man/systemd.generator.xml

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   9   SPDX-License-Identifier: LGPL-2.1+
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  11   This file is part of systemd.
  12
  13   Copyright 2015 Zbigniew Jędrzejewski-Szmek
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  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>
  38     <refpurpose>systemd unit generators</refpurpose>
  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>
  51 <filename>/etc/systemd/system-generators/*</filename>
  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>
  58 <filename>/etc/systemd/user-generators/*</filename>
  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>
  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>
  93
  94     <para>Generators are loaded from a set of paths determined during
  95     compilation, as listed above. System and user generators are loaded
  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
 104     reversed with respect to the unit load path, and generators in
 105     <filename>/run</filename> overwrite those in
 106     <filename>/etc</filename>.</para>
 107
 108     <para>After installing new generators or updating the
 109     configuration, <command>systemctl daemon-reload</command> may be
 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>
 119     <title>Output directories</title>
 120
 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>
 126
 127     <orderedlist>
 128       <listitem>
 129         <para><parameter>normal-dir</parameter></para>
 130         <para>In normal use this is <filename>/run/systemd/generator</filename> in
 131         case of the system generators and
 132         <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user
 133         generators. Unit files placed in this directory take precedence over vendor
 134         unit configuration but not over native user/administrator unit configuration.
 135         </para>
 136       </listitem>
 137
 138       <listitem>
 139         <para><parameter>early-dir</parameter></para>
 140         <para>In normal use this is <filename>/run/systemd/generator.early</filename>
 141         in case of the system generators and
 142         <filename>$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>
 148       </listitem>
 149
 150       <listitem>
 151         <para><parameter>late-dir</parameter></para>
 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>
 158       </listitem>
 159     </orderedlist>
 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>
 228
 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>
 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>
 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>
 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
 293       <filename>system-update.target</filename>, if a system update is
 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
 297       <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 298       </para>
 299     </example>
 300
 301     <example>
 302       <title>Debugging a generator</title>
 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>,
 318       <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 319       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 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>,
 323       <citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 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>,
 327       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 328       <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 329     </para>
 330   </refsect1>
 331 </refentry>