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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Development</title>
<para>This chapter has some random notes on hacking on
NixOS.</para>
<section>
<title>Extending NixOS</title>
<para>A unique syntax is used to express all system, hardware, computer and
service configurations. This syntax helps for reading and writing new
configuration files. It is coming with some extra strategies defined in
NixPkgs which are used to merge and evaluate all configuration files.</para>
<para>A configuration file is the same as your own computer
configuration.</para>
<example xml:id='conf-syntax'><title>Usual configuration file</title>
<programlisting>
{config, modulesPath, pkgs, ...}: <co xml:id='conf-syntax-1' />
let
inherit (pkgs.lib) mkOption mkIf mkThenElse types; <co xml:id='conf-syntax-2' />
cfg = config.services.locate; <co xml:id='conf-syntax-4' />
locatedb = "/var/cache/locatedb";
logfile = "/var/log/updatedb";
cmd = "root updatedb --localuser=nobody --output=${locatedb} > ${logfile}";
in
{
imports = [ <co xml:id='conf-syntax-6' />
(modulesPath + /services/scheduling/cron.nix)
];
options = { <co xml:id='conf-syntax-3' />
services.locate = {
enable = mkOption {
default = false;
example = true;
type = types.bool;
description = ''
If enabled, NixOS will periodically update the database of
files used by the <command>locate</command> command.
'';
};
period = mkOption {
default = "15 02 * * *";
type = types.uniq type.string;
description = ''
This option defines (in the format used by cron) when the
locate database is updated.
The default is to update at 02:15 (at night) every day.
'';
};
};
};
config = mkIf cfg.enable { <co xml:id='conf-syntax-5' />
services.cron = {
systemCronJobs = mkThenElse { <co xml:id='conf-syntax-7' />
thenPart = "${cfg.period} root ${cmd}";
elsePart = "";
};
};
};
}</programlisting>
</example>
<para><xref linkend='conf-syntax' /> shows the <emphasis>configuration
file</emphasis> for the locate service which uses cron to update the
database at some dates which can be defined by the user. This nix
expression is coming
from <filename>upstart-jobs/cron/locate.nix</filename>. It shows a simple
example of a service that can be either distributed on many computer that
are using the same configuration or to shared with the community. This
file is divided in two with the interface and the implementation. Both
the interface and the implementation declare a <emphasis>configuration
set</emphasis>.
<calloutlist>
<callout arearefs='conf-syntax-1'>
<para>This line declares the arguments of the configuration file. You
can omit this line if there is no reference to <varname>pkgs</varname>
and <varname>config</varname> inside the configuration file.</para>
<para>The argument <varname>pkgs</varname> refers to NixPkgs and allow
you to access all attributes contained inside it. In this
example <varname>pkgs</varname> is used to retrieve common functions to
ease the writing of configuration files
like <varname>mkOption</varname>, <varname>mkIf</varname>
and <varname>mkThenElse</varname>.</para>
<para>The argument <varname>config</varname> corresponds to the whole
NixOS configuration. This is a set which is build by merging all
configuration files imported to set up the system. Thus all options
declared are contained inside this variable. In this
example <varname>config</varname> is used to retrieve the status of
the <varname>enable</varname> flag. The important point of this
argument is that it contains either the result of the merge of different
settings or the default value, therefore you cannot assume
that <option>config.services.locate.enable</option> is always false
because it may have been defined in another configuration file.</para>
</callout>
<callout arearefs='conf-syntax-2'>
<para>This line is used to import a functions that are useful for
writing this configuration file.</para>
</callout>
<callout arearefs='conf-syntax-3'>
<para>The variable <varname>options</varname> is
a <emphasis>configuration set</emphasis> which is only used to declare
options with the function <varname>mkOption</varname> imported
from <filename>pkgs/lib/default.nix</filename>. Options may contained
any attribute but only the following have a special
meaning: <varname>default</varname>, <varname>example</varname>,
<varname>description</varname>, <varname>merge</varname>
and <varname>apply</varname>.</para>
<para>The <varname>merge</varname> attribute is used to merge all values
defined in all configuration files and this function return a value
which has the same type as the default value. If the merge function is
not defined, then a default function
(<varname>pkgs.lib.mergeDefaultOption</varname>) is used to merge
values. The <varname>merge</varname> attribute is a function which
expect two arguments: the location of the option and the list of values
which have to be merged.</para>
<para>The <varname>apply</varname> attribute is a function used to
process the option. Thus the value return
by <option>config.<replaceable>option</replaceable></option> would be
the result of the <varname>apply</varname> function called with either
the <varname>default</varname> value or the result of
the <varname>merge</varname> function.</para>
</callout>
<callout arearefs='conf-syntax-4'>
<para>This line is a common trick used to reduce the amount of
writing. In this case <varname>cfg</varname> is just a sugar over
<option>config.services.locate</option></para>
</callout>
<callout arearefs='conf-syntax-5'>
<para>This line is used to declare a special <emphasis>IF</emphasis>
statement. If you had put a usual <emphasis>IF</emphasis> statement
here, with the same condition, then you will get an infinite loop. The
