.@: @file{/tmp/virbr0.xml}, containing the following:

@example
<network>
  <name>virbr0</name>
  <bridge name="virbr0" />
  <forward mode="route"/>
  <ip address="192.168.2.0" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.2.1" end="192.168.2.254"/>
    </dhcp>
  </ip>
</network>
@end example

Then create and configure the interface using the @command{virsh}
command, as root:

@example
virsh net-define /tmp/virbr0.xml
virsh net-autostart virbr0
virsh net-start virbr0
@end example

The @samp{virbr0} interface should now be visible e.g.@: via the
@samp{ip address} command.  It will be automatically started every time
your libvirt virtual machine is started.

@subsection Configuring the static routes for your virtual bridge

If you configured your virtual machine to use your newly created
@samp{virbr0} virtual bridge interface, it should already receive an IP
via DHCP such as @samp{192.168.2.15} and be reachable from the server
hosting it, e.g.@: via @samp{ping 192.168.2.15}.  There's one last
configuration needed so that the VM can reach the external network:
adding static routes to the network's router.

In this example, the LAN network is @samp{192.168.1.0/24} and the router
configuration web page may be accessible via e.g.@: the
@url{http://192.168.1.1} page.  On a router running the
@url{https://librecmc.org/, libreCMC} firmware, you would navigate to
the @clicksequence{Network @click{} Static Routes} page
(@url{https://192.168.1.1/cgi-bin/luci/admin/network/routes}), and you
would add a new entry to the @samp{Static IPv4 Routes} with the
following information:

@table @samp
@item Interface
lan
@item Target
192.168.2.0
@item IPv4-Netmask
255.255.255.0
@item IPv4-Gateway
@var{server-ip}
@item Route type
unicast
@end table

where @var{server-ip} is the IP address of the machine hosting the VMs,
which should be static.

After saving/applying this new static route, external connectivity
should work from within your VM; you can e.g.@: run @samp{ping gnu.org}
to verify that it functions correctly.

@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:
@dfn{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.

@quotation Note
This section is an opinionated guide on the use of multiple profiles.
It predates @command{guix shell} and its fast profile cache
(@pxref{Invoking guix shell,,, guix, GNU Guix Reference Manual}).

In many cases, you may find that using @command{guix shell} to set up
the environment you need, when you need it, is less work that
maintaining a dedicated profile.  Your call!
@end quotation

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 frictionless.
@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!

@menu
* Basic setup with manifests::
* Required packages::
* Default profile::
* The benefits of manifests::
* Reproducible profiles::
@end menu

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

A Guix profile can be set up @i{via} a @dfn{manifest}.  A manifest is a
snippet of Scheme code that specifies the set of packages you want to
have in your profile; it 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

@xref{Writing Manifests,,, guix, GNU Guix Reference Manual}, for
more information about the syntax.

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 @file{~/.config/guix/current}.

To enable all profiles on login, add this to your @file{~/.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
@file{~/.guix-profile} is activated from @file{/etc/profile}, that latter being loaded by
@file{~/.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
@file{~/.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 @file{~/.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 let you @dfn{declare} the set of packages you'd like to have
in a profile (@pxref{Writing Manifests,,, guix, GNU Guix Reference Manual}).
They 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.  The @command{guix shell} command also protects
recently-used profiles from garbage collection; profiles that have not
been used for a while may be garbage-collected though, along with the
packages they refer to.

To be 100% sure that a given profile will never be collected,
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 (@pxref{Writing Manifests,,, guix, GNU Guix Reference Manual});
@item
a Guix channel specification (@pxref{Replicating Guix,,, guix, GNU Guix
Reference Manual}).
@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} (@pxref{Invoking guix describe,,, guix, GNU Guix
Reference Manual}).
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 -p "$GUIX_EXTRA"/my-project
guix pull --channels=channel-specs.scm --profile="$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.

@node Software Development
@chapter Software Development

@cindex development, with Guix
@cindex software development, with Guix
Guix is a handy tool for developers; @command{guix shell}, in
particular, gives a standalone development environment for your package,
no matter what language(s) it's written in (@pxref{Invoking guix
shell,,, guix, GNU Guix Reference Manual}).  To benefit from it, you
have to initially write a package definition and have it either in Guix
proper, or in a channel, or directly in your project's source tree as a
@file{guix.scm} file.  This last option is appealing: all developers
have to do to get set up is clone the project's repository and run
@command{guix shell}, with no arguments.

Development needs go beyond development environments though.  How can
developers perform continuous integration of their code in Guix build
environments? How can they deliver their code straight to adventurous
users? This chapter describes a set of files developers can add to their
repository to set up Guix-based development environments, continuous
integration, and continuous delivery---all at once@footnote{This chapter
is adapted from a
@uref{https://guix.gnu.org/en/blog/2023/from-development-environments-to-continuous-integrationthe-ultimate-guide-to-software-development-with-guix/,
blog post} published in June 2023 on the Guix web site.}.

@menu
* Getting Started::             Step 0: using `guix shell'.
* Building with Guix::          Step 1: building your code.
* The Repository as a Channel::  Step 2: turning the repo in a channel.
* Package Variants::            Bonus: Defining variants.
* Setting Up Continuous Integration::  Step 3: continuous integration.
* Build Manifest::              Bonus: Manifest.
* Wrapping Up::                 Recap.
@end menu

@node Getting Started
@section Getting Started

How do we go about ``Guixifying'' a repository? The first step, as we've
seen, will be to add a @file{guix.scm} at the root of the repository in
question. We'll take @uref{https://www.gnu.org/software/guile,Guile} as
an example in this chapter: it's written in Scheme (mostly) and C, and
has a number of dependencies---a C compilation tool chain, C libraries,
Autoconf and its friends, LaTeX, and so on. The resulting
@file{guix.scm} looks like the usual package definition (@pxref{Defining
Packages,,, guix, GNU Guix Reference Manual}), just without the
@code{define-public} bit:

@lisp
;; The ‘guix.scm’ file for Guile, for use by ‘guix shell’.

