]>
Commit | Line | Data |
---|---|---|
432d5965 | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
432d5965 ZJS |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [ |
4 | <!ENTITY % entities SYSTEM "custom-entities.ent" > | |
5 | %entities; | |
6 | ]> | |
db9ecf05 | 7 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> |
432d5965 ZJS |
8 | |
9 | <refentry id="systemd.generator"> | |
10 | <refentryinfo> | |
11 | <title>systemd.generator</title> | |
12 | <productname>systemd</productname> | |
432d5965 ZJS |
13 | </refentryinfo> |
14 | ||
15 | <refmeta> | |
16 | <refentrytitle>systemd.generator</refentrytitle> | |
17 | <manvolnum>7</manvolnum> | |
18 | </refmeta> | |
19 | ||
20 | <refnamediv> | |
21 | <refname>systemd.generator</refname> | |
f95b0be7 | 22 | <refpurpose>systemd unit generators</refpurpose> |
432d5965 ZJS |
23 | </refnamediv> |
24 | ||
25 | <refsynopsisdiv> | |
26 | <cmdsynopsis> | |
f8b68539 | 27 | <command index='false'>/path/to/generator</command> |
432d5965 | 28 | <arg choice="plain"><replaceable>normal-dir</replaceable></arg> |
17021368 ZJS |
29 | <arg choice="option"><replaceable>early-dir</replaceable></arg> |
30 | <arg choice="option"><replaceable>late-dir</replaceable></arg> | |
432d5965 ZJS |
31 | </cmdsynopsis> |
32 | ||
33 | <para> | |
34 | <literallayout><filename>/run/systemd/system-generators/*</filename> | |
12b42c76 | 35 | <filename>/etc/systemd/system-generators/*</filename> |
432d5965 | 36 | <filename>/usr/local/lib/systemd/system-generators/*</filename> |
23b8071d | 37 | <filename>&SYSTEM_GENERATOR_DIR;/*</filename></literallayout> |
432d5965 ZJS |
38 | </para> |
39 | ||
40 | <para> | |
41 | <literallayout><filename>/run/systemd/user-generators/*</filename> | |
12b42c76 | 42 | <filename>/etc/systemd/user-generators/*</filename> |
432d5965 | 43 | <filename>/usr/local/lib/systemd/user-generators/*</filename> |
23b8071d | 44 | <filename>&USER_GENERATOR_DIR;/*</filename></literallayout> |
432d5965 ZJS |
45 | </para> |
46 | </refsynopsisdiv> | |
47 | ||
48 | <refsect1> | |
49 | <title>Description</title> | |
23b8071d | 50 | <para>Generators are small executables placed in <filename>&SYSTEM_GENERATOR_DIR;/</filename> and other |
3acf00a5 LP |
51 | directories listed above. |
52 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will execute | |
53 | these binaries very early at bootup and at configuration reload time — before unit files are | |
a1d05574 LP |
54 | loaded. Their main purpose is to convert configuration and execution context parameters that are not |
55 | native to the service manager into dynamically generated unit files, symlinks or unit file drop-ins, so | |
56 | that they can extend the unit file hierarchy the service manager subsequently loads and operates | |
57 | on.</para> | |
58 | ||
17021368 ZJS |
59 | <para><command>systemd</command> will call each generator with three directory paths that are to be used |
60 | for generator output. In these three directories, generators may dynamically generate unit files (regular | |
61 | ones, instances, as well as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic | |
62 | links to unit files to add additional dependencies, create aliases, or instantiate existing templates. | |
63 | Those directories are included in the unit load path, allowing generated configuration to extend or | |
64 | override existing definitions. For tests, generators may be called with just one argument; the generator | |
65 | should assume that all three paths are the same in that case.</para> | |
a1d05574 LP |
66 | |
67 | <para>Directory paths for generator output differ by priority: <filename>…/generator.early</filename> has | |
68 | priority higher than the admin configuration in <filename>/etc/</filename>, while | |
69 | <filename>…/generator</filename> has lower priority than <filename>/etc/</filename> but higher than | |
70 | vendor configuration in <filename>/usr/</filename>, and <filename>…/generator.late</filename> has | |
71 | priority lower than all other configuration. See the next section and the discussion of unit load paths | |
72 | and unit overriding in | |
b82f27e7 ZJS |
73 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. |
74 | </para> | |
432d5965 | 75 | |
a1d05574 LP |
76 | <para>Generators are loaded from a set of paths determined during compilation, as listed above. System |
77 | and user generators are loaded from directories with names ending in | |
78 | <filename>system-generators/</filename> and <filename>user-generators/</filename>, | |
79 | respectively. Generators found in directories listed earlier override the ones with the same name in | |
80 | directories lower in the list. A symlink to <filename>/dev/null</filename> or an empty file can be used | |
81 | to mask a generator, thereby preventing it from running. Please note that the order of the two | |
82 | directories with the highest priority is reversed with respect to the unit load path, and generators in | |
83 | <filename>/run/</filename> overwrite those in <filename>/etc/</filename>.</para> | |
84 | ||
85 | <para>After installing new generators or updating the configuration, <command>systemctl | |
86 | daemon-reload</command> may be executed. This will delete the previous configuration created by | |
