Chunk NixOS manual
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Mikey Ariel
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Eelco Dolstra
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<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-writing-modules">
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<title>Writing NixOS Modules</title>
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<para>NixOS has a modular system for declarative configuration. This
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system combines multiple <emphasis>modules</emphasis> to produce the
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full system configuration. One of the modules that constitute the
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configuration is <filename>/etc/nixos/configuration.nix</filename>.
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Most of the others live in the <link
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xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/modules"><filename>nixos/modules</filename></link>
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subdirectory of the Nixpkgs tree.</para>
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<para>Each NixOS module is a file that handles one logical aspect of
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the configuration, such as a specific kind of hardware, a service, or
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network settings. A module configuration does not have to handle
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everything from scratch; it can use the functionality provided by
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other modules for its implementation. Thus a module can
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<emphasis>declare</emphasis> options that can be used by other
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modules, and conversely can <emphasis>define</emphasis> options
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provided by other modules in its own implementation. For example, the
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module <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/security/pam.nix"><filename>pam.nix</filename></link>
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declares the option <option>security.pam.services</option> that allows
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other modules (e.g. <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/ssh/sshd.nix"><filename>sshd.nix</filename></link>)
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to define PAM services; and it defines the option
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<option>environment.etc</option> (declared by <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/etc/etc.nix"><filename>etc.nix</filename></link>)
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to cause files to be created in
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<filename>/etc/pam.d</filename>.</para>
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<para xml:id="para-module-syn">In <xref
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linkend="sec-configuration-syntax"/>, we saw the following structure
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of NixOS modules:
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<programlisting>
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{ config, pkgs, ... }:
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{ <replaceable>option definitions</replaceable>
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}
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</programlisting>
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This is actually an <emphasis>abbreviated</emphasis> form of module
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that only defines options, but does not declare any. The structure of
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full NixOS modules is shown in <xref linkend='ex-module-syntax' />.</para>
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<example xml:id='ex-module-syntax'><title>Structure of NixOS Modules</title>
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<programlisting>
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{ config, pkgs, ... }: <co xml:id='module-syntax-1' />
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{
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imports =
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[ <replaceable>paths of other modules</replaceable> <co xml:id='module-syntax-2' />
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];
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options = {
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<replaceable>option declarations</replaceable> <co xml:id='module-syntax-3' />
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};
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config = {
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<replaceable>option definitions</replaceable> <co xml:id='module-syntax-4' />
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};
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}</programlisting>
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</example>
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<para>The meaning of each part is as follows.
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<calloutlist>
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<callout arearefs='module-syntax-1'>
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<para>This line makes the current Nix expression a function. The
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variable <varname>pkgs</varname> contains Nixpkgs, while
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<varname>config</varname> contains the full system configuration.
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This line can be omitted if there is no reference to
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<varname>pkgs</varname> and <varname>config</varname> inside the
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module.</para>
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</callout>
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<callout arearefs='module-syntax-2'>
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<para>This list enumerates the paths to other NixOS modules that
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should be included in the evaluation of the system configuration.
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A default set of modules is defined in the file
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<filename>modules/module-list.nix</filename>. These don't need to
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be added in the import list.</para>
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</callout>
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<callout arearefs='module-syntax-3'>
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<para>The attribute <varname>options</varname> is a nested set of
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<emphasis>option declarations</emphasis> (described below).</para>
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</callout>
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<callout arearefs='module-syntax-4'>
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<para>The attribute <varname>config</varname> is a nested set of
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<emphasis>option definitions</emphasis> (also described
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below).</para>
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</callout>
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</calloutlist>
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</para>
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<para><xref linkend='locate-example' /> shows a module that handles
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the regular update of the “locate” database, an index of all files in
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the file system. This module declares two options that can be defined
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by other modules (typically the user’s
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<filename>configuration.nix</filename>):
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<option>services.locate.enable</option> (whether the database should
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be updated) and <option>services.locate.period</option> (when the
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update should be done). It implements its functionality by defining
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two options declared by other modules:
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<option>systemd.services</option> (the set of all systemd services)
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and <option>services.cron.systemCronJobs</option> (the list of
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commands to be executed periodically by <command>cron</command>).</para>
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<example xml:id='locate-example'><title>NixOS Module for the “locate” Service</title>
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<programlisting>
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{ config, lib, pkgs, ... }:
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with lib;
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let locatedb = "/var/cache/locatedb"; in
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{
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options = {
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services.locate = {
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enable = mkOption {
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type = types.bool;
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default = false;
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description = ''
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If enabled, NixOS will periodically update the database of
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files used by the <command>locate</command> command.
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'';
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};
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period = mkOption {
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type = types.str;
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default = "15 02 * * *";
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description = ''
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This option defines (in the format used by cron) when the
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locate database is updated. The default is to update at
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02:15 at night every day.
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'';
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};
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};
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};
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config = {
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systemd.services.update-locatedb =
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{ description = "Update Locate Database";
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path = [ pkgs.su ];
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script =
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''
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mkdir -m 0755 -p $(dirname ${locatedb})
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exec updatedb --localuser=nobody --output=${locatedb} --prunepaths='/tmp /var/tmp /media /run'
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'';
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};
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services.cron.systemCronJobs = optional config.services.locate.enable
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"${config.services.locate.period} root ${config.systemd.package}/bin/systemctl start update-locatedb.service";
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};
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}</programlisting>
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</example>
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<xi:include href="option-declarations.xml" />
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<xi:include href="option-def.xml" />
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</chapter>
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