(use-modules (guix)
             (guix build-system gnu)
             ((guix licenses) #:prefix license:)
             (gnu packages autotools)
             (gnu packages base)
             (gnu packages bash)
             (gnu packages bdw-gc)
             (gnu packages compression)
             (gnu packages flex)
             (gnu packages gdb)
             (gnu packages gettext)
             (gnu packages gperf)
             (gnu packages libffi)
             (gnu packages libunistring)
             (gnu packages linux)
             (gnu packages pkg-config)
             (gnu packages readline)
             (gnu packages tex)
             (gnu packages texinfo)
             (gnu packages version-control))

(package
  (name "guile")
  (version "3.0.99-git")                          ;funky version number
  (source #f)                                     ;no source
  (build-system gnu-build-system)
  (native-inputs
   (append (list autoconf
                 automake
                 libtool
                 gnu-gettext
                 flex
                 texinfo
                 texlive-base                 ;for "make pdf"
                 texlive-epsf
                 gperf
                 git
                 gdb
                 strace
                 readline
                 lzip
                 pkg-config)

           ;; When cross-compiling, a native version of Guile itself is
           ;; needed.
           (if (%current-target-system)
               (list this-package)
               '())))
  (inputs
   (list libffi bash-minimal))
  (propagated-inputs
   (list libunistring libgc))

  (native-search-paths
   (list (search-path-specification
          (variable "GUILE_LOAD_PATH")
          (files '("share/guile/site/3.0")))
         (search-path-specification
          (variable "GUILE_LOAD_COMPILED_PATH")
          (files '("lib/guile/3.0/site-ccache")))))
  (synopsis "Scheme implementation intended especially for extensions")
  (description
   "Guile is the GNU Ubiquitous Intelligent Language for Extensions,
and it's actually a full-blown Scheme implementation!")
  (home-page "https://www.gnu.org/software/guile/")
  (license license:lgpl3+))
@end lisp

Quite a bit of boilerplate, but now someone who'd like to hack on Guile
now only needs to run:

@lisp
guix shell
@end lisp

That gives them a shell containing all the dependencies of Guile: those
listed above, but also @emph{implicit dependencies} such as the GCC tool
chain, GNU@ Make, sed, grep, and so on.  @xref{Invoking guix shell,,,
guix, GNU Guix Reference Manual}, for more info on @command{guix shell}.

@quotation The chef's recommendation
Our suggestion is to create development environments like this:

@example
guix shell --container --link-profile
@end example

@noindent
... or, for short:

@example
guix shell -CP
@end example

That gives a shell in an isolated container, and all the dependencies
show up in @code{$HOME/.guix-profile}, which plays well with caches such
as @file{config.cache} (@pxref{Cache Files,,, autoconf, Autoconf}) and
absolute file names recorded in generated @code{Makefile}s and the
likes. The fact that the shell runs in a container brings peace of mind:
nothing but the current directory and Guile's dependencies is visible
inside the container; nothing from the system can possibly interfere
with your development.
@end quotation

@node Building with Guix
@section Level 1: Building with Guix

Now that we have a package definition (@pxref{Getting Started}), why not
also take advantage of it so we can build Guile with Guix? We had left
the @code{source} field empty, because @command{guix shell} above only
cares about the @emph{inputs} of our package---so it can set up the
development environment---not about the package itself.

To build the package with Guix, we'll need to fill out the @code{source}
field, along these lines:

@lisp
(use-modules (guix)
             (guix git-download)  ;for ‘git-predicate’
             @dots{})

(define vcs-file?
  ;; Return true if the given file is under version control.
  (or (git-predicate (current-source-directory))
      (const #t)))                                ;not in a Git checkout

(package
  (name "guile")
  (version "3.0.99-git")                          ;funky version number
  (source (local-file "." "guile-checkout"
                      #:recursive? #t
                      #:select? vcs-file?))
  @dots{})
@end lisp

Here's what we changed compared to the previous section:

@enumerate 
@item
We added @code{(guix git-download)} to our set of imported modules, so
we can use its @code{git-predicate} procedure.
@item
We defined @code{vcs-file?} as a procedure that returns true when passed
a file that is under version control. For good measure, we add a
fallback case for when we're not in a Git checkout: always return true.
@item
We set @code{source} to a
@uref{https://guix.gnu.org/manual/devel/en/html_node/G_002dExpressions.html#index-local_002dfile,@code{local-file}}---a
recursive copy of the current directory (@code{"."}), limited to files
under version control (the @code{#:select?} bit).
@end enumerate

From there on, our @file{guix.scm} file serves a second purpose: it lets
us build the software with Guix. The whole point of building with Guix
is that it's a ``clean'' build---you can be sure nothing from your
working tree or system interferes with the build result---and it lets
you test a variety of things. First, you can do a plain native build:

@example
guix build -f guix.scm
@end example

But you can also build for another system (possibly after setting up
@pxref{Daemon Offload Setup, offloading,, guix, GNU Guix Reference Manual}
or
@pxref{Virtualization Services, transparent emulation,, guix, GNU Guix
Reference Manual}):

@lisp
guix build -f guix.scm -s aarch64-linux -s riscv64-linux
@end lisp

@noindent
@dots{} or cross-compile:

@lisp
guix build -f guix.scm --target=x86_64-w64-mingw32
@end lisp

You can also use @dfn{package transformations} to test package variants
(@pxref{Package Transformation Options,,, guix, GNU Guix Reference Manual}):

@example
# What if we built with Clang instead of GCC?
guix build -f guix.scm \
  --with-c-toolchain=guile@@3.0.99-git=clang-toolchain

# What about that under-tested configure flag?
guix build -f guix.scm \
  --with-configure-flag=guile@@3.0.99-git=--disable-networking
@end example

Handy!

