summaryrefslogtreecommitdiffstats
path: root/man/systemd.generator.xml
blob: 5007563e062e13c5f8ccc0df0d50ceb1d9bca559 (plain)
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
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY % entities SYSTEM "custom-entities.ent" >
%entities;
]>

<!--
  SPDX-License-Identifier: LGPL-2.1+
-->

<refentry id="systemd.generator">
  <refentryinfo>
    <title>systemd.generator</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.generator</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.generator</refname>
    <refpurpose>systemd unit generators</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>/path/to/generator</command>
      <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
      <arg choice="plain"><replaceable>early-dir</replaceable></arg>
      <arg choice="plain"><replaceable>late-dir</replaceable></arg>
    </cmdsynopsis>

    <para>
      <literallayout><filename>/run/systemd/system-generators/*</filename>
<filename>/etc/systemd/system-generators/*</filename>
<filename>/usr/local/lib/systemd/system-generators/*</filename>
<filename>&systemgeneratordir;/*</filename></literallayout>
    </para>

    <para>
      <literallayout><filename>/run/systemd/user-generators/*</filename>
<filename>/etc/systemd/user-generators/*</filename>
<filename>/usr/local/lib/systemd/user-generators/*</filename>
<filename>&usergeneratordir;/*</filename></literallayout>
    </para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>
    <para>Generators are small executables that live in
    <filename>&systemgeneratordir;/</filename> and other directories listed above.
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    will execute those binaries very early at bootup and at configuration reload time
     before unit files are loaded. Their main purpose is to convert configuration
    that is not native into dynamically generated unit files.</para>

    <para>Each generator is called with three directory paths that are to be used for
    generator output. In these three directories, generators may dynamically generate
    unit files (regular ones, instances, as well as templates), unit file
    <filename>.d/</filename> drop-ins, and create symbolic links to unit files to add
    additional dependencies, create aliases, or instantiate existing templates. Those
    directories are included in the unit load path of
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
    allowing generated configuration to extend or override existing
    definitions.</para>

    <para>Directory paths for generator output differ by priority:
    <filename>…/generator.early</filename> has priority higher than the admin
    configuration in <filename>/etc</filename>, while
    <filename>…/generator</filename> has lower priority than
    <filename>/etc</filename> but higher than vendor configuration in
    <filename>/usr</filename>, and <filename>…/generator.late</filename> has priority
    lower than all other configuration. See the next section and the discussion of
    unit load paths and unit overriding in
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
    </para>

    <para>Generators are loaded from a set of paths determined during
    compilation, as listed above. System and user generators are loaded
    from directories with names ending in
    <filename>system-generators/</filename> and
    <filename>user-generators/</filename>, respectively. Generators
    found in directories listed earlier override the ones with the
    same name in directories lower in the list. A symlink to
    <filename>/dev/null</filename> or an empty file can be used to
    mask a generator, thereby preventing it from running. Please note
    that the order of the two directories with the highest priority is
    reversed with respect to the unit load path, and generators in
    <filename>/run</filename> overwrite those in
    <filename>/etc</filename>.</para>

    <para>After installing new generators or updating the
    configuration, <command>systemctl daemon-reload</command> may be
    executed. This will delete the previous configuration created by
    generators, re-run all generators, and cause
    <command>systemd</command> to reload units from disk. See
    <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    for more information.
    </para>
  </refsect1>

  <refsect1>
    <title>Output directories</title>

    <para>Generators are invoked with three arguments: paths to directories where
    generators can place their generated unit files or symlinks. By default those
    paths are runtime directories that are included in the search path of
    <command>systemd</command>, but a generator may be called with different paths
    for debugging purposes.</para>

    <orderedlist>
      <listitem>
        <para><parameter>normal-dir</parameter></para>
        <para>In normal use this is <filename>/run/systemd/generator</filename> in
        case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user
        generators. Unit files placed in this directory take precedence over vendor
        unit configuration but not over native user/administrator unit configuration.
        </para>
      </listitem>

      <listitem>
        <para><parameter>early-dir</parameter></para>
        <para>In normal use this is <filename>/run/systemd/generator.early</filename>
        in case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user
        generators. Unit files placed in this directory override unit files in
        <filename>/usr</filename>, <filename>/run</filename> and
        <filename>/etc</filename>. This means that unit files placed in this
        directory take precedence over all normal configuration, both vendor and
        user/administrator.</para>
      </listitem>

      <listitem>
        <para><parameter>late-dir</parameter></para>
        <para>In normal use this is <filename>/run/systemd/generator.late</filename>
        in case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user
        generators. This directory may be used to extend the unit file tree without
        overriding any other unit files. Any native configuration files supplied by
        the vendor or user/administrator take precedence.</para>
      </listitem>
    </orderedlist>
  </refsect1>