87 | generators, re-run all generators, and cause <command>systemd</command> to reload units from disk. See | |
88 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for more | |
89 | information. | |
432d5965 ZJS |
90 | </para> |
91 | </refsect1> | |
92 | ||
93 | <refsect1> | |
b82f27e7 | 94 | <title>Output directories</title> |
432d5965 | 95 | |
a1d05574 LP |
96 | <para>Generators are invoked with three arguments: paths to directories where generators can place their |
97 | generated unit files or symlinks. By default those paths are runtime directories that are included in the | |
98 | search path of <command>systemd</command>, but a generator may be called with different paths for | |
17021368 | 99 | debugging purposes. If only one argument is provided, the generator should use the same directory as the |
835b781f | 100 | three output paths.</para> |
432d5965 ZJS |
101 | |
102 | <orderedlist> | |
103 | <listitem> | |
104 | <para><parameter>normal-dir</parameter></para> | |
a1d05574 LP |
105 | <para>In normal use this is <filename>/run/systemd/generator</filename> in case of the system |
106 | generators and <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user generators. Unit | |
107 | files placed in this directory take precedence over vendor unit configuration but not over native | |
108 | user/administrator unit configuration. | |
b82f27e7 | 109 | </para> |
432d5965 ZJS |
110 | </listitem> |
111 | ||
112 | <listitem> | |
113 | <para><parameter>early-dir</parameter></para> | |
a1d05574 LP |
114 | <para>In normal use this is <filename>/run/systemd/generator.early</filename> in case of the system |
115 | generators and <filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user | |
116 | generators. Unit files placed in this directory override unit files in <filename>/usr/</filename>, | |
117 | <filename>/run/</filename> and <filename>/etc/</filename>. This means that unit files placed in this | |
118 | directory take precedence over all normal configuration, both vendor and user/administrator.</para> | |
432d5965 ZJS |
119 | </listitem> |
120 | ||
121 | <listitem> | |
122 | <para><parameter>late-dir</parameter></para> | |
a1d05574 LP |
123 | <para>In normal use this is <filename>/run/systemd/generator.late</filename> in case of the system |
124 | generators and <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user | |
125 | generators. This directory may be used to extend the unit file tree without overriding any other unit | |
126 | files. Any native configuration files supplied by the vendor or user/administrator take | |
127 | precedence.</para> | |
432d5965 ZJS |
128 | </listitem> |
129 | </orderedlist> | |
2bd88a29 ZJS |
130 | |
131 | <para>Note: generators <emphasis>must not</emphasis> write to other locations or otherwise make changes | |
132 | to system state. Generator output is supposed to last only until the next | |
133 | <command>daemon-reload</command> or <command>daemon-reexec</command>; if the generator is replaced | |
134 | or masked, its effects should vanish.</para> | |
80efdacd ZJS |
135 | </refsect1> |
136 | ||
5b9d199a LP |
137 | <refsect1> |
138 | <title>Environment</title> | |
139 | ||
140 | <para>The service manager sets a number of environment variables when invoking generator | |
141 | executables. They carry information about the execution context of the generator, in order to simplify | |
142 | conditionalizing generators to specific environments. The following environment variables are set:</para> | |
143 | ||
144 | <variablelist class='environment-variables'> | |
145 | <varlistentry> | |
146 | <term><varname>$SYSTEMD_SCOPE</varname></term> | |
147 | ||
148 | <listitem><para>If the generator is invoked from the system service manager this variable is set to | |
149 | <literal>system</literal>; if invoked from the per-user service manager it is set to | |
150 | <literal>user</literal>.</para></listitem> | |
151 | </varlistentry> | |
152 | ||
153 | <varlistentry> | |
154 | <term><varname>$SYSTEMD_IN_INITRD</varname></term> | |
155 | ||
55c041b4 LP |
156 | <listitem><para>If the generator is run as part of an initrd this is set to <literal>1</literal>. If |
157 | it is run from the regular host (i.e. after the transition from initrd to host) it is set to | |
158 | <literal>0</literal>. This environment variable is only set for system generators.</para></listitem> | |
5b9d199a LP |
159 | </varlistentry> |
160 | ||
161 | <varlistentry> | |
162 | <term><varname>$SYSTEMD_FIRST_BOOT</varname></term> | |
163 | ||
164 | <listitem><para>If this boot-up cycle is considered a "first boot", this is set to | |
165 | <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For | |
166 | details see the documentation of <varname>ConditionFirstBoot=</varname> in | |
167 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This | |
168 | environment variable is only set for system generators.</para></listitem> | |
169 | </varlistentry> | |
170 | ||
171 | <varlistentry> | |
172 | <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term> | |
173 | ||
174 | <listitem><para>If the service manager is run in a virtualized environment, | |
175 | <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The | |
176 | first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type | |
