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[thirdparty/systemd.git] / man / systemd.service.xml

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

  <refmeta>
    <refentrytitle>systemd.service</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.service</refname>
    <refpurpose>Service unit configuration</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename><replaceable>service</replaceable>.service</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>A unit configuration file whose name ends in
    <literal>.service</literal> encodes information about a process
    controlled and supervised by systemd.</para>

    <para>This man page lists the configuration options specific to
    this unit type. See
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for the common options of all unit configuration files. The common
    configuration items are configured in the generic
    <literal>[Unit]</literal> and <literal>[Install]</literal>
    sections. The service specific configuration options are
    configured in the <literal>[Service]</literal> section.</para>

    <para>Additional options are listed in
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    which define the execution environment the commands are executed
    in, and in
    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    which define the way the processes of the service are terminated,
    and in
    <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    which configure resource control settings for the processes of the
    service.</para>

    <para>If a service is requested under a certain name but no unit
    configuration file is found, systemd looks for a SysV init script
    by the same name (with the <filename>.service</filename> suffix
    removed) and dynamically creates a service unit from that script.
    This is useful for compatibility with SysV. Note that this
    compatibility is quite comprehensive but not 100%. For details
    about the incompatibilities, see the <ulink
    url="https://www.freedesktop.org/wiki/Software/systemd/Incompatibilities">Incompatibilities
    with SysV</ulink> document.</para>

    <para>The <citerefentry><refentrytitle>systemd-run</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    command allows creating <filename>.service</filename> and <filename>.scope</filename> units dynamically
    and transiently from the command line.</para>
  </refsect1>

  <refsect1>
    <title>Service Templates</title>

    <para>It is possible for <command>systemd</command> services to take a single argument via the
    <literal><replaceable>service</replaceable>@<replaceable>argument</replaceable>.service</literal>
    syntax. Such services are called "instantiated" services, while the unit definition without the
    <replaceable>argument</replaceable> parameter is called a "template". An example could be a
    <filename>dhcpcd@.service</filename> service template which takes a network interface as a
    parameter to form an instantiated service. Within the service file, this parameter or "instance
    name" can be accessed with %-specifiers. See
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details.</para>
  </refsect1>

  <refsect1>
    <title>Automatic Dependencies</title>

    <refsect2>
      <title>Implicit Dependencies</title>

      <para>The following dependencies are implicitly added:</para>

      <itemizedlist>
        <listitem><para>Services with <varname>Type=dbus</varname> set automatically
        acquire dependencies of type <varname>Requires=</varname> and
        <varname>After=</varname> on
        <filename>dbus.socket</filename>.</para></listitem>

        <listitem><para>Socket activated services are automatically ordered after
        their activating <filename>.socket</filename> units via an
        automatic <varname>After=</varname> dependency.
        Services also pull in all <filename>.socket</filename> units
        listed in <varname>Sockets=</varname> via automatic
        <varname>Wants=</varname> and <varname>After=</varname> dependencies.</para></listitem>
      </itemizedlist>

      <para>Additional implicit dependencies may be added as result of
      execution and resource control parameters as documented in
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      and
      <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
    </refsect2>

    <refsect2>
      <title>Default Dependencies</title>

      <para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para>

      <itemizedlist>
        <listitem><para>Service units will have dependencies of type <varname>Requires=</varname> and
        <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>After=</varname> on
        <filename>basic.target</filename> as well as dependencies of type <varname>Conflicts=</varname> and
        <varname>Before=</varname> on <filename>shutdown.target</filename>. These ensure that normal service units pull in
        basic system initialization, and are terminated cleanly prior to system shutdown. Only services involved with early
        boot or late system shutdown should disable this option.</para></listitem>

        <listitem><para>Instanced service units (i.e. service units with an <literal>@</literal> in their name) are assigned by
        default a per-template slice unit (see
        <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>), named after the
        template unit, containing all instances of the specific template. This slice is normally stopped at shutdown,
        together with all template instances. If that is not desired, set <varname>DefaultDependencies=no</varname> in the
        template unit, and either define your own per-template slice unit file that also sets
        <varname>DefaultDependencies=no</varname>, or set <varname>Slice=system.slice</varname> (or another suitable slice)
        in the template unit. Also see
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para></listitem>
      </itemizedlist>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>Service files must include a <literal>[Service]</literal>
    section, which carries information about the service and the
    process it supervises. A number of options that may be used in
    this section are shared with other unit types. These options are
    documented in
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    and
    <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
    The options specific to the <literal>[Service]</literal> section
    of service units are the following:</para>

