systemd/man/systemd.generator.xml

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<refentry id="systemd.generator">
  <refentryinfo>
    <title>systemd.generator</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.generator</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.generator</refname>
    <refpurpose>Systemd unit generators</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>/path/to/generator</command>
      <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
      <arg choice="plain"><replaceable>early-dir</replaceable></arg>
      <arg choice="plain"><replaceable>late-dir</replaceable></arg>
    </cmdsynopsis>

    <para>
      <literallayout><filename>/run/systemd/system-generators/*</filename>
<filename>/etc/systemd/system-generators/*</filename>
<filename>/usr/local/lib/systemd/system-generators/*</filename>
<filename>&systemgeneratordir;/*</filename></literallayout>
    </para>

    <para>
      <literallayout><filename>/run/systemd/user-generators/*</filename>
<filename>/etc/systemd/user-generators/*</filename>
<filename>/usr/local/lib/systemd/user-generators/*</filename>
<filename>&usergeneratordir;/*</filename></literallayout>
    </para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>
    <para>Generators are small binaries that live in
    <filename>&usergeneratordir;/</filename> and other directories
    listed above.
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    will execute those binaries very early at bootup and at
    configuration reload time — before unit files are loaded.
    Generators can dynamically generate unit files or create symbolic
    links to unit files to add additional dependencies, thus extending
    or overriding existing definitions. Their main purpose is to
    convert configuration files that are not native unit files
    dynamically into native unit files.</para>

    <para>Generators are loaded from a set of paths determined during
    compilation, as listed above. System and user generators are loaded
    from directories with names ending in
    <filename>system-generators/</filename> and
    <filename>user-generators/</filename>, respectively. Generators
    found in directories listed earlier override the ones with the
    same name in directories lower in the list. A symlink to
    <filename>/dev/null</filename> or an empty file can be used to
    mask a generator, thereby preventing it from running. Please note
    that the order of the two directories with the highest priority is
    reversed with respect to the unit load path, and generators in
    <filename>/run</filename> overwrite those in
    <filename>/etc</filename>.</para>

    <para>After installing new generators or updating the
    configuration, <command>systemctl daemon-reload</command> may be
    executed. This will delete the previous configuration created by
    generators, re-run all generators, and cause
    <command>systemd</command> to reload units from disk. See
    <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    for more information.
    </para>
  </refsect1>

  <refsect1>
    <title>Writing generators</title>

    <para>Generators are invoked with three arguments: paths to
    runtime directories where generators can place their generated
    unit files or symlinks.</para>

    <orderedlist>
      <listitem>
        <para><parameter>normal-dir</parameter></para>
        <para>argv[1] may be used to override unit files in
        <filename>/usr</filename>, but not those in
        <filename>/etc</filename>. This means that unit files placed
        in this directory take precedence over vendor unit
        configuration but not over native user/administrator unit
        configuration.</para>
      </listitem>

      <listitem>
        <para><parameter>early-dir</parameter></para>
        <para>argv[2] may be used to override unit files in
        <filename>/usr</filename> and in
        <filename>/etc</filename>. This means that unit files placed
        in this directory take precedence over all configuration,
        both vendor and user/administrator.</para>
      </listitem>

      <listitem>
        <para><parameter>late-dir</parameter></para>
        <para>argv[3] may be used to extend the unit file tree without
        overriding any other unit files. Any native configuration
        files supplied by the vendor or user/administrator take
        precedence over the generated ones placed in this directory.
        </para>
      </listitem>
    </orderedlist>

    <refsect2>
      <title>Notes</title>

      <itemizedlist>
        <listitem>
          <para>
            All generators are executed in parallel. That means all
            executables are started at the very same time and need to
            be able to cope with this parallelism.
          </para>
        </listitem>

        <listitem>
          <para>
            Generators are run very early at boot and cannot rely on
            any external services. They may not talk to any other
            process. That includes simple things such as logging to
            <citerefentry
            project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
            or <command>systemd</command> itself (this means: no
            <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>!). They
            can however rely on the most basic kernel functionality to
            be available, including a mounted <filename>/sys</filename>,
            <filename>/proc</filename>, <filename>/dev</filename>.
          </para>
        </listitem>

