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191 lines
7.1 KiB
191 lines
7.1 KiB
<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>
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NixOS has a modular system for declarative configuration. This system
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combines multiple <emphasis>modules</emphasis> to produce the full system
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configuration. One of the modules that constitute the configuration is
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<filename>/etc/nixos/configuration.nix</filename>. Most of the others live in
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the
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<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.
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</para>
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<para>
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Each NixOS module is a file that handles one logical aspect of the
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configuration, such as a specific kind of hardware, a service, or network
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settings. A module configuration does not have to handle everything from
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scratch; it can use the functionality provided by other modules for its
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implementation. Thus a module can <emphasis>declare</emphasis> options that
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can be used by other modules, and conversely can <emphasis>define</emphasis>
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options provided by other modules in its own implementation. For example, the
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module
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<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 other
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modules (e.g.
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<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
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<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 <filename>/etc/pam.d</filename>.
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</para>
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<para xml:id="para-module-syn">
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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 that only
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defines options, but does not declare any. The structure of full NixOS
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modules is shown in <xref linkend='ex-module-syntax' />.
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</para>
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<example xml:id='ex-module-syntax'>
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<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>
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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>
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This line makes the current Nix expression a function. The variable
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<varname>pkgs</varname> contains Nixpkgs (by default, it takes the
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<varname>nixpkgs</varname> entry of <envar>NIX_PATH</envar>, see the <link
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xlink:href="https://nixos.org/manual/nix/stable/#sec-common-env">Nix
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manual</link> for further details), while <varname>config</varname>
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contains the full system configuration. This line can be omitted if there
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is no reference to <varname>pkgs</varname> and <varname>config</varname>
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inside the module.
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</para>
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</callout>
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<callout arearefs='module-syntax-2'>
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<para>
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This list enumerates the paths to other NixOS modules that should be
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included in the evaluation of the system configuration. A default set of
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modules is defined in the file
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<filename>modules/module-list.nix</filename>. These don't need to be added
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in the import list.
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</para>
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</callout>
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<callout arearefs='module-syntax-3'>
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<para>
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The attribute <varname>options</varname> is a nested set of
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<emphasis>option declarations</emphasis> (described below).
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</para>
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</callout>
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<callout arearefs='module-syntax-4'>
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<para>
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The attribute <varname>config</varname> is a nested set of
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<emphasis>option definitions</emphasis> (also described below).
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</para>
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</callout>
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</calloutlist>
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</para>
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<para>
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<xref linkend='locate-example' /> shows a module that handles the regular
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update of the “locate” database, an index of all files in the file
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system. This module declares two options that can be defined by other modules
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(typically the user’s <filename>configuration.nix</filename>):
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<option>services.locate.enable</option> (whether the database should be
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updated) and <option>services.locate.interval</option> (when the update
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should be done). It implements its functionality by defining two options
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declared by other modules: <option>systemd.services</option> (the set of all
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systemd services) and <option>systemd.timers</option> (the list of commands
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to be executed periodically by <command>systemd</command>).
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</para>
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<example xml:id='locate-example'>
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<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
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cfg = config.services.locate;
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in {
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options.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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interval = mkOption {
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type = types.str;
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default = "02:15";
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example = "hourly";
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description = ''
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Update the locate database at this interval. Updates by
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default at 2:15 AM every day.
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The format is described in
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<citerefentry><refentrytitle>systemd.time</refentrytitle>
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<manvolnum>7</manvolnum></citerefentry>.
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'';
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};
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# Other options omitted for documentation
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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 ${toString cfg.output})
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exec updatedb \
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--localuser=${cfg.localuser} \
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${optionalString (!cfg.includeStore) "--prunepaths='/nix/store'"} \
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--output=${toString cfg.output} ${concatStringsSep " " cfg.extraFlags}
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'';
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};
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systemd.timers.update-locatedb = mkIf cfg.enable
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{ description = "Update timer for locate database";
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partOf = [ "update-locatedb.service" ];
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wantedBy = [ "timers.target" ];
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timerConfig.OnCalendar = cfg.interval;
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};
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};
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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-types.xml" />
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<xi:include href="option-def.xml" />
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<xi:include href="assertions.xml" />
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<xi:include href="meta-attributes.xml" />
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<xi:include href="importing-modules.xml" />
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<xi:include href="replace-modules.xml" />
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<xi:include href="freeform-modules.xml" />
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<xi:include href="settings-options.xml" />
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</chapter>
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