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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 | <!-- | |
9 | This file is part of systemd. | |
10 | ||
11 | Copyright 2015 Zbigniew Jędrzejewski-Szmek | |
12 | ||
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. | |
17 | ||
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. | |
22 | ||
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/>. | |
25 | --> | |
26 | ||
27 | <refentry id="systemd.generator"> | |
28 | <refentryinfo> | |
29 | <title>systemd.generator</title> | |
30 | <productname>systemd</productname> | |
31 | ||
32 | <authorgroup> | |
33 | <author> | |
34 | <contrib>Developer</contrib> | |
35 | <firstname>Lennart</firstname> | |
36 | <surname>Poettering</surname> | |
37 | <email>lennart@poettering.net</email> | |
38 | </author> | |
39 | </authorgroup> | |
40 | </refentryinfo> | |
41 | ||
42 | <refmeta> | |
43 | <refentrytitle>systemd.generator</refentrytitle> | |
44 | <manvolnum>7</manvolnum> | |
45 | </refmeta> | |
46 | ||
47 | <refnamediv> | |
48 | <refname>systemd.generator</refname> | |
49 | <refpurpose>Systemd unit generators</refpurpose> | |
50 | </refnamediv> | |
51 | ||
52 | <refsynopsisdiv> | |
53 | <cmdsynopsis> | |
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> | |
58 | </cmdsynopsis> | |
59 | ||
60 | <para> | |
61 | <literallayout><filename>/run/systemd/system-generators/*</filename> | |
12b42c76 | 62 | <filename>/etc/systemd/system-generators/*</filename> |
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63 | <filename>/usr/local/lib/systemd/system-generators/*</filename> |
64 | <filename>&systemgeneratordir;/*</filename></literallayout> | |
65 | </para> | |
66 | ||
67 | <para> | |
68 | <literallayout><filename>/run/systemd/user-generators/*</filename> | |
12b42c76 | 69 | <filename>/etc/systemd/user-generators/*</filename> |
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70 | <filename>/usr/local/lib/systemd/user-generators/*</filename> |
71 | <filename>&usergeneratordir;/*</filename></literallayout> | |
72 | </para> | |
73 | </refsynopsisdiv> | |
74 | ||
75 | <refsect1> | |
76 | <title>Description</title> | |
77 | <para>Generators are small binaries that live in | |
78 | <filename>&usergeneratordir;/</filename> and other directories | |
79 | listed above. | |
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> | |
88 | ||
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> | |
102 | ||
103 | <para>After installing new generators or updating the | |
c4c086a2 | 104 | configuration, <command>systemctl daemon-reload</command> may be |
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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. | |
110 | </para> | |
111 | </refsect1> | |
112 | ||
113 | <refsect1> | |
114 | <title>Writing generators</title> | |
115 | ||
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> | |
119 | ||
120 | <orderedlist> | |
121 | <listitem> | |
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> | |
129 | </listitem> | |
130 | ||
131 | <listitem> | |
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> | |
138 | </listitem> | |
139 | ||
140 | <listitem> | |
141 | <para><parameter>late-dir</parameter></para> | |
142 | <para>argv[3] may be used to extend the unit file tree without | |
ff9b60f3 | 143 | overriding any other unit files. Any native configuration |
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144 | files supplied by the vendor or user/administrator take |
145 | precedence over the generated ones placed in this directory. | |
146 | </para> | |
147 | </listitem> | |
148 | </orderedlist> | |
149 | ||
150 | <refsect2> | |
151 | <title>Notes</title> | |
152 | ||
153 | <itemizedlist> | |
154 | <listitem> | |
155 | <para> | |
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. | |
159 | </para> | |
160 | </listitem> | |
161 | ||
162 | <listitem> | |
163 | <para> | |
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 | |
167 | <citerefentry | |
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>. | |
174 | </para> | |
175 | </listitem> | |
176 | ||
177 | <listitem> | |
178 | <para> | |
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. | |
183 | </para> | |
184 | </listitem> | |
185 | ||
186 | <listitem> | |
187 | <para> | |
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 | |
194 | question. | |
195 | </para> | |
196 | </listitem> | |
197 | ||
198 | <listitem> | |
199 | <para> | |
200 | Since | |
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. | |
204 | </para> | |
205 | </listitem> | |
206 | ||
207 | <listitem> | |
208 | <para> | |
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. | |
216 | </para> | |
217 | </listitem> | |
218 | ||
219 | <listitem> | |
220 | <para> | |
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. | |
229 | </para> | |
230 | </listitem> | |
231 | ||
232 | <listitem> | |
233 | <para> | |
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. | |
238 | </para> | |
239 | </listitem> | |
240 | ||
241 | <listitem> | |
242 | <para>Regarding overriding semantics: there are two rules we | |
243 | try to follow when thinking about the overriding semantics: | |
244 | </para> | |
245 | ||
246 | <orderedlist numeration="lowerroman"> | |
247 | <listitem> | |
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> | |
252 | </listitem> | |
253 | ||
254 | <listitem> | |
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 | |
258 | same purpose.</para> | |
259 | </listitem> | |
260 | </orderedlist> | |
261 | ||
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> | |
266 | </listitem> | |
267 | ||
268 | <listitem> | |
269 | <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. | |
274 | </para> | |
275 | </listitem> | |
276 | </itemizedlist> | |
277 | </refsect2> | |
278 | </refsect1> | |
279 | ||
280 | <refsect1> | |
281 | <title>Examples</title> | |
282 | <example> | |
283 | <title>systemd-fstab-generator</title> | |
284 | ||
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>. | |
292 | </para> | |
293 | ||
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. | |
301 | </para> | |
302 | </example> | |
303 | ||
304 | <example> | |
305 | <title>systemd-system-update-generator</title> | |
306 | ||
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> | |
315 | </example> | |
316 | ||
317 | <example> | |
318 | <title>Debuging a generator</title> | |
319 | ||
320 | <programlisting>dir=$(mktemp -d) | |
321 | SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \ | |
322 | "$dir" "$dir" "$dir" | |
323 | find $dir</programlisting> | |
324 | </example> | |
325 | </refsect1> | |
326 | ||
327 | <refsect1> | |
328 | <title>See also</title> | |
329 | ||
330 | <para> | |
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>, | |
432d5965 | 334 | <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
3ba3a79d | 335 | <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
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336 | <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
337 | <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
338 | <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
339 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
340 | <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
341 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
342 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
343 | </para> | |
344 | </refsect1> | |
345 | </refentry> |