]>
Commit | Line | Data |
---|---|---|
432d5965 ZJS |
1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [ | |
4 | <!ENTITY % entities SYSTEM "custom-entities.ent" > | |
5 | %entities; | |
6 | ]> | |
7 | ||
8 | <!-- | |
572eb058 ZJS |
9 | SPDX-License-Identifier: LGPL-2.1+ |
10 | ||
432d5965 ZJS |
11 | This file is part of systemd. |
12 | ||
13 | Copyright 2015 Zbigniew Jędrzejewski-Szmek | |
14 | ||
15 | systemd is free software; you can redistribute it and/or modify it | |
16 | under the terms of the GNU Lesser General Public License as published by | |
17 | the Free Software Foundation; either version 2.1 of the License, or | |
18 | (at your option) any later version. | |
19 | ||
20 | systemd is distributed in the hope that it will be useful, but | |
21 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
22 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
23 | Lesser General Public License for more details. | |
24 | ||
25 | You should have received a copy of the GNU Lesser General Public License | |
26 | along with systemd; If not, see <http://www.gnu.org/licenses/>. | |
27 | --> | |
28 | ||
29 | <refentry id="systemd.generator"> | |
30 | <refentryinfo> | |
31 | <title>systemd.generator</title> | |
32 | <productname>systemd</productname> | |
33 | ||
34 | <authorgroup> | |
35 | <author> | |
36 | <contrib>Developer</contrib> | |
37 | <firstname>Lennart</firstname> | |
38 | <surname>Poettering</surname> | |
39 | <email>lennart@poettering.net</email> | |
40 | </author> | |
41 | </authorgroup> | |
42 | </refentryinfo> | |
43 | ||
44 | <refmeta> | |
45 | <refentrytitle>systemd.generator</refentrytitle> | |
46 | <manvolnum>7</manvolnum> | |
47 | </refmeta> | |
48 | ||
49 | <refnamediv> | |
50 | <refname>systemd.generator</refname> | |
f95b0be7 | 51 | <refpurpose>systemd unit generators</refpurpose> |
432d5965 ZJS |
52 | </refnamediv> |
53 | ||
54 | <refsynopsisdiv> | |
55 | <cmdsynopsis> | |
56 | <command>/path/to/generator</command> | |
57 | <arg choice="plain"><replaceable>normal-dir</replaceable></arg> | |
58 | <arg choice="plain"><replaceable>early-dir</replaceable></arg> | |
59 | <arg choice="plain"><replaceable>late-dir</replaceable></arg> | |
60 | </cmdsynopsis> | |
61 | ||
62 | <para> | |
63 | <literallayout><filename>/run/systemd/system-generators/*</filename> | |
12b42c76 | 64 | <filename>/etc/systemd/system-generators/*</filename> |
432d5965 ZJS |
65 | <filename>/usr/local/lib/systemd/system-generators/*</filename> |
66 | <filename>&systemgeneratordir;/*</filename></literallayout> | |
67 | </para> | |
68 | ||
69 | <para> | |
70 | <literallayout><filename>/run/systemd/user-generators/*</filename> | |
12b42c76 | 71 | <filename>/etc/systemd/user-generators/*</filename> |
432d5965 ZJS |
72 | <filename>/usr/local/lib/systemd/user-generators/*</filename> |
73 | <filename>&usergeneratordir;/*</filename></literallayout> | |
74 | </para> | |
75 | </refsynopsisdiv> | |
76 | ||
77 | <refsect1> | |
78 | <title>Description</title> | |
b82f27e7 ZJS |
79 | <para>Generators are small executables that live in |
80 | <filename>&systemgeneratordir;/</filename> and other directories listed above. | |
81 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
82 | will execute those binaries very early at bootup and at configuration reload time | |
83 | — before unit files are loaded. Their main purpose is to convert configuration | |
84 | that is not native into dynamically generated unit files.</para> | |
85 | ||
86 | <para>Each generator is called with three directory paths that are to be used for | |
87 | generator output. In these three directories, generators may dynamically generate | |
88 | unit files (regular ones, instances, as well as templates), unit file | |
89 | <filename>.d/</filename> drop-ins, and create symbolic links to unit files to add | |
90 | additional dependencies, create aliases, or instantiate existing templates. Those | |
91 | directories are included in the unit load path of | |
92 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
93 | allowing generated configuration to extend or override existing | |
94 | definitions.</para> | |
95 | ||
96 | <para>Directory paths for generator output differ by priority: | |