@node The Repository as a Channel
@section Level 2: The Repository as a Channel

We now have a Git repository containing (among other things) a package
definition (@pxref{Building with Guix}).  Can't we turn it into a
@dfn{channel} (@pxref{Channels,,, guix, GNU Guix Reference Manual})?
After all, channels are designed to ship package definitions to users,
and that's exactly what we're doing with our @file{guix.scm}.

Turns out we can indeed turn it into a channel, but with one caveat: we
must create a separate directory for the @code{.scm} file(s) of our
channel so that @command{guix pull} doesn't load unrelated @code{.scm}
files when someone pulls the channel---and in Guile, there are lots of
them! So we'll start like this, keeping a top-level @file{guix.scm}
symlink for the sake of @command{guix shell}:

@lisp
mkdir -p .guix/modules
mv guix.scm .guix/modules/guile-package.scm
ln -s .guix/modules/guile-package.scm guix.scm
@end lisp

To make it usable as part of a channel, we need to turn our
@file{guix.scm} file into a @dfn{package module} (@pxref{Package
Modules,,, guix, GNU Guix Reference Manual}):
we do that by changing the @code{use-modules} form at the top to a
@code{define-module} form. We also need to actually @emph{export} a
package variable, with @code{define-public}, while still returning the
package value at the end of the file so we can still use
@command{guix shell} and @command{guix build -f guix.scm}. The end result
looks like this (not repeating things that haven't changed):

@lisp
(define-module (guile-package)
  #:use-module (guix)
  #:use-module (guix git-download)   ;for ‘git-predicate’
  @dots{})

(define vcs-file?
  ;; Return true if the given file is under version control.
  (or (git-predicate (dirname (dirname (current-source-directory))))
      (const #t)))                                ;not in a Git checkout

(define-public guile
  (package
    (name "guile")
    (version "3.0.99-git")                          ;funky version number
    (source (local-file "../.." "guile-checkout"
                        #:recursive? #t
                        #:select? vcs-file?))
    @dots{}))

;; Return the package object define above at the end of the module.
guile
@end lisp

We need one last thing: a
@uref{https://guix.gnu.org/manual/devel/en/html_node/Package-Modules-in-a-Sub_002ddirectory.html,@code{.guix-channel}
file} so Guix knows where to look for package modules in our repository:

@lisp
;; This file lets us present this repo as a Guix channel.

(channel
  (version 0)
  (directory ".guix/modules"))  ;look for package modules under .guix/modules/
@end lisp

To recap, we now have these files:

@lisp
.
├── .guix-channel
├── guix.scm → .guix/modules/guile-package.scm
└── .guix
    └── modules
       └── guile-package.scm
@end lisp

And that's it: we have a channel! (We could do better and support
@uref{https://guix.gnu.org/manual/devel/en/html_node/Specifying-Channel-Authorizations.html,@emph{channel
authentication}} so users know they're pulling genuine code. We'll spare
you the details here but it's worth considering!) Users can pull from
this channel by
@uref{https://guix.gnu.org/manual/devel/en/html_node/Specifying-Additional-Channels.html,adding
it to @code{~/.config/guix/channels.scm}}, along these lines:

@lisp
(append (list (channel
                (name 'guile)
                (url "https://git.savannah.gnu.org/git/guile.git")
                (branch "main")))
        %default-channels)
@end lisp

After running @command{guix pull}, we can see the new package:

@example
$ guix describe
Generation 264  May 26 2023 16:00:35    (current)
  guile 36fd2b4
    repository URL: https://git.savannah.gnu.org/git/guile.git
    branch: main
    commit: 36fd2b4920ae926c79b936c29e739e71a6dff2bc
  guix c5bc698
    repository URL: https://git.savannah.gnu.org/git/guix.git
    commit: c5bc698e8922d78ed85989985cc2ceb034de2f23
$ guix package -A ^guile$
guile   3.0.99-git      out,debug       guile-package.scm:51:4
guile   3.0.9           out,debug       gnu/packages/guile.scm:317:2
guile   2.2.7           out,debug       gnu/packages/guile.scm:258:2
guile   2.2.4           out,debug       gnu/packages/guile.scm:304:2
guile   2.0.14          out,debug       gnu/packages/guile.scm:148:2
guile   1.8.8           out             gnu/packages/guile.scm:77:2
$ guix build guile@@3.0.99-git
[@dots{}]
/gnu/store/axnzbl89yz7ld78bmx72vpqp802dwsar-guile-3.0.99-git-debug
/gnu/store/r34gsij7f0glg2fbakcmmk0zn4v62s5w-guile-3.0.99-git
@end example

That's how, as a developer, you get your software delivered directly
into the hands of users! No intermediaries, yet no loss of transparency
and provenance tracking.