        <listitem>
          <para>
            Units written by generators are removed when the configuration
            is reloaded. That means the lifetime of the generated
            units is closely bound to the reload cycles of
            <command>systemd</command> itself.
          </para>
        </listitem>

        <listitem>
          <para>
            Generators should only be used to generate unit files, not
            any other kind of configuration. Due to the lifecycle
            logic mentioned above, generators are not a good fit to
            generate dynamic configuration for other services. If you
            need to generate dynamic configuration for other services,
            do so in normal services you order before the service in
            question.
          </para>
        </listitem>

        <listitem>
          <para>
            Since
            <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
            is not available (see above), log messages have to be
            written to <filename>/dev/kmsg</filename> instead.
          </para>
        </listitem>

        <listitem>
          <para>
            It is a good idea to use the
            <varname>SourcePath=</varname> directive in generated unit
            files to specify the source configuration file you are
            generating the unit from. This makes things more easily
            understood by the user and also has the benefit that
            systemd can warn the user about configuration files that
            changed on disk but have not been read yet by systemd.
          </para>
        </listitem>

        <listitem>
          <para>
            Generators may write out dynamic unit files or just hook
            unit files into other units with the usual
            <filename>.wants/</filename> or
            <filename>.requires/</filename> symlinks. Often, it is
            nicer to simply instantiate a template unit file from
            <filename>/usr</filename> with a generator instead of
            writing out entirely dynamic unit files. Of course, this
            works only if a single parameter is to be used.
          </para>
        </listitem>

        <listitem>
          <para>
            If you are careful, you can implement generators in shell
            scripts. We do recommend C code however, since generators
            are executed synchronously and hence delay the
            entire boot if they are slow.
          </para>
        </listitem>

        <listitem>
          <para>Regarding overriding semantics: there are two rules we
          try to follow when thinking about the overriding semantics:
          </para>

          <orderedlist numeration="lowerroman">
            <listitem>
              <para>User configuration should override vendor
              configuration. This (mostly) means that stuff from
              <filename>/etc</filename> should override stuff from
              <filename>/usr</filename>.</para>
            </listitem>

            <listitem>
              <para>Native configuration should override non-native
              configuration. This (mostly) means that stuff you
              generate should never override native unit files for the
              same purpose.</para>
            </listitem>
          </orderedlist>

          <para>Of these two rules the first rule is probably the more
          important one and breaks the second one sometimes. Hence,
          when deciding whether to user argv[1], argv[2], or argv[3],
          your default choice should probably be argv[1].</para>
        </listitem>

        <listitem>
          <para>
            Instead of heading off now and writing all kind of
            generators for legacy configuration file formats, please
            think twice! It is often a better idea to just deprecate
            old stuff instead of keeping it artificially alive.
          </para>
        </listitem>
      </itemizedlist>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Examples</title>
    <example>
      <title>systemd-fstab-generator</title>

      <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      converts <filename>/etc/fstab</filename> into native mount
      units. It uses argv[1] as location to place the generated unit
      files in order to allow the user to override
      <filename>/etc/fstab</filename> with her own native unit files,
      but also to ensure that <filename>/etc/fstab</filename>
      overrides any vendor default from <filename>/usr</filename>.
      </para>

      <para>After editing <filename>/etc/fstab</filename>, the user
      should invoke <command>systemctl daemon-reload</command>. This
      will re-run all generators and cause <command>systemd</command>
      to reload units from disk. To actually mount new directories
      added to <filename>fstab</filename>, <command>systemctl start
      <replaceable>/path/to/mountpoint</replaceable></command> or
      <command>systemctl start local-fs.target</command> may be used.
      </para>
    </example>

    <example>
      <title>systemd-system-update-generator</title>

      <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      temporarily redirects <filename>default.target</filename> to
      <filename>system-update.target</filename> if a system update is
      scheduled. Since this needs to override the default user
      configuration for <filename>default.target</filename>, it uses
      argv[2]. For details about this logic, see
      <ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates">Implementing
      Offline System Updates</ulink>.</para>
    </example>

    <example>
      <title>Debuging a generator</title>

      <programlisting>dir=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
        "$dir" "$dir" "$dir"
find $dir</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>