97 | <filename>…/generator.early</filename> has priority higher than the admin | |
98 | configuration in <filename>/etc</filename>, while | |
99 | <filename>…/generator</filename> has lower priority than | |
100 | <filename>/etc</filename> but higher than vendor configuration in | |
101 | <filename>/usr</filename>, and <filename>…/generator.late</filename> has priority | |
102 | lower than all other configuration. See the next section and the discussion of | |
103 | unit load paths and unit overriding in | |
104 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
105 | </para> | |
432d5965 ZJS |
106 | |
107 | <para>Generators are loaded from a set of paths determined during | |
a8eaaee7 | 108 | compilation, as listed above. System and user generators are loaded |
432d5965 ZJS |
109 | from directories with names ending in |
110 | <filename>system-generators/</filename> and | |
111 | <filename>user-generators/</filename>, respectively. Generators | |
112 | found in directories listed earlier override the ones with the | |
113 | same name in directories lower in the list. A symlink to | |
114 | <filename>/dev/null</filename> or an empty file can be used to | |
115 | mask a generator, thereby preventing it from running. Please note | |
116 | that the order of the two directories with the highest priority is | |
b938cb90 | 117 | reversed with respect to the unit load path, and generators in |
432d5965 ZJS |
118 | <filename>/run</filename> overwrite those in |
119 | <filename>/etc</filename>.</para> | |
120 | ||
121 | <para>After installing new generators or updating the | |
c4c086a2 | 122 | configuration, <command>systemctl daemon-reload</command> may be |
432d5965 ZJS |
123 | executed. This will delete the previous configuration created by |
124 | generators, re-run all generators, and cause | |
125 | <command>systemd</command> to reload units from disk. See | |
126 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
127 | for more information. | |
128 | </para> | |
129 | </refsect1> | |
130 | ||
131 | <refsect1> | |
b82f27e7 | 132 | <title>Output directories</title> |
432d5965 | 133 | |
b82f27e7 ZJS |
134 | <para>Generators are invoked with three arguments: paths to directories where |
135 | generators can place their generated unit files or symlinks. By default those | |
136 | paths are runtime directories that are included in the search path of | |
137 | <command>systemd</command>, but a generator may be called with different paths | |
138 | for debugging purposes.</para> | |
432d5965 ZJS |
139 | |
140 | <orderedlist> | |
141 | <listitem> | |
142 | <para><parameter>normal-dir</parameter></para> | |
b82f27e7 ZJS |
143 | <para>In normal use this is <filename>/run/systemd/generator</filename> in |
144 | case of the system generators and | |
145 | <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user | |
146 | generators. Unit files placed in this directory take precedence over vendor | |
147 | unit configuration but not over native user/administrator unit configuration. | |
148 | </para> | |
432d5965 ZJS |
149 | </listitem> |
150 | ||
151 | <listitem> | |
152 | <para><parameter>early-dir</parameter></para> | |
b82f27e7 ZJS |
153 | <para>In normal use this is <filename>/run/systemd/generator.early</filename> |
154 | in case of the system generators and | |
155 | <filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user | |
156 | generators. Unit files placed in this directory override unit files in | |
157 | <filename>/usr</filename>, <filename>/run</filename> and | |
158 | <filename>/etc</filename>. This means that unit files placed in this | |
159 | directory take precedence over all normal configuration, both vendor and | |
160 | user/administrator.</para> | |
432d5965 ZJS |
161 | </listitem> |
162 | ||
163 | <listitem> | |
164 | <para><parameter>late-dir</parameter></para> | |
b82f27e7 ZJS |
165 | <para>In normal use this is <filename>/run/systemd/generator.late</filename> |
166 | in case of the system generators and | |
167 | <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user | |
168 | generators. This directory may be used to extend the unit file tree without | |
169 | overriding any other unit files. Any native configuration files supplied by | |
