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