    <variablelist class='unit-directives'>
      <varlistentry>
        <term><varname>Type=</varname></term>

        <listitem>
          <para>Configures the process start-up type for this service unit. One of <option>simple</option>,
          <option>exec</option>, <option>forking</option>, <option>oneshot</option>, <option>dbus</option>,
          <option>notify</option> or <option>idle</option>:</para>

          <itemizedlist>
            <listitem><para>If set to <option>simple</option> (the default if <varname>ExecStart=</varname> is
            specified but neither <varname>Type=</varname> nor <varname>BusName=</varname> are), the service manager
            will consider the unit started immediately after the main service process has been forked off. It is
            expected that the process configured with <varname>ExecStart=</varname> is the main process of the
            service. In this mode, if the process offers functionality to other processes on the system, its
            communication channels should be installed before the service is started up (e.g.  sockets set up by
            systemd, via socket activation), as the service manager will immediately proceed starting follow-up units,
            right after creating the main service process, and before executing the service's binary. Note that this
            means <command>systemctl start</command> command lines for <option>simple</option> services will report
            success even if the service's binary cannot be invoked successfully (for example because the selected
            <varname>User=</varname> doesn't exist, or the service binary is missing).</para></listitem>

            <listitem><para>The <option>exec</option> type is similar to <option>simple</option>, but the service
            manager will consider the unit started immediately after the main service binary has been executed. The service
            manager will delay starting of follow-up units until that point. (Or in other words:
            <option>simple</option> proceeds with further jobs right after <function>fork()</function> returns, while
            <option>exec</option> will not proceed before both <function>fork()</function> and
            <function>execve()</function> in the service process succeeded.) Note that this means <command>systemctl
            start</command> command lines for <option>exec</option> services will report failure when the service's
            binary cannot be invoked successfully (for example because the selected <varname>User=</varname> doesn't
            exist, or the service binary is missing).</para></listitem>

            <listitem><para>If set to <option>forking</option>, it is expected that the process configured with
            <varname>ExecStart=</varname> will call <function>fork()</function> as part of its start-up. The parent
            process is expected to exit when start-up is complete and all communication channels are set up. The child
            continues to run as the main service process, and the service manager will consider the unit started when
            the parent process exits. This is the behavior of traditional UNIX services. If this setting is used, it is
            recommended to also use the <varname>PIDFile=</varname> option, so that systemd can reliably identify the
            main process of the service. systemd will proceed with starting follow-up units as soon as the parent
            process exits.</para></listitem>

            <listitem><para>Behavior of <option>oneshot</option> is similar to <option>simple</option>;
            however, the service manager will consider the unit up after the main process exits. It will then
            start follow-up units. <varname>RemainAfterExit=</varname> is particularly useful for this type
            of service. <varname>Type=</varname><option>oneshot</option> is the implied default if neither
            <varname>Type=</varname> nor <varname>ExecStart=</varname> are specified. Note that if this
            option is used without <varname>RemainAfterExit=</varname> the service will never enter
            <literal>active</literal> unit state, but directly transition from <literal>activating</literal>
            to <literal>deactivating</literal> or <literal>dead</literal> since no process is configured that
            shall run continously. In particular this means that after a service of this type ran (and which
            has <varname>RemainAfterExit=</varname> not set) it will not show up as started afterwards, but
            as dead.</para></listitem>

            <listitem><para>Behavior of <option>dbus</option> is similar to <option>simple</option>; however, it is
            expected that the service acquires a name on the D-Bus bus, as configured by
            <varname>BusName=</varname>. systemd will proceed with starting follow-up units after the D-Bus bus name
            has been acquired. Service units with this option configured implicitly gain dependencies on the
            <filename>dbus.socket</filename> unit. This type is the default if <varname>BusName=</varname> is
            specified.</para></listitem>