170 | the vendor or user/administrator take precedence.</para> | |
432d5965 ZJS |
171 | </listitem> |
172 | </orderedlist> | |
80efdacd ZJS |
173 | </refsect1> |
174 | ||
175 | <refsect1> | |
176 | <title>Notes about writing generators</title> | |
177 | ||
178 | <itemizedlist> | |
179 | <listitem> | |
180 | <para>All generators are executed in parallel. That means all executables are | |
181 | started at the very same time and need to be able to cope with this | |
182 | parallelism. | |
183 | </para> | |
184 | </listitem> | |
185 | ||
186 | <listitem> | |
187 | <para>Generators are run very early at boot and cannot rely on any external | |
188 | services. They may not talk to any other process. That includes simple things | |
189 | such as logging to | |
190 | <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
191 | or <command>systemd</command> itself (this means: no | |
192 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)! | |
193 | Non-essential file systems like <filename>/var</filename> and | |
194 | <filename>/home</filename> are mounted after generators have run. Generators | |
195 | can however rely on the most basic kernel functionality to be available, | |
196 | including a mounted <filename>/sys</filename>, <filename>/proc</filename>, | |
197 | <filename>/dev</filename>, <filename>/usr</filename>. | |
198 | </para> | |
199 | </listitem> | |
200 | ||
201 | <listitem> | |
202 | <para>Units written by generators are removed when the configuration is | |
203 | reloaded. That means the lifetime of the generated units is closely bound to | |
204 | the reload cycles of <command>systemd</command> itself.</para> | |
205 | </listitem> | |
206 | ||
207 | <listitem> | |
208 | <para>Generators should only be used to generate unit files and symlinks to | |
209 | them, not any other kind of configuration. Due to the lifecycle logic | |
210 | mentioned above, generators are not a good fit to generate dynamic | |
211 | configuration for other services. If you need to generate dynamic | |
212 | configuration for other services, do so in normal services you order before | |
213 | the service in question.</para> | |
214 | </listitem> | |
215 | ||
216 | <listitem> | |
217 | <para>Since | |
218 | <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
219 | ||
220 | is not available (see above), log messages have to be written to | |
221 | <filename>/dev/kmsg</filename> instead.</para> | |
222 | </listitem> | |
223 | ||
224 | <listitem> | |
225 | <para>It is a good idea to use the <varname>SourcePath=</varname> directive | |
226 | in generated unit files to specify the source configuration file you are | |
227 | generating the unit from. This makes things more easily understood by the | |
228 | user and also has the benefit that systemd can warn the user about | |
229 | configuration files that changed on disk but have not been read yet by | |
230 | systemd.</para> | |
231 | </listitem> | |
232 | ||
233 | <listitem> | |
234 | <para>Generators may write out dynamic unit files or just hook unit files | |
235 | into other units with the usual <filename>.wants/</filename> or | |
236 | <filename>.requires/</filename> symlinks. Often, it is nicer to simply | |
237 | instantiate a template unit file from <filename>/usr</filename> with a | |
238 | generator instead of writing out entirely dynamic unit files. Of course, this | |
239 | works only if a single parameter is to be used.</para> | |
240 | </listitem> | |
432d5965 | 241 | |
80efdacd ZJS |
242 | <listitem> |
243 | <para>If you are careful, you can implement generators in shell scripts. We | |
244 | do recommend C code however, since generators are executed synchronously and | |
245 | hence delay the entire boot if they are slow.</para> | |
246 | </listitem> | |
247 | ||
248 | <listitem> | |
249 | <para>Regarding overriding semantics: there are two rules we try to follow | |
250 | when thinking about the overriding semantics:</para> | |
251 | ||
252 | <orderedlist numeration="lowerroman"> | |
253 | <listitem> | |
254 | <para>User configuration should override vendor configuration. This | |
255 | (mostly) means that stuff from <filename>/etc</filename> should override | |
256 | stuff from <filename>/usr</filename>.</para> | |