With that in place, it also becomes trivial for anyone to create Docker
images, Deb/RPM packages, or a plain tarball with @command{guix pack}
(@pxref{Invoking guix pack,,, guix, GNU Guix Reference Manual}):

@example
# How about a Docker image of our Guile snapshot?
guix pack -f docker -S /bin=bin guile@@3.0.99-git

# And a relocatable RPM?
guix pack -f rpm -R -S /bin=bin guile@@3.0.99-git
@end example

@node Package Variants
@section Bonus: Package Variants

We now have an actual channel, but it contains only one package
(@pxref{The Repository as a Channel}).  While we're at it, we can define
@dfn{package variants} (@pxref{Defining Package Variants,,, guix, GNU
Guix Reference Manual}) in our @file{guile-package.scm} file, variants
that we want to be able to test as Guile developers---similar to what we
did above with transformation options. We can add them like so:

@lisp
;; This is the ‘.guix/modules/guile-package.scm’ file.

(define-module (guile-package)
  @dots{})

(define-public guile
  @dots{})

(define (package-with-configure-flags p flags)
  "Return P with FLAGS as additional 'configure' flags."
  (package/inherit p
    (arguments
     (substitute-keyword-arguments (package-arguments p)
       ((#:configure-flags original-flags #~(list))
        #~(append #$original-flags #$flags))))))

(define-public guile-without-threads
  (package
    (inherit (package-with-configure-flags guile
                                           #~(list "--without-threads")))
    (name "guile-without-threads")))

(define-public guile-without-networking
  (package
    (inherit (package-with-configure-flags guile
                                           #~(list "--disable-networking")))
    (name "guile-without-networking")))


;; Return the package object defined above at the end of the module.
guile
@end lisp

We can build these variants as regular packages once we've pulled the
channel. Alternatively, from a checkout of Guile, we can run a command
like this one from the top level:

@lisp
guix build -L $PWD/.guix/modules guile-without-threads
@end lisp

@node Setting Up Continuous Integration
@section Level 3: Setting Up Continuous Integration

@cindex continuous integration (CI)
The channel we defined above (@pxref{The Repository as a Channel})
becomes even more interesting once we set up
@uref{https://en.wikipedia.org/wiki/Continuous_integration,
@dfn{continuous integration}} (CI). There are several ways to do that.

You can use one of the mainstream continuous integration tools, such as
GitLab-CI. To do that, you need to make sure you run jobs in a Docker
image or virtual machine that has Guix installed. If we were to do that
in the case of Guile, we'd have a job that runs a shell command like
this one:

@lisp
guix build -L $PWD/.guix/modules guile@@3.0.99-git
@end lisp

Doing this works great and has the advantage of being easy to achieve on
your favorite CI platform.

That said, you'll really get the most of it by using
@uref{https://guix.gnu.org/en/cuirass,Cuirass}, a CI tool designed for
and tightly integrated with Guix. Using it is more work than using a
hosted CI tool because you first need to set it up, but that setup phase
is greatly simplified if you use its Guix System service
(@pxref{Continuous Integration,,, guix, GNU Guix Reference Manual}).
Going back to our example, we give Cuirass a spec file that goes like
this:

@lisp
;; Cuirass spec file to build all the packages of the ‘guile’ channel.
(list (specification
        (name "guile")
        (build '(channels guile))
        (channels
         (append (list (channel
                         (name 'guile)
                         (url "https://git.savannah.gnu.org/git/guile.git")
                         (branch "main")))
                 %default-channels))))
@end lisp

It differs from what you'd do with other CI tools in two important ways:

@itemize
@item
Cuirass knows it's tracking @emph{two} channels, @code{guile} and
@code{guix}. Indeed, our own @code{guile} package depends on many
packages provided by the @code{guix} channel---GCC, the GNU libc,
libffi, and so on. Changes to packages from the @code{guix} channel can
potentially influence our @code{guile} build and this is something we'd
like to see as soon as possible as Guile developers.
@item
Build results are not thrown away: they can be distributed as
@dfn{substitutes} so that users of our @code{guile} channel
transparently get pre-built binaries!  (@pxref{Substitutes,,, guix, GNU
Guix Reference Manual}, for background info on substitutes.)
@end itemize

From a developer's viewpoint, the end result is this
@uref{https://ci.guix.gnu.org/jobset/guile,status page} listing
@emph{evaluations}: each evaluation is a combination of commits of the
@code{guix} and @code{guile} channels providing a number of
@emph{jobs}---one job per package defined in @file{guile-package.scm}
times the number of target architectures.

As for substitutes, they come for free! As an example, since our
@code{guile} jobset is built on ci.guix.gnu.org, which runs
@command{guix publish} (@pxref{Invoking guix publish,,, guix, GNU Guix
Reference Manual}) in addition to Cuirass, one automatically gets
substitutes for @code{guile} builds from ci.guix.gnu.org; no additional
work is needed for that.

@node Build Manifest
@section Bonus: Build manifest

The Cuirass spec above is convenient: it builds every package in our
channel, which includes a few variants (@pxref{Setting Up Continuous
Integration}).  However, this might be insufficiently expressive in some
cases: one might want specific cross-compilation jobs, transformations,
Docker images, RPM/Deb packages, or even system tests.

To achieve that, you can write a @dfn{manifest} (@pxref{Writing
Manifests,,, guix, GNU Guix Reference Manual}).  The one we have for
Guile has entries for the package variants we defined above, as well as
additional variants and cross builds:

@lisp
;; This is ‘.guix/manifest.scm’.

