systemd/man/systemd.generator.xml

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

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

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

  <refsynopsisdiv>
    <cmdsynopsis>
      <command index='false'>/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 executables that live in
    <filename>&systemgeneratordir;/</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. Their main purpose is to convert configuration
    that is not native into dynamically generated unit files.</para>

    <para>Each generator is called with three directory paths that are to be used for
    generator output. In these three directories, generators may dynamically generate
    unit files (regular ones, instances, as well as templates), unit file
    <filename>.d/</filename> drop-ins, and create symbolic links to unit files to add
    additional dependencies, create aliases, or instantiate existing templates. Those
    directories are included in the unit load path of
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
    allowing generated configuration to extend or override existing
    definitions.</para>

    <para>Directory paths for generator output differ by priority:
    <filename>…/generator.early</filename> has priority higher than the admin
    configuration in <filename>/etc/</filename>, while
    <filename>…/generator</filename> has lower priority than
    <filename>/etc/</filename> but higher than vendor configuration in
    <filename>/usr/</filename>, and <filename>…/generator.late</filename> has priority
    lower than all other configuration. See the next section and the discussion of
    unit load paths and unit overriding in
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
    </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>Output directories</title>

    <para>Generators are invoked with three arguments: paths to directories where
    generators can place their generated unit files or symlinks. By default those
    paths are runtime directories that are included in the search path of
    <command>systemd</command>, but a generator may be called with different paths
    for debugging purposes.</para>

    <orderedlist>
      <listitem>
        <para><parameter>normal-dir</parameter></para>
        <para>In normal use this is <filename>/run/systemd/generator</filename> in
        case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user
        generators. 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>In normal use this is <filename>/run/systemd/generator.early</filename>
        in case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user
        generators. Unit files placed in this directory override unit files in
        <filename>/usr/</filename>, <filename>/run/</filename> and
        <filename>/etc/</filename>. This means that unit files placed in this
        directory take precedence over all normal configuration, both vendor and
        user/administrator.</para>
      </listitem>

      <listitem>
        <para><parameter>late-dir</parameter></para>
        <para>In normal use this is <filename>/run/systemd/generator.late</filename>
        in case of the system generators and
        <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user
        generators. This directory 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.</para>
      </listitem>
    </orderedlist>
  </refsect1>

  <refsect1>
    <title>Notes about writing generators</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>)!
        Non-essential file systems like <filename>/var/</filename> and
        <filename>/home/</filename> are mounted after generators have run. Generators
        can however rely on the most basic kernel functionality to be available,
        including a mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
        <filename>/dev/</filename>, <filename>/usr/</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 and symlinks to
        them, 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>The generator should always include its own name in a comment at the top of the generated file,
        so that the user can easily figure out which component created or amended a particular unit.</para>

        <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
        source configuration file they are generated 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. The <varname>SourcePath=</varname> value does not have
        to be a file in a physical filesystem. For example, in the common case of the generator looking at
        the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</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 use
        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>
  </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 their 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
      <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
      </para>
    </example>

    <example>
      <title>Debugging 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-rc-local-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-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>