257 | </listitem> | |
258 | ||
259 | <listitem> | |
260 | <para>Native configuration should override non-native configuration. This | |
261 | (mostly) means that stuff you generate should never override native unit | |
262 | files for the same purpose.</para> | |
263 | </listitem> | |
264 | </orderedlist> | |
265 | ||
266 | <para>Of these two rules the first rule is probably the more important one | |
267 | and breaks the second one sometimes. Hence, when deciding whether to use | |
268 | argv[1], argv[2], or argv[3], your default choice should probably be | |
269 | argv[1].</para> | |
270 | </listitem> | |
271 | ||
272 | <listitem> | |
273 | <para>Instead of heading off now and writing all kind of generators for | |
274 | legacy configuration file formats, please think twice! It is often a better | |
275 | idea to just deprecate old stuff instead of keeping it artificially alive. | |
276 | </para> | |
277 | </listitem> | |
278 | </itemizedlist> | |
432d5965 ZJS |
279 | </refsect1> |
280 | ||
281 | <refsect1> | |
282 | <title>Examples</title> | |
283 | <example> | |
284 | <title>systemd-fstab-generator</title> | |
285 | ||
286 | <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
80efdacd ZJS |
287 | converts <filename>/etc/fstab</filename> into native mount units. It uses |
288 | argv[1] as location to place the generated unit files in order to allow the | |
289 | user to override <filename>/etc/fstab</filename> with her own native unit | |
290 | files, but also to ensure that <filename>/etc/fstab</filename> overrides any | |
291 | vendor default from <filename>/usr</filename>.</para> | |
292 | ||
293 | <para>After editing <filename>/etc/fstab</filename>, the user should invoke | |
294 | <command>systemctl daemon-reload</command>. This will re-run all generators and | |
295 | cause <command>systemd</command> to reload units from disk. To actually mount | |
296 | new directories added to <filename>fstab</filename>, <command>systemctl start | |
297 | <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl | |
298 | start local-fs.target</command> may be used.</para> | |
432d5965 ZJS |
299 | </example> |
300 | ||
301 | <example> | |
302 | <title>systemd-system-update-generator</title> | |
303 | ||
304 | <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
305 | temporarily redirects <filename>default.target</filename> to | |
0663a4a6 | 306 | <filename>system-update.target</filename>, if a system update is |
80efdacd ZJS |
307 | scheduled. Since this needs to override the default user configuration for |
308 | <filename>default.target</filename>, it uses argv[2]. For details about this | |
309 | logic, see | |
49174f75 ZJS |
310 | <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>. |
311 | </para> | |
432d5965 ZJS |
312 | </example> |
313 | ||
314 | <example> | |
b8e1d4d1 | 315 | <title>Debugging a generator</title> |
432d5965 ZJS |
316 | |
317 | <programlisting>dir=$(mktemp -d) | |
318 | SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \ | |
319 | "$dir" "$dir" "$dir" | |
320 | find $dir</programlisting> | |
321 | </example> | |
322 | </refsect1> | |
323 | ||
324 | <refsect1> | |
325 | <title>See also</title> | |
326 | ||
327 | <para> | |
328 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
329 | <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
330 | <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
432d5965 | 331 | <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
3ba3a79d | 332 | <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
432d5965 ZJS |
333 | <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
334 | <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
335 | <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
245992a0 | 336 | <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
432d5965 ZJS |
337 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
338 | <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
339 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
1bd2d4e3 ZJS |
340 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
341 | <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
432d5965 ZJS |
342 | </para> |
343 | </refsect1> | |
344 | </refentry> |