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9 This file is part of systemd.
11 Copyright 2015 Zbigniew Jędrzejewski-Szmek
13 systemd is free software; you can redistribute it and/or modify it
14 under the terms of the GNU Lesser General Public License as published by
15 the Free Software Foundation; either version 2.1 of the License, or
16 (at your option) any later version.
18 systemd is distributed in the hope that it will be useful, but
19 WITHOUT ANY WARRANTY; without even the implied warranty of
20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
21 Lesser General Public License for more details.
23 You should have received a copy of the GNU Lesser General Public License
24 along with systemd; If not, see <http://www.gnu.org/licenses/>.
27 <refentry id="systemd.generator">
29 <title>systemd.generator</title>
30 <productname>systemd</productname>
34 <contrib>Developer</contrib>
35 <firstname>Lennart</firstname>
36 <surname>Poettering</surname>
37 <email>lennart@poettering.net</email>
43 <refentrytitle>systemd.generator</refentrytitle>
44 <manvolnum>7</manvolnum>
48 <refname>systemd.generator</refname>
49 <refpurpose>Systemd unit generators</refpurpose>
54 <command>/path/to/generator</command>
55 <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
56 <arg choice="plain"><replaceable>early-dir</replaceable></arg>
57 <arg choice="plain"><replaceable>late-dir</replaceable></arg>
61 <literallayout><filename>/run/systemd/system-generators/*</filename>
62 <filename>/etc/systemd/system-generators/*</filename>
63 <filename>/usr/local/lib/systemd/system-generators/*</filename>
64 <filename>&systemgeneratordir;/*</filename></literallayout>
68 <literallayout><filename>/run/systemd/user-generators/*</filename>
69 <filename>/etc/systemd/user-generators/*</filename>
70 <filename>/usr/local/lib/systemd/user-generators/*</filename>
71 <filename>&usergeneratordir;/*</filename></literallayout>
76 <title>Description</title>
77 <para>Generators are small binaries that live in
78 <filename>&usergeneratordir;/</filename> and other directories
80 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
81 will execute those binaries very early at bootup and at
82 configuration reload time — before unit files are loaded.
83 Generators can dynamically generate unit files or create symbolic
84 links to unit files to add additional dependencies, thus extending
85 or overriding existing definitions. Their main purpose is to
86 convert configuration files that are not native unit files
87 dynamically into native unit files.</para>
89 <para>Generators are loaded from a set of paths determined during
90 compilation, listed above. System and user generators are loaded
91 from directories with names ending in
92 <filename>system-generators/</filename> and
93 <filename>user-generators/</filename>, respectively. Generators
94 found in directories listed earlier override the ones with the
95 same name in directories lower in the list. A symlink to
96 <filename>/dev/null</filename> or an empty file can be used to
97 mask a generator, thereby preventing it from running. Please note
98 that the order of the two directories with the highest priority is
99 reversed with respect to the unit load path and generators in
100 <filename>/run</filename> overwrite those in
101 <filename>/etc</filename>.</para>
103 <para>After installing new generators or updating the
104 configuration, <command>systemctl daemon-reload</command> may be
105 executed. This will delete the previous configuration created by
106 generators, re-run all generators, and cause
107 <command>systemd</command> to reload units from disk. See
108 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
109 for more information.
114 <title>Writing generators</title>
116 <para>Generators are invoked with three arguments: paths to
117 runtime directories where generators can place their generated
118 unit files or symlinks.</para>
122 <para><parameter>normal-dir</parameter></para>
123 <para>argv[1] may be used to override unit files in
124 <filename>/usr</filename>, but not those in
125 <filename>/etc</filename>. This means that unit files placed
126 in this directory take precedence over vendor unit
127 configuration but not over native user/administrator unit
128 configuration.</para>
132 <para><parameter>early-dir</parameter></para>
133 <para>argv[2] may be used to override unit files in
134 <filename>/usr</filename> and in
135 <filename>/etc</filename>. This means that unit files placed
136 in this directory take precedence over all configuration,
137 both vendor and user/administrator.</para>
141 <para><parameter>late-dir</parameter></para>
142 <para>argv[3] may be used to extend the unit file tree without
143 overridding any other unit files. Any native configuration
144 files supplied by the vendor or user/administrator take
145 precedence over the generated ones placed in this directory.