            <listitem><para>Behavior of <option>notify</option> is similar to <option>exec</option>; however, it is
            expected that the service sends a notification message via
            <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> or an
            equivalent call when it has finished starting up. systemd will proceed with starting follow-up units after
            this notification message has been sent. If this option is used, <varname>NotifyAccess=</varname> (see
            below) should be set to open access to the notification socket provided by systemd. If
            <varname>NotifyAccess=</varname> is missing or set to <option>none</option>, it will be forcibly set to
            <option>main</option>. Note that currently <varname>Type=</varname><option>notify</option> will not work if
            used in combination with <varname>PrivateNetwork=</varname><option>yes</option>.</para></listitem>

            <listitem><para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however,
            actual execution of the service program is delayed until all active jobs are dispatched. This may be used
            to avoid interleaving of output of shell services with the status output on the console. Note that this
            type is useful only to improve console output, it is not useful as a general unit ordering tool, and the
            effect of this service type is subject to a 5s timeout, after which the service program is invoked
            anyway.</para></listitem>
          </itemizedlist>

          <para>It is generally recommended to use <varname>Type=</varname><option>simple</option> for long-running
          services whenever possible, as it is the simplest and fastest option. However, as this service type won't
          propagate service start-up failures and doesn't allow ordering of other units against completion of
          initialization of the service (which for example is useful if clients need to connect to the service through
          some form of IPC, and the IPC channel is only established by the service itself — in contrast to doing this
          ahead of time through socket or bus activation or similar), it might not be sufficient for many cases. If so,
          <option>notify</option> or <option>dbus</option> (the latter only in case the service provides a D-Bus
          interface) are the preferred options as they allow service program code to precisely schedule when to
          consider the service started up successfully and when to proceed with follow-up units. The
          <option>notify</option> service type requires explicit support in the service codebase (as
          <function>sd_notify()</function> or an equivalent API needs to be invoked by the service at the appropriate
          time) — if it's not supported, then <option>forking</option> is an alternative: it supports the traditional
          UNIX service start-up protocol. Finally, <option>exec</option> might be an option for cases where it is
          enough to ensure the service binary is invoked, and where the service binary itself executes no or little
          initialization on its own (and its initialization is unlikely to fail). Note that using any type other than
          <option>simple</option> possibly delays the boot process, as the service manager needs to wait for service
          initialization to complete. It is hence recommended not to needlessly use any types other than
          <option>simple</option>. (Also note it is generally not recommended to use <option>idle</option> or
          <option>oneshot</option> for long-running services.)</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RemainAfterExit=</varname></term>

        <listitem><para>Takes a boolean value that specifies whether
        the service shall be considered active even when all its
        processes exited. Defaults to <option>no</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>GuessMainPID=</varname></term>

        <listitem><para>Takes a boolean value that specifies whether
        systemd should try to guess the main PID of a service if it
        cannot be determined reliably. This option is ignored unless
        <option>Type=forking</option> is set and
        <option>PIDFile=</option> is unset because for the other types
        or with an explicitly configured PID file, the main PID is
        always known. The guessing algorithm might come to incorrect
        conclusions if a daemon consists of more than one process. If
        the main PID cannot be determined, failure detection and
        automatic restarting of a service will not work reliably.
        Defaults to <option>yes</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>PIDFile=</varname></term>

        <listitem><para>Takes a path referring to the PID file of the service. Usage of this option is recommended for
        services where <varname>Type=</varname> is set to <option>forking</option>. The path specified typically points
        to a file below <filename>/run/</filename>. If a relative path is specified it is hence prefixed with
        <filename>/run/</filename>. The service manager will read the PID of the main process of the service from this
        file after start-up of the service. The service manager will not write to the file configured here, although it
        will remove the file after the service has shut down if it still exists. The PID file does not need to be owned
        by a privileged user, but if it is owned by an unprivileged user additional safety restrictions are enforced:
        the file may not be a symlink to a file owned by a different user (neither directly nor indirectly), and the
        PID file must refer to a process already belonging to the service.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>BusName=</varname></term>

        <listitem><para>Takes a D-Bus bus name that this service is
        reachable as. This option is mandatory for services where
        <varname>Type=</varname> is set to
        <option>dbus</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExecStart=</varname></term>
        <listitem><para>Commands with their arguments that are
        executed when this service is started. The value is split into
        zero or more command lines according to the rules described
        below (see section "Command Lines" below).
        </para>