  <refsect1>
    <title>Notes about writing generators</title>

    <itemizedlist>
      <listitem>
        <para>All generators are executed in parallel. That means all executables are
        started at the very same time and need to be able to cope with this
        parallelism.
        </para>
      </listitem>

      <listitem>
        <para>Generators are run very early at boot and cannot rely on any external
        services. They may not talk to any other process. That includes simple things
        such as logging to
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        or <command>systemd</command> itself (this means: no
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
        Non-essential file systems like <filename>/var</filename> and
        <filename>/home</filename> are mounted after generators have run. Generators
        can however rely on the most basic kernel functionality to be available,
        including a mounted <filename>/sys</filename>, <filename>/proc</filename>,
        <filename>/dev</filename>, <filename>/usr</filename>.
        </para>
      </listitem>

      <listitem>
        <para>Units written by generators are removed when the configuration is
        reloaded. That means the lifetime of the generated units is closely bound to
        the reload cycles of <command>systemd</command> itself.</para>
      </listitem>

      <listitem>
        <para>Generators should only be used to generate unit files and symlinks to
        them, not any other kind of configuration. Due to the lifecycle logic
        mentioned above, generators are not a good fit to generate dynamic
        configuration for other services. If you need to generate dynamic
        configuration for other services, do so in normal services you order before
        the service in question.</para>
      </listitem>

      <listitem>
        <para>Since
        <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>

        is not available (see above), log messages have to be written to
        <filename>/dev/kmsg</filename> instead.</para>
      </listitem>

      <listitem>
        <para>It is a good idea to use the <varname>SourcePath=</varname> directive
        in generated unit files to specify the source configuration file you are
        generating the unit from. This makes things more easily understood by the
        user and also has the benefit that systemd can warn the user about
        configuration files that changed on disk but have not been read yet by
        systemd.</para>
      </listitem>

      <listitem>
        <para>Generators may write out dynamic unit files or just hook unit files
        into other units with the usual <filename>.wants/</filename> or
        <filename>.requires/</filename> symlinks. Often, it is nicer to simply
        instantiate a template unit file from <filename>/usr</filename> with a
        generator instead of writing out entirely dynamic unit files. Of course, this
        works only if a single parameter is to be used.</para>
      </listitem>

      <listitem>
        <para>If you are careful, you can implement generators in shell scripts. We
        do recommend C code however, since generators are executed synchronously and
        hence delay the entire boot if they are slow.</para>
      </listitem>

      <listitem>
        <para>Regarding overriding semantics: there are two rules we try to follow
        when thinking about the overriding semantics:</para>

        <orderedlist numeration="lowerroman">
          <listitem>
            <para>User configuration should override vendor configuration. This
            (mostly) means that stuff from <filename>/etc</filename> should override
            stuff from <filename>/usr</filename>.</para>
          </listitem>

          <listitem>
            <para>Native configuration should override non-native configuration. This
            (mostly) means that stuff you generate should never override native unit
            files for the same purpose.</para>
          </listitem>
        </orderedlist>

        <para>Of these two rules the first rule is probably the more important one
        and breaks the second one sometimes. Hence, when deciding whether to use
        argv[1], argv[2], or argv[3], your default choice should probably be
        argv[1].</para>
      </listitem>

      <listitem>
        <para>Instead of heading off now and writing all kind of generators for
        legacy configuration file formats, please think twice! It is often a better
        idea to just deprecate old stuff instead of keeping it artificially alive.
        </para>
      </listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Examples</title>
    <example>
      <title>systemd-fstab-generator</title>

      <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      converts <filename>/etc/fstab</filename> into native mount units. It uses
      argv[1] as location to place the generated unit files in order to allow the
      user to override <filename>/etc/fstab</filename> with their own native unit
      files, but also to ensure that <filename>/etc/fstab</filename> overrides any
      vendor default from <filename>/usr</filename>.</para>

      <para>After editing <filename>/etc/fstab</filename>, the user should invoke
      <command>systemctl daemon-reload</command>. This will re-run all generators and
      cause <command>systemd</command> to reload units from disk. To actually mount
      new directories added to <filename>fstab</filename>, <command>systemctl start
      <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl
      start local-fs.target</command> may be used.</para>
    </example>

    <example>
      <title>systemd-system-update-generator</title>

      <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      temporarily redirects <filename>default.target</filename> to
      <filename>system-update.target</filename>, if a system update is
      scheduled. Since this needs to override the default user configuration for
      <filename>default.target</filename>, it uses argv[2]. For details about this
      logic, see
      <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
      </para>
    </example>

    <example>
      <title>Debugging a generator</title>

      <programlisting>dir=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
        "$dir" "$dir" "$dir"
find $dir</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>