177 | of virtualization. The second string identifies the implementation of the virtualization | |
178 | technology. If no virtualization is detected this variable will not be set. This data is identical to | |
179 | what | |
180 | <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
181 | detects and reports, and uses the same vocabulary of virtualization implementation | |
182 | identifiers.</para></listitem> | |
183 | </varlistentry> | |
184 | ||
185 | <varlistentry> | |
186 | <term><varname>$SYSTEMD_ARCHITECTURE</varname></term> | |
187 | ||
188 | <listitem><para>This variable is set to a short identifier of the reported architecture of the | |
189 | system. For details about defined values, see documentation of | |
190 | <varname>ConditionArchitecture=</varname> in | |
191 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem> | |
192 | </varlistentry> | |
193 | </variablelist> | |
194 | </refsect1> | |
195 | ||
80efdacd ZJS |
196 | <refsect1> |
197 | <title>Notes about writing generators</title> | |
198 | ||
199 | <itemizedlist> | |
200 | <listitem> | |
a1d05574 LP |
201 | <para>All generators are executed in parallel. That means all executables are started at the very |
202 | same time and need to be able to cope with this parallelism. | |
80efdacd ZJS |
203 | </para> |
204 | </listitem> | |
205 | ||
206 | <listitem> | |
3acf00a5 LP |
207 | <para>Generators are run very early at boot and cannot rely on any external services. They may not |
208 | talk to any other process. That includes simple things such as logging to <citerefentry | |
209 | project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, or | |
210 | <command>systemd</command> itself (this means: no | |
80efdacd | 211 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)! |
3acf00a5 LP |
212 | Non-essential file systems like <filename>/var/</filename> and <filename>/home/</filename> are |
213 | mounted after generators have run. Generators can however rely on the most basic kernel functionality | |
214 | to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>, | |
215 | <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems. | |
80efdacd ZJS |
216 | </para> |
217 | </listitem> | |
218 | ||
219 | <listitem> | |
a1d05574 LP |
220 | <para>Units written by generators are removed when the configuration is reloaded. That means the |
221 | lifetime of the generated units is closely bound to the reload cycles of <command>systemd</command> | |
222 | itself.</para> | |
80efdacd ZJS |
223 | </listitem> |
224 | ||
225 | <listitem> | |
3acf00a5 LP |
226 | <para>Generators should only be used to generate unit files, <filename>.d/*.conf</filename> drop-ins |
227 | for them and symlinks to them, not any other kind of non-unit related configuration. Due to the | |
228 | lifecycle logic mentioned above, generators are not a good fit to generate dynamic configuration for | |
229 | other services. If you need to generate dynamic configuration for other services, do so in normal | |
230 | services you order before the service in question.</para> | |
231 | ||
232 | <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname> | |
233 | settings of service unit files (see | |
234 | <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it | |
235 | is possible to make arbitrary input data (including daemon-specific configuration) part of the unit | |
236 | definitions, which often might be sufficient to embed data or configuration for other programs into | |
237 | unit files in a native fashion.</para> | |
80efdacd ZJS |
238 | </listitem> |
239 | ||
240 | <listitem> | |
241 | <para>Since | |
242 | <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
243 | ||
a1d05574 LP |
244 | is not available (see above), log messages have to be written to <filename>/dev/kmsg</filename> |
245 | instead.</para> | |
80efdacd ZJS |
246 | </listitem> |
247 | ||
248 | <listitem> | |
9c5ac5eb ZJS |
249 | <para>The generator should always include its own name in a comment at the top of the generated file, |
250 | so that the user can easily figure out which component created or amended a particular unit.</para> | |
251 | ||
252 | <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the | |
253 | source configuration file they are generated from. This makes things more easily understood by the | |
254 | user and also has the benefit that systemd can warn the user about configuration files that changed | |
255 | on disk but have not been read yet by systemd. The <varname>SourcePath=</varname> value does not have | |
256 | to be a file in a physical filesystem. For example, in the common case of the generator looking at | |
257 | the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</para> | |
80efdacd ZJS |
258 | </listitem> |
259 | ||
260 | <listitem> | |
a1d05574 LP |
261 | <para>Generators may write out dynamic unit files or just hook unit files into other units with the |
262 | usual <filename>.wants/</filename> or <filename>.requires/</filename> symlinks. Often, it is nicer to | |
263 | simply instantiate a template unit file from <filename>/usr/</filename> with a generator instead of | |
264 | writing out entirely dynamic unit files. Of course, this works only if a single parameter is to be | |