(use-modules (guix)
             (guix profiles)
             (guile-package))   ;import our own package module

(define* (package->manifest-entry* package system
                                   #:key target)
  "Return a manifest entry for PACKAGE on SYSTEM, optionally cross-compiled to
TARGET."
  (manifest-entry
    (inherit (package->manifest-entry package))
    (name (string-append (package-name package) "." system
                         (if target
                             (string-append "." target)
                             "")))
    (item (with-parameters ((%current-system system)
                            (%current-target-system target))
            package))))

(define native-builds
  (manifest
   (append (map (lambda (system)
                  (package->manifest-entry* guile system))

                '("x86_64-linux" "i686-linux"
                  "aarch64-linux" "armhf-linux"
                  "powerpc64le-linux"))
           (map (lambda (guile)
                  (package->manifest-entry* guile "x86_64-linux"))
                (cons (package
                        (inherit (package-with-c-toolchain
                                  guile
                                  `(("clang-toolchain"
                                     ,(specification->package
                                       "clang-toolchain")))))
                        (name "guile-clang"))
                      (list guile-without-threads
                            guile-without-networking
                            guile-debug
                            guile-strict-typing))))))

(define cross-builds
  (manifest
   (map (lambda (target)
          (package->manifest-entry* guile "x86_64-linux"
                                    #:target target))
        '("i586-pc-gnu"
          "aarch64-linux-gnu"
          "riscv64-linux-gnu"
          "i686-w64-mingw32"
          "x86_64-linux-gnu"))))

(concatenate-manifests (list native-builds cross-builds))
@end lisp

We won't go into the details of this manifest; suffice to say that it
provides additional flexibility. We now need to tell Cuirass to build
this manifest, which is done with a spec slightly different from the
previous one:

@lisp
;; Cuirass spec file to build all the packages of the ‘guile’ channel.
(list (specification
        (name "guile")
        (build '(manifest ".guix/manifest.scm"))
        (channels
         (append (list (channel
                         (name 'guile)
                         (url "https://git.savannah.gnu.org/git/guile.git")
                         (branch "main")))
                 %default-channels))))
@end lisp

We changed the @code{(build @dots{})} part of the spec to
@code{'(manifest ".guix/manifest.scm")} so that it would pick our
manifest, and that's it!

@node Wrapping Up
@section Wrapping Up

We picked Guile as the running example in this chapter and you can see
the result here:

@itemize
@item
@uref{https://git.savannah.gnu.org/cgit/guile.git/tree/.guix-channel?id=cd57379b3df636198d8cd8e76c1bfbc523762e79,@code{.guix-channel}};
@item
@uref{https://git.savannah.gnu.org/cgit/guile.git/tree/.guix/modules/guile-package.scm?id=cd57379b3df636198d8cd8e76c1bfbc523762e79,@code{.guix/modules/guile-package.scm}}
with the top-level @file{guix.scm} symlink;
@item
@uref{https://git.savannah.gnu.org/cgit/guile.git/tree/.guix/manifest.scm?id=cd57379b3df636198d8cd8e76c1bfbc523762e79,@code{.guix/manifest.scm}}.
@end itemize

These days, repositories are commonly peppered with dot files for
various tools: @code{.envrc}, @code{.gitlab-ci.yml},
@code{.github/workflows}, @code{Dockerfile}, @code{.buildpacks},
@code{Aptfile}, @code{requirements.txt}, and whatnot. It may sound like
we're proposing a bunch of @emph{additional} files, but in fact those
files are expressive enough to @emph{supersede} most or all of those
listed above.

With a couple of files, we get support for:

@itemize
@item
development environments (@command{guix shell});
@item
pristine test builds, including for package variants and for
cross-compilation (@command{guix build});
@item
continuous integration (with Cuirass or with some other tool);
@item
continuous delivery to users (@emph{via} the channel and with pre-built
binaries);
@item
generation of derivative build artifacts such as Docker images or
Deb/RPM packages (@command{guix pack}).
@end itemize

This a nice (in our view!) unified tool set for reproducible software
deployment, and an illustration of how you as a developer can benefit
from it!


@c *********************************************************************
@node Environment management
@chapter Environment management

Guix provides multiple tools to manage environment.  This chapter
demonstrate such utilities.

@menu
* Guix environment via direnv::  Setup Guix environment with direnv
@end menu

@node Guix environment via direnv
@section Guix environment via direnv

Guix provides a @samp{direnv} package, which could extend shell after
directory change.  This tool could be used to prepare a pure Guix
environment.

The following example provides a shell function for @file{~/.direnvrc}
file, which could be used from Guix Git repository in
@file{~/src/guix/.envrc} file to setup a build environment similar to
described in @pxref{Building from Git,,, guix, GNU Guix Reference
Manual}.

Create a @file{~/.direnvrc} with a Bash code:

@example
# Thanks <https://github.com/direnv/direnv/issues/73#issuecomment-152284914>
export_function()
@{
  local name=$1
  local alias_dir=$PWD/.direnv/aliases
  mkdir -p "$alias_dir"
  PATH_add "$alias_dir"
  local target="$alias_dir/$name"
  if declare -f "$name" >/dev/null; then
    echo "#!$SHELL" > "$target"
    declare -f "$name" >> "$target" 2>/dev/null
    # Notice that we add shell variables to the function trigger.
    echo "$name \$*" >> "$target"
    chmod +x "$target"
  fi
@}

use_guix()
@{
    # Set GitHub token.
    export GUIX_GITHUB_TOKEN="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

    # Unset 'GUIX_PACKAGE_PATH'.
    export GUIX_PACKAGE_PATH=""

    # Recreate a garbage collector root.
    gcroots="$HOME/.config/guix/gcroots"
    mkdir -p "$gcroots"
    gcroot="$gcroots/guix"
    if [ -L "$gcroot" ]
    then
        rm -v "$gcroot"
    fi

    # Miscellaneous packages.
    PACKAGES_MAINTENANCE=(
        direnv
        git
        git:send-email
        git-cal
        gnupg
        guile-colorized
        guile-readline
        less
        ncurses
        openssh
        xdot
    )

    # Environment packages.
    PACKAGES=(help2man guile-sqlite3 guile-gcrypt)

    # Thanks <https://lists.gnu.org/archive/html/guix-devel/2016-09/msg00859.html>
    eval "$(guix shell --search-paths --root="$gcroot" --pure \
     --development guix $@{PACKAGES[@@]@} $@{PACKAGES_MAINTENANCE[@@]@} "$@@")"

    # Predefine configure flags.
    configure()
    @{
        ./configure
    @}
    export_function configure

    # Run make and optionally build something.
    build()
    @{
        make -j 2
        if [ $# -gt 0 ]
        then
            ./pre-inst-env guix build "$@@"
        fi
    @}
    export_function build

    # Predefine push Git command.
    push()
    @{
        git push --set-upstream origin
    @}
    export_function push

    clear                        # Clean up the screen.
    git-cal --author='Your Name' # Show contributions calendar.

    # Show commands help.
    echo "
build          build a package or just a project if no argument provided
configure      run ./configure with predefined parameters
push           push to upstream Git repository
"
@}
@end example

Every project containing @file{.envrc} with a string @code{use guix}
will have predefined environment variables and procedures.

Run @command{direnv allow} to setup the environment for the first time.


@c *********************************************************************
@node Installing Guix on a Cluster
@chapter Installing Guix on a Cluster

@cindex cluster installation
@cindex high-performance computing, HPC
@cindex HPC, high-performance computing
Guix is appealing to scientists and @acronym{HPC, high-performance
computing} practitioners: it makes it easy to deploy potentially complex
software stacks, and it lets you do so in a reproducible fashion---you
can redeploy the exact same software on different machines and at
different points in time.

In this chapter we look at how a cluster sysadmin can install Guix for
system-wide use, such that it can be used on all the cluster nodes, and
discuss the various tradeoffs@footnote{This chapter is adapted from a
@uref{https://hpc.guix.info/blog/2017/11/installing-guix-on-a-cluster/,
blog post published on the Guix-HPC web site in 2017}.}.

@quotation Note
Here we assume that the cluster is running a GNU/Linux distro other than
Guix System and that we are going to install Guix on top of it.
@end quotation

@menu
* Setting Up a Head Node::      The node that runs the daemon.
* Setting Up Compute Nodes::    Client nodes.
* Cluster Network Access::      Dealing with network access restrictions.
* Cluster Disk Usage::          Disk usage considerations.
* Cluster Security Considerations::  Keeping the cluster secure.
@end menu

@node Setting Up a Head Node
@section Setting Up a Head Node

The recommended approach is to set up one @emph{head node} running
@command{guix-daemon} and exporting @file{/gnu/store} over NFS to
compute nodes.

Remember that @command{guix-daemon} is responsible for spawning build
processes and downloads on behalf of clients (@pxref{Invoking
guix-daemon,,, guix, GNU Guix Reference Manual}), and more generally
accessing @file{/gnu/store}, which contains all the package binaries
built by all the users (@pxref{The Store,,, guix, GNU Guix Reference
Manual}).  ``Client'' here refers to all the Guix commands that users
see, such as @code{guix install}.  On a cluster, these commands may be
running on the compute nodes and we'll want them to talk to the head
node's @code{guix-daemon} instance.

To begin with, the head node can be installed following the usual binary
installation instructions (@pxref{Binary Installation,,, guix, GNU Guix
Reference Manual}).  Thanks to the installation script, this should be
quick.  Once installation is complete, we need to make some adjustments.

Since we want @code{guix-daemon} to be reachable not just from the head
node but also from the compute nodes, we need to arrange so that it
listens for connections over TCP/IP.  To do that, we'll edit the systemd
startup file for @command{guix-daemon},
@file{/etc/systemd/system/guix-daemon.service}, and add a
@code{--listen} argument to the @code{ExecStart} line so that it looks
something like this:

@c Since Debian Buster, \ is documented to split lines.
@c https://manpages.debian.org/buster/systemd/systemd.exec.5.en.html
@c Use it in PDF and Info versions to avoid cut-off at the page border.
@ifnothtml
@example
ExecStart=/var/guix/profiles/per-user/root/current-guix/bin/guix-daemon \
    --build-users-group=guixbuild \
    --listen=/var/guix/daemon-socket/socket --listen=0.0.0.0
@end example
@end ifnothtml
@ifhtml
@example
ExecStart=/var/guix/profiles/per-user/root/current-guix/bin/guix-daemon --build-users-group=guixbuild --listen=/var/guix/daemon-socket/socket --listen=0.0.0.0
@end example
@end ifhtml

For these changes to take effect, the service needs to be restarted:

@example
systemctl daemon-reload
systemctl restart guix-daemon
@end example

@quotation Note
The @code{--listen=0.0.0.0} bit means that @code{guix-daemon} will
process @emph{all} incoming TCP connections on port 44146
(@pxref{Invoking guix-daemon,,, guix, GNU Guix Reference Manual}). This
is usually fine in a cluster setup where the head node is reachable
exclusively from the cluster's local area network---you don't want that
to be exposed to the Internet!
@end quotation

The next step is to define our NFS exports in
@uref{https://linux.die.net/man/5/exports,@file{/etc/exports}} by adding
something along these lines:

@example
/gnu/store    *(ro)
/var/guix     *(rw, async)
/var/log/guix *(ro)
@end example

The @file{/gnu/store} directory can be exported read-only since only
@command{guix-daemon} on the master node will ever modify it.
@file{/var/guix} contains @emph{user profiles} as managed by @code{guix
package}; thus, to allow users to install packages with @code{guix
package}, this must be read-write.

Users can create as many profiles as they like in addition to the
default profile, @file{~/.guix-profile}.  For instance, @code{guix
package -p ~/dev/python-dev -i python} installs Python in a profile
reachable from the @code{~/dev/python-dev} symlink.  To make sure that
this profile is protected from garbage collection---i.e., that Python
will not be removed from @file{/gnu/store} while this profile exists---,
@emph{home directories should be mounted on the head node} as well so
that @code{guix-daemon} knows about these non-standard profiles and
avoids collecting software they refer to.

It may be a good idea to periodically remove unused bits from
@file{/gnu/store} by running @command{guix gc} (@pxref{Invoking guix
gc,,, guix, GNU Guix Reference Manual}).  This can be done by adding a
crontab entry on the head node:

@example
root@@master# crontab -e
@end example

@noindent
... with something like this:

@example
# Every day at 5AM, run the garbage collector to make sure
# at least 10 GB are free on /gnu/store.
0 5 * * 1  /usr/local/bin/guix gc -F10G
@end example

We're done with the head node! Let's look at compute nodes now.

@node Setting Up Compute Nodes
@section Setting Up Compute Nodes

First of all, we need compute nodes to mount those NFS directories that
the head node exports.  This can be done by adding the following lines
to @uref{https://linux.die.net/man/5/fstab,@file{/etc/fstab}}:

@example
@var{head-node}:/gnu/store    /gnu/store    nfs  defaults,_netdev,vers=3 0 0
@var{head-node}:/var/guix     /var/guix     nfs  defaults,_netdev,vers=3 0 0
@var{head-node}:/var/log/guix /var/log/guix nfs  defaults,_netdev,vers=3 0 0
@end example

@noindent
... where @var{head-node} is the name or IP address of your head node.
From there on, assuming the mount points exist, you should be able to
mount each of these on the compute nodes.

Next, we need to provide a default @command{guix} command that users can
run when they first connect to the cluster (eventually they will invoke
@command{guix pull}, which will provide them with their ``own''
@command{guix} command).  Similar to what the binary installation script
did on the head node, we'll store that in @file{/usr/local/bin}:

@example
mkdir -p /usr/local/bin
ln -s /var/guix/profiles/per-user/root/current-guix/bin/guix \
      /usr/local/bin/guix
@end example

We then need to tell @code{guix} to talk to the daemon running on our
master node, by adding these lines to @code{/etc/profile}:

@example
GUIX_DAEMON_SOCKET="guix://@var{head-node}"
export GUIX_DAEMON_SOCKET
@end example

To avoid warnings and make sure @code{guix} uses the right locale, we
need to tell it to use locale data provided by Guix (@pxref{Application
Setup,,, guix, GNU Guix Reference Manual}):

@example
GUIX_LOCPATH=/var/guix/profiles/per-user/root/guix-profile/lib/locale
export GUIX_LOCPATH

# Here we must use a valid locale name.  Try "ls $GUIX_LOCPATH/*"
# to see what names can be used.
LC_ALL=fr_FR.utf8
export LC_ALL
@end example

For convenience, @code{guix package} automatically generates
@file{~/.guix-profile/etc/profile}, which defines all the environment
variables necessary to use the packages---@code{PATH},
@code{C_INCLUDE_PATH}, @code{PYTHONPATH}, etc.  Likewise, @command{guix
pull} does that under @file{~/.config/guix/current}.  Thus it's a good
idea to source both from @code{/etc/profile}:

@example
for GUIX_PROFILE in "$HOME/.config/guix/current" "$HOME/.guix-profile"
do
  if [ -f "$GUIX_PROFILE/etc/profile" ]; then
    . "$GUIX_PROFILE/etc/profile"
  fi
done
@end example

Last but not least, Guix provides command-line completion notably for
Bash and zsh.  In @code{/etc/bashrc}, consider adding this line:

@verbatim
. /var/guix/profiles/per-user/root/current-guix/etc/bash_completion.d/guix
@end verbatim

Voilà!

You can check that everything's in place by logging in on a compute node
and running:

@example
guix install hello
@end example

The daemon on the head node should download pre-built binaries on your
behalf and unpack them in @file{/gnu/store}, and @command{guix install}
should create @file{~/.guix-profile} containing the
@file{~/.guix-profile/bin/hello} command.

@node Cluster Network Access
@section Network Access

Guix requires network access to download source code and pre-built
binaries.  The good news is that only the head node needs that since
compute nodes simply delegate to it.

It is customary for cluster nodes to have access at best to a
@emph{white list} of hosts.  Our head node needs at least
@code{ci.guix.gnu.org} in this white list since this is where it gets
pre-built binaries from by default, for all the packages that are in
Guix proper.

Incidentally, @code{ci.guix.gnu.org} also serves as a
@emph{content-addressed mirror} of the source code of those packages.
Consequently, it is sufficient to have @emph{only}
@code{ci.guix.gnu.org} in that white list.

Software packages maintained in a separate repository such as one of the
various @uref{https://hpc.guix.info/channels, HPC channels} are of
course unavailable from @code{ci.guix.gnu.org}.  For these packages, you
may want to extend the white list such that source and pre-built
binaries (assuming this-party servers provide binaries for these
packages) can be downloaded.  As a last resort, users can always
download source on their workstation and add it to the cluster's
@file{/gnu/store}, like this:

@verbatim
GUIX_DAEMON_SOCKET=ssh://compute-node.example.org \
  guix download http://starpu.gforge.inria.fr/files/starpu-1.2.3/starpu-1.2.3.tar.gz
@end verbatim

The above command downloads @code{starpu-1.2.3.tar.gz} @emph{and} sends
it to the cluster's @code{guix-daemon} instance over SSH.

Air-gapped clusters require more work.  At the moment, our suggestion
would be to download all the necessary source code on a workstation
running Guix.  For instance, using the @option{--sources} option of
@command{guix build} (@pxref{Invoking guix build,,, guix, GNU Guix
Reference Manual}), the example below downloads all the source code the
@code{openmpi} package depends on:

@example
$ guix build --sources=transitive openmpi

@dots{}

/gnu/store/xc17sm60fb8nxadc4qy0c7rqph499z8s-openmpi-1.10.7.tar.bz2
/gnu/store/s67jx92lpipy2nfj5cz818xv430n4b7w-gcc-5.4.0.tar.xz
/gnu/store/npw9qh8a46lrxiwh9xwk0wpi3jlzmjnh-gmp-6.0.0a.tar.xz
/gnu/store/hcz0f4wkdbsvsdky3c0vdvcawhdkyldb-mpfr-3.1.5.tar.xz
/gnu/store/y9akh452n3p4w2v631nj0injx7y0d68x-mpc-1.0.3.tar.gz
/gnu/store/6g5c35q8avfnzs3v14dzl54cmrvddjm2-glibc-2.25.tar.xz
/gnu/store/p9k48dk3dvvk7gads7fk30xc2pxsd66z-hwloc-1.11.8.tar.bz2
/gnu/store/cry9lqidwfrfmgl0x389cs3syr15p13q-gcc-5.4.0.tar.xz
/gnu/store/7ak0v3rzpqm2c5q1mp3v7cj0rxz0qakf-libfabric-1.4.1.tar.bz2
/gnu/store/vh8syjrsilnbfcf582qhmvpg1v3rampf-rdma-core-14.tar.gz
…
@end example

(In case you're wondering, that's more than 320@ MiB of
@emph{compressed} source code.)

We can then make a big archive containing all of this (@pxref{Invoking
guix archive,,, guix, GNU Guix Reference Manual}):

@verbatim
$ guix archive --export \
    `guix build --sources=transitive openmpi` \
    > openmpi-source-code.nar
@end verbatim

@dots{} and we can eventually transfer that archive to the cluster on
removable storage and unpack it there:

@verbatim
$ guix archive --import < openmpi-source-code.nar
@end verbatim

This process has to be repeated every time new source code needs to be
brought to the cluster.

As we write this, the research institutes involved in Guix-HPC do not
have air-gapped clusters though. If you have experience with such
setups, we would like to hear feedback and suggestions.

@node Cluster Disk Usage
@section Disk Usage

@cindex disk usage, on a cluster
A common concern of sysadmins' is whether this is all going to eat a lot
of disk space.  If anything, if something is going to exhaust disk
space, it's going to be scientific data sets rather than compiled
software---that's our experience with almost ten years of Guix usage on
HPC clusters.  Nevertheless, it's worth taking a look at how Guix
contributes to disk usage.

First, having several versions or variants of a given package in
@file{/gnu/store} does not necessarily cost much, because
@command{guix-daemon} implements deduplication of identical files, and
package variants are likely to have a number of common files.

As mentioned above, we recommend having a cron job to run @code{guix gc}
periodically, which removes @emph{unused} software from
@file{/gnu/store}. However, there's always a possibility that users will
keep lots of software in their profiles, or lots of old generations of
their profiles, which is ``live'' and cannot be deleted from the
viewpoint of @command{guix gc}.

The solution to this is for users to regularly remove old generations of
their profile. For instance, the following command removes generations
that are more than two-month old:

@example
guix package --delete-generations=2m
@end example

Likewise, it's a good idea to invite users to regularly upgrade their
profile, which can reduce the number of variants of a given piece of
software stored in @file{/gnu/store}:

@example
guix pull
guix upgrade
@end example

As a last resort, it is always possible for sysadmins to do some of this
on behalf of their users. Nevertheless, one of the strengths of Guix is
the freedom and control users get on their software environment, so we
strongly recommend leaving users in control.

@node Cluster Security Considerations
@section Security Considerations

@cindex security, on a cluster
On an HPC cluster, Guix is typically used to manage scientific software.
Security-critical software such as the operating system kernel and
system services such as @code{sshd} and the batch scheduler remain under
control of sysadmins.

The Guix project has a good track record delivering security updates in
a timely fashion (@pxref{Security Updates,,, guix, GNU Guix Reference
Manual}).  To get security updates, users have to run @code{guix pull &&
guix upgrade}.

Because Guix uniquely identifies software variants, it is easy to see if
a vulnerable piece of software is in use. For instance, to check whether
the glibc@ 2.25 variant without the mitigation patch against
``@uref{https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt,Stack
Clash}'', one can check whether user profiles refer to it at all:

@example
guix gc --referrers /gnu/store/…-glibc-2.25
@end example

This will report whether profiles exist that refer to this specific
glibc variant.


@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} and on the Guix-HPC blog at
@uref{https://hpc.guix.info/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

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