1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
|
<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" xml:id="sec-writing-modules">
<title>Writing NixOS Modules</title>
<para>
NixOS has a modular system for declarative configuration. This
system combines multiple <emphasis>modules</emphasis> to produce the
full system configuration. One of the modules that constitute the
configuration is <literal>/etc/nixos/configuration.nix</literal>.
Most of the others live in the
<link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/modules"><literal>nixos/modules</literal></link>
subdirectory of the Nixpkgs tree.
</para>
<para>
Each NixOS module is a file that handles one logical aspect of the
configuration, such as a specific kind of hardware, a service, or
network settings. A module configuration does not have to handle
everything from scratch; it can use the functionality provided by
other modules for its implementation. Thus a module can
<emphasis>declare</emphasis> options that can be used by other
modules, and conversely can <emphasis>define</emphasis> options
provided by other modules in its own implementation. For example,
the module
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/security/pam.nix"><literal>pam.nix</literal></link>
declares the option <literal>security.pam.services</literal> that
allows other modules (e.g.
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/ssh/sshd.nix"><literal>sshd.nix</literal></link>)
to define PAM services; and it defines the option
<literal>environment.etc</literal> (declared by
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/etc/etc.nix"><literal>etc.nix</literal></link>)
to cause files to be created in <literal>/etc/pam.d</literal>.
</para>
<para>
In <xref linkend="sec-configuration-syntax" />, we saw the following
structure of NixOS modules:
</para>
<programlisting language="bash">
{ config, pkgs, ... }:
{ option definitions
}
</programlisting>
<para>
This is actually an <emphasis>abbreviated</emphasis> form of module
that only defines options, but does not declare any. The structure
of full NixOS modules is shown in
<link linkend="ex-module-syntax">Example: Structure of NixOS
Modules</link>.
</para>
<anchor xml:id="ex-module-syntax" />
<para>
<emphasis role="strong">Example: Structure of NixOS
Modules</emphasis>
</para>
<programlisting language="bash">
{ config, pkgs, ... }:
{
imports =
[ paths of other modules
];
options = {
option declarations
};
config = {
option definitions
};
}
</programlisting>
<para>
The meaning of each part is as follows.
</para>
<itemizedlist>
<listitem>
<para>
The first line makes the current Nix expression a function. The
variable <literal>pkgs</literal> contains Nixpkgs (by default,
it takes the <literal>nixpkgs</literal> entry of
<literal>NIX_PATH</literal>, see the
<link xlink:href="https://nixos.org/manual/nix/stable/#sec-common-env">Nix
manual</link> for further details), while
<literal>config</literal> contains the full system
configuration. This line can be omitted if there is no reference
to <literal>pkgs</literal> and <literal>config</literal> inside
the module.
</para>
</listitem>
<listitem>
<para>
This <literal>imports</literal> list enumerates the paths to
other NixOS modules that should be included in the evaluation of
the system configuration. A default set of modules is defined in
the file <literal>modules/module-list.nix</literal>. These don't
need to be added in the import list.
</para>
</listitem>
<listitem>
<para>
The attribute <literal>options</literal> is a nested set of
<emphasis>option declarations</emphasis> (described below).
</para>
</listitem>
<listitem>
<para>
The attribute <literal>config</literal> is a nested set of
<emphasis>option definitions</emphasis> (also described below).
</para>
</listitem>
</itemizedlist>
<para>
<link linkend="locate-example">Example: NixOS Module for the
<quote>locate</quote> Service</link> shows a module that handles the
regular update of the <quote>locate</quote> database, an index of
all files in the file system. This module declares two options that
can be defined by other modules (typically the user’s
<literal>configuration.nix</literal>):
<literal>services.locate.enable</literal> (whether the database
should be updated) and <literal>services.locate.interval</literal>
(when the update should be done). It implements its functionality by
defining two options declared by other modules:
<literal>systemd.services</literal> (the set of all systemd
services) and <literal>systemd.timers</literal> (the list of
commands to be executed periodically by <literal>systemd</literal>).
</para>
<anchor xml:id="locate-example" />
<para>
<emphasis role="strong">Example: NixOS Module for the
<quote>locate</quote> Service</emphasis>
</para>
<programlisting language="bash">
{ config, lib, pkgs, ... }:
with lib;
let
cfg = config.services.locate;
in {
options.services.locate = {
enable = mkOption {
type = types.bool;
default = false;
description = ''
If enabled, NixOS will periodically update the database of
files used by the locate command.
'';
};
interval = mkOption {
type = types.str;
default = "02:15";
example = "hourly";
description = ''
Update the locate database at this interval. Updates by
default at 2:15 AM every day.
The format is described in
systemd.time(7).
'';
};
# Other options omitted for documentation
};
config = {
systemd.services.update-locatedb =
{ description = "Update Locate Database";
path = [ pkgs.su ];
script =
''
mkdir -m 0755 -p $(dirname ${toString cfg.output})
exec updatedb \
--localuser=${cfg.localuser} \
${optionalString (!cfg.includeStore) "--prunepaths='/nix/store'"} \
--output=${toString cfg.output} ${concatStringsSep " " cfg.extraFlags}
'';
};
systemd.timers.update-locatedb = mkIf cfg.enable
{ description = "Update timer for locate database";
partOf = [ "update-locatedb.service" ];
wantedBy = [ "timers.target" ];
timerConfig.OnCalendar = cfg.interval;
};
};
}
</programlisting>
<xi:include href="option-declarations.section.xml" />
<xi:include href="option-types.section.xml" />
<xi:include href="option-def.section.xml" />
<xi:include href="assertions.section.xml" />
<xi:include href="meta-attributes.section.xml" />
<xi:include href="importing-modules.section.xml" />
<xi:include href="replace-modules.section.xml" />
<xi:include href="freeform-modules.section.xml" />
<xi:include href="settings-options.section.xml" />
</chapter>
|