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="option"><replaceable>early-dir</replaceable></arg>
      <arg choice="option"><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>&SYSTEM_GENERATOR_DIR;/*</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>&USER_GENERATOR_DIR;/*</filename></literallayout>
    </para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>
    <para>Generators are small executables placed in <filename>&SYSTEM_GENERATOR_DIR;/</filename> and other
    directories listed above.
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will execute
    these binaries very early at bootup and at configuration reload time — before unit files are
    loaded. Their main purpose is to convert configuration and execution context parameters that are not
    native to the service manager into dynamically generated unit files, symlinks or unit file drop-ins, so
    that they can extend the unit file hierarchy the service manager subsequently loads and operates
    on.</para>

    <para><command>systemd</command> will call each generator 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, allowing generated configuration to extend or
    override existing definitions. For tests, generators may be called with just one argument; the generator
    should assume that all three paths are the same in that case.</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. If only one argument is provided, the generator should use the same directory as the
    the three output paths.</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/systemd/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/systemd/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/systemd/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>

    <para>Note: generators <emphasis>must not</emphasis> write to other locations or otherwise make changes
    to system state. Generator output is supposed to last only until the next
    <command>daemon-reload</command> or <command>daemon-reexec</command>; if the generator is replaced
    or masked, its effects should vanish.</para>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <para>The service manager sets a number of environment variables when invoking generator
    executables. They carry information about the execution context of the generator, in order to simplify
    conditionalizing generators to specific environments. The following environment variables are set:</para>

    <variablelist class='environment-variables'>
      <varlistentry>
        <term><varname>$SYSTEMD_SCOPE</varname></term>

        <listitem><para>If the generator is invoked from the system service manager this variable is set to
        <literal>system</literal>; if invoked from the per-user service manager it is set to
        <literal>user</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_IN_INITRD</varname></term>

        <listitem><para>If the generator is run as part of an initrd this is set to <literal>1</literal>. If
        it is run from the regular host (i.e. after the transition from initrd to host) it is set to
        <literal>0</literal>. This environment variable is only set for system generators.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_FIRST_BOOT</varname></term>

        <listitem><para>If this boot-up cycle is considered a "first boot", this is set to
        <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
        details see the documentation of <varname>ConditionFirstBoot=</varname> in
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
        environment variable is only set for system generators.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>

        <listitem><para>If the service manager is run in a virtualized environment,
        <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
        first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
        of virtualization. The second string identifies the implementation of the virtualization
        technology. If no virtualization is detected this variable will not be set. This data is identical to
        what
        <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        detects and reports, and uses the same vocabulary of virtualization implementation
        identifiers.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_ARCHITECTURE</varname></term>

        <listitem><para>This variable is set to a short identifier of the reported architecture of the
        system. For details about defined values, see documentation of
        <varname>ConditionArchitecture=</varname> in
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
      </varlistentry>
    </variablelist>
  </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, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
        <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
        </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, <filename>.d/*.conf</filename> drop-ins
        for them and symlinks to them, not any other kind of non-unit related 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>

        <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
        settings of service unit files (see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
        is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
        definitions, which often might be sufficient to embed data or configuration for other programs into
        unit files in a native fashion.</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=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/systemd-fstab-generator \
        "$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>