        <para>Unless <varname>Type=</varname> is <option>oneshot</option>, exactly one command must be given. When
        <varname>Type=oneshot</varname> is used, zero or more commands may be specified. Commands may be specified by
        providing multiple command lines in the same directive, or alternatively, this directive may be specified more
        than once with the same effect. If the empty string is assigned to this option, the list of commands to start
        is reset, prior assignments of this option will have no effect. If no <varname>ExecStart=</varname> is
        specified, then the service must have <varname>RemainAfterExit=yes</varname> and at least one
        <varname>ExecStop=</varname> line set. (Services lacking both <varname>ExecStart=</varname> and
        <varname>ExecStop=</varname> are not valid.)</para>

        <para>For each of the specified commands, the first argument must be either an absolute path to an executable
        or a simple file name without any slashes. Optionally, this filename may be prefixed with a number of special
        characters:</para>

        <table>
          <title>Special executable prefixes</title>

          <tgroup cols='2'>
            <colspec colname='prefix'/>
            <colspec colname='meaning'/>

            <thead>
              <row>
                <entry>Prefix</entry>
                <entry>Effect</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>@</literal></entry>
                <entry>If the executable path is prefixed with <literal>@</literal>, the second specified token will be passed as <literal>argv[0]</literal> to the executed process (instead of the actual filename), followed by the further arguments specified.</entry>
              </row>

              <row>
                <entry><literal>-</literal></entry>
                <entry>If the executable path is prefixed with <literal>-</literal>, an exit code of the command normally considered a failure (i.e. non-zero exit status or abnormal exit due to signal) is recorded, but has no further effect and is considered equivalent to success.</entry>
              </row>

              <row>
                <entry><literal>:</literal></entry>
                <entry>If the executable path is prefixed with <literal>:</literal>, environment variable substitution (as described by the "Command Lines" section below) is not applied.</entry>
              </row>

              <row>
                <entry><literal>+</literal></entry>
                <entry>If the executable path is prefixed with <literal>+</literal> then the process is executed with full privileges. In this mode privilege restrictions configured with <varname>User=</varname>, <varname>Group=</varname>, <varname>CapabilityBoundingSet=</varname> or the various file system namespacing options (such as <varname>PrivateDevices=</varname>, <varname>PrivateTmp=</varname>) are not applied to the invoked command line (but still affect any other <varname>ExecStart=</varname>, <varname>ExecStop=</varname>, … lines).</entry>
              </row>

              <row>
                <entry><literal>!</literal></entry>

                <entry>Similar to the <literal>+</literal> character discussed above this permits invoking command lines with elevated privileges. However, unlike <literal>+</literal> the <literal>!</literal> character exclusively alters the effect of <varname>User=</varname>, <varname>Group=</varname> and <varname>SupplementaryGroups=</varname>, i.e. only the stanzas that affect user and group credentials. Note that this setting may be combined with <varname>DynamicUser=</varname>, in which case a dynamic user/group pair is allocated before the command is invoked, but credential changing is left to the executed process itself.</entry>
              </row>

              <row>
                <entry><literal>!!</literal></entry>

                <entry>This prefix is very similar to <literal>!</literal>, however it only has an effect on systems lacking support for ambient process capabilities, i.e. without support for <varname>AmbientCapabilities=</varname>. It's intended to be used for unit files that take benefit of ambient capabilities to run processes with minimal privileges wherever possible while remaining compatible with systems that lack ambient capabilities support. Note that when <literal>!!</literal> is used, and a system lacking ambient capability support is detected any configured <varname>SystemCallFilter=</varname> and <varname>CapabilityBoundingSet=</varname> stanzas are implicitly modified, in order to permit spawned processes to drop credentials and capabilities themselves, even if this is configured to not be allowed. Moreover, if this prefix is used and a system lacking ambient capability support is detected <varname>AmbientCapabilities=</varname> will be skipped and not be applied. On systems supporting ambient capabilities, <literal>!!</literal> has no effect and is redundant.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        <para><literal>@</literal>, <literal>-</literal>, <literal>:</literal>, and one of
        <literal>+</literal>/<literal>!</literal>/<literal>!!</literal> may be used together and they can appear in any
        order. However, only one of <literal>+</literal>, <literal>!</literal>, <literal>!!</literal> may be used at a
        time. Note that these prefixes are also supported for the other command line settings,
        i.e. <varname>ExecStartPre=</varname>, <varname>ExecStartPost=</varname>, <