265 | used.</para> | |
80efdacd | 266 | </listitem> |
432d5965 | 267 | |
80efdacd | 268 | <listitem> |
a1d05574 LP |
269 | <para>If you are careful, you can implement generators in shell scripts. We do recommend C code |
270 | however, since generators are executed synchronously and hence delay the entire boot if they are | |
271 | slow.</para> | |
80efdacd ZJS |
272 | </listitem> |
273 | ||
274 | <listitem> | |
a1d05574 LP |
275 | <para>Regarding overriding semantics: there are two rules we try to follow when thinking about the |
276 | overriding semantics:</para> | |
80efdacd ZJS |
277 | |
278 | <orderedlist numeration="lowerroman"> | |
279 | <listitem> | |
a1d05574 LP |
280 | <para>User configuration should override vendor configuration. This (mostly) means that stuff |
281 | from <filename>/etc/</filename> should override stuff from <filename>/usr/</filename>.</para> | |
80efdacd ZJS |
282 | </listitem> |
283 | ||
284 | <listitem> | |
a1d05574 LP |
285 | <para>Native configuration should override non-native configuration. This (mostly) means that |
286 | stuff you generate should never override native unit files for the same purpose.</para> | |
80efdacd ZJS |
287 | </listitem> |
288 | </orderedlist> | |
289 | ||
a1d05574 LP |
290 | <para>Of these two rules the first rule is probably the more important one and breaks the second one |
291 | sometimes. Hence, when deciding whether to use argv[1], argv[2], or argv[3], your default choice | |
292 | should probably be argv[1].</para> | |
80efdacd ZJS |
293 | </listitem> |
294 | ||
295 | <listitem> | |
a1d05574 LP |
296 | <para>Instead of heading off now and writing all kind of generators for legacy configuration file |
297 | formats, please think twice! It is often a better idea to just deprecate old stuff instead of keeping | |
298 | it artificially alive. | |
80efdacd ZJS |
299 | </para> |
300 | </listitem> | |
301 | </itemizedlist> | |
432d5965 ZJS |
302 | </refsect1> |
303 | ||
304 | <refsect1> | |
305 | <title>Examples</title> | |
306 | <example> | |
307 | <title>systemd-fstab-generator</title> | |
308 | ||
309 | <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
a1d05574 LP |
310 | converts <filename>/etc/fstab</filename> into native mount units. It uses argv[1] as location to place |
311 | the generated unit files in order to allow the user to override <filename>/etc/fstab</filename> with | |
312 | their own native unit files, but also to ensure that <filename>/etc/fstab</filename> overrides any | |
3b121157 | 313 | vendor default from <filename>/usr/</filename>.</para> |
80efdacd | 314 | |
a1d05574 LP |
315 | <para>After editing <filename>/etc/fstab</filename>, the user should invoke <command>systemctl |
316 | daemon-reload</command>. This will re-run all generators and cause <command>systemd</command> to reload | |
317 | units from disk. To actually mount new directories added to <filename>fstab</filename>, | |
318 | <command>systemctl start <replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl | |
80efdacd | 319 | start local-fs.target</command> may be used.</para> |
432d5965 ZJS |
320 | </example> |
321 | ||
322 | <example> | |
323 | <title>systemd-system-update-generator</title> | |
324 | ||
325 | <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
a1d05574 LP |
326 | temporarily redirects <filename>default.target</filename> to <filename>system-update.target</filename>, |
327 | if a system update is scheduled. Since this needs to override the default user configuration for | |
328 | <filename>default.target</filename>, it uses argv[2]. For details about this logic, see | |
49174f75 ZJS |
329 | <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>. |
330 | </para> | |
432d5965 ZJS |
331 | </example> |
332 | ||
333 | <example> | |
b8e1d4d1 | 334 | <title>Debugging a generator</title> |
432d5965 ZJS |
335 | |
336 | <programlisting>dir=$(mktemp -d) | |
23b8071d | 337 | SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/systemd-fstab-generator \ |
432d5965 ZJS |
338 | "$dir" "$dir" "$dir" |
339 | find $dir</programlisting> | |
340 | </example> | |
341 | </refsect1> | |
342 | ||
343 | <refsect1> | |
344 | <title>See also</title> | |
345 | ||
346 | <para> | |
347 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
348 | <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
349 | <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
432d5965 | 350 | <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
3ba3a79d | 351 | <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
432d5965 ZJS |
352 | <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
353 | <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
354 | <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
245992a0 | 355 | <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
432d5965 ZJS |
356 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
357 | <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
4540e698 | 358 | <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
432d5965 | 359 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
1bd2d4e3 ZJS |
360 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
361 | <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
432d5965 ZJS |
362 | </para> |
363 | </refsect1> | |
364 | </refentry> |