156 All generators are executed in parallel. That means all
157 executables are started at the very same time and need to
158 be able to cope with this parallelism.
164 Generators are run very early at boot and cannot rely on
165 any external services. They may not talk to any other
166 process. That includes simple things such as logging to
168 project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
169 or <command>systemd</command> itself (this means: no
170 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>!). They
171 can however rely on the most basic kernel functionality to
172 be available, including mounted <filename>/sys</filename>,
173 <filename>/proc</filename>, <filename>/dev</filename>.
179 Units written by generators are removed when configuration
180 is reloaded. That means the lifetime of the generated
181 units is closely bound to the reload cycles of
182 <command>systemd</command> itself.
188 Generators should only be used to generate unit files, not
189 any other kind of configuration. Due to the lifecycle
190 logic mentioned above generators are not a good fit to
191 generate dynamic configuration for other services. If you
192 need to generate dynamic configuration for other services
193 do so in normal services you order before the service in
201 <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
202 is not available (see above) log messages have to be
203 written to <filename>/dev/kmsg</filename> instead.
209 It is a good idea to use the
210 <varname>SourcePath=</varname> directive in generated unit
211 files to specify the source configuration file you are
212 generating the unit from. This makes things more easily
213 understood by the user and also has the benefit that
214 systemd can warn the user about configuration files that
215 changed on disk but have not been read yet by systemd.
221 Generators may write out dynamic unit files or just hook
222 unit files into other units with the usual
223 <filename>.wants/</filename> or
224 <filename>.requires/</filename> symlinks. Often it is
225 nicer to simply instantiate a template unit file from
226 <filename>/usr</filename> with a generator instead of
227 writing out entirely dynamic unit files. Of course this
228 works only if a single parameter is to be used.
234 If you are careful you can implement generators in shell
235 scripts. We do recommend C code however, since generators
236 delay are executed synchronously and hence delay the
237 entire boot if they are slow.
242 <para>Regarding overriding semantics: there are two rules we
243 try to follow when thinking about the overriding semantics:
246 <orderedlist numeration="lowerroman">
248 <para>User configuration should override vendor
249 configuration. This (mostly) means that stuff from
250 <filename>/etc</filename> should override stuff from
251 <filename>/usr</filename>.</para>
255 <para>Native configuration should override non-native
256 configuration. This (mostly) means that stuff you
257 generate should never override native unit files for the
262 <para>Of these two rules the first rule is probably the more
263 important one and breaks the second one sometimes. Hence,
264 when deciding whether to user argv[1], argv[2], or argv[3],
265 your default choice should probably be argv[1].</para>
270 Instead of heading off now and writing all kind of
271 generators for legacy configuration file formats, please
272 think twice! It's often a better idea to just deprecate
273 old stuff instead of keeping it artificially alive.
281 <title>Examples</title>
283 <title>systemd-fstab-generator</title>
285 <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
286 converts <filename>/etc/fstab</filename> into native mount
287 units. It uses argv[1] as location to place the generated unit
288 files in order to allow the user to override
289 <filename>/etc/fstab</filename> with her own native unit files,
290 but also to ensure that <filename>/etc/fstab</filename>
291 overrides any vendor default from <filename>/usr</filename>.
294 <para>After editing <filename>/etc/fstab</filename>, the user
295 should invoke <command>systemctl daemon-reload</command>. This
296 will re-run all generators and cause <command>systemd</command>
297 to reload units from disk. To actually mount new directories
298 added to <filename>fstab</filename>, <command>systemctl start
299 <replaceable>/path/to/mountpoint</replaceable></command> or
300 <command>systemctl start local-fs.target</command> may be used.
305 <title>systemd-system-update-generator</title>
307 <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
308 temporarily redirects <filename>default.target</filename> to
309 <filename>system-update.target</filename> if a system update is
310 scheduled. Since this needs to override the default user
311 configuration for <filename>default.target</filename> it uses
312 argv[2]. For details about this logic, see
313 <ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates">Implementing
314 Offline System Updates</ulink>.</para>
318 <title>Debuging a generator</title>
320 <programlisting>dir=$(mktemp -d)
321 SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
323 find $dir</programlisting>
328 <title>See also</title>
331 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
332 <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
333 <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
334 <citerefentry><refentrytitle>systemd-efi-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
335 <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
336 <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
337 <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
338 <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
339 <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
340 <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
341 <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
342 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
343 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>