reason is that your condition ask for the value of the
option <option>config.services.locate.enable</option> but in order to
get this value you have to evaluate all configuration sets including the
configuration set contained inside your file.</para>
<para>To remove this extra complexity, <varname>mkIf</varname> has been
introduced to get rid of possible infinite loop and to factor your
writing.</para>
</callout>
<callout arearefs='conf-syntax-6'>
<para>The attribute <varname>imports</varname> contains a list of other
module location. These modules should provide option declarations for
the current module in order to be evaluated safely.</para>
<para>When a dependence on a NixOS module has to be made, then you
should use the argument <varname>modulesPath</varname> to prefix the
location of your NixOS repository.</para>
</callout>
<callout arearefs='conf-syntax-7'>
<para>The attribute <varname>config</varname>, which should not be
confused with the argument of the same name, contains a set of option
definitions defined by this module. When there is
neither <varname>imports</varname> nor <varname>options</varname>, then
this attribute set can be return without any enclosing. This feature
allow you to write your <filename>configuration.nix</filename> as a
module which does not contains any option declarations.</para>
<para>As <varname>mkIf</varname> does not need
any <emphasis>then</emphasis> part or <emphasis>else</emphasis> part,
then you can specify one on each option definition with the
function <varname>mkThenElse</varname>.</para>
<para>To avoid a lot of <varname>mkThenElse</varname> with empty
<emphasis>else</emphasis> part, a sugar has been added to infer the
corresponding <emphasis>empty</emphasis>-value of your option when the
function <varname>mkThenElse</varname> is not used.</para>
<para>If your <emphasis>then</emphasis> part
and <emphasis>else</emphasis> part are identical, then you should use
the function <varname>mkAlways</varname> to ignore the condition.</para>
<para>If you need to add another condition, then you can add <varname>mkIf</varname> to on
the appropriate location. Thus the <emphasis>then</emphasis> part will
only be used if all conditions declared with <varname>mkIf</varname>
are satisfied.</para>
</callout>
</calloutlist>
</para>
</section>
<section>
<title>Building specific parts of NixOS</title>
<para>
<screen>
$ nix-build /etc/nixos/nixos -A <replaceable>attr</replaceable></screen>
where <replaceable>attr</replaceable> is an attribute in
<filename>/etc/nixos/nixos/default.nix</filename>. Attributes of interest include:
<variablelist>
<varlistentry>
<term><varname>config</varname></term>
<listitem><para>The computer configuration generated from
the <envar>NIXOS_CONFIG</envar> environment variable (default
is <filename>/etc/nixos/configuration.nix</filename>) with the NixOS
default set of modules.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>system</varname></term>
<listitem><para>The derivation which build your computer system. It is
built by the command <command>nixos-rebuild
build</command></para></listitem>
</varlistentry>
<varlistentry>
<term><varname>vm</varname></term>
<listitem><para>The derivation which build your computer system inside a
virtual machine. It is built by the command <command>nixos-rebuild
build-vm</command></para></listitem>
</varlistentry>
</variablelist>
</para>
<para>
Most parts of NixOS can be build through the <varname>config</varname>
attribute set. This attribute set allows you to have a view of the merged
option definitions and all its derivations. Important derivations are store
inside the option <option>system.build</option> and can be listed with the
command <command>nix-instantiate --xml --eval-only /etc/nixos/nixos -A
config.system.build</command>
</para>
</section>
<section>
<title>Building your own NixOS CD</title>
<para>Building a NixOS CD is as easy as configuring your own computer. The
idea is to use another module which will replace
your <filename>configuration.nix</filename> to configure the system that
would be install on the CD.</para>
<para>Default CD/DVD configurations are available
inside <filename>nixos/modules/installer/cd-dvd</filename>. To build them
you have to set <envar>NIXOS_CONFIG</envar> before
running <command>nix-build</command> to build the ISO.
<screen>
$ export NIXOS_CONFIG=/etc/nixos/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix
$ nix-build /etc/nixos/nixos -A config.system.build.isoImage</screen>
</para>
<para>Before burning your CD/DVD, you can check the content of the image by mounting anywhere like
suggested by the following command:
<screen>
$ mount -o loop -t iso9660 ./result/iso/cd.iso /mnt/iso</screen>
</para>
</section>
<section>
<title>Testing the installer</title>
<para>Building, burning, and booting from an installation CD is rather
tedious, so here is a quick way to see if the installer works
properly:
<screen>
$ export NIXOS_CONFIG=/etc/nixos/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix
$ nix-build /etc/nixos/nixos -A config.system.build.nixosInstall
$ dd if=/dev/zero of=diskimage seek=2G count=0 bs=1
$ yes | mke2fs -j diskimage
$ mount -o loop diskimage /mnt
$ ./result/bin/nixos-install</screen>
</para>
</section>
<section>
<title>Testing the <literal>initrd</literal></title>
<para>A quick way to test whether the kernel and the initial ramdisk
boot correctly is to use QEMUs <option>-kernel</option> and
<option>-initrd</option> options:
<screen>
$ nix-build /etc/nixos/nixos -A config.system.build.initialRamdisk -o initrd
$ nix-build /etc/nixos/nixos -A config.system.build.kernel -o kernel
$ qemu-system-x86_64 -kernel ./kernel/vmlinuz -initrd ./initrd/initrd -hda /dev/null
</screen>
</para>
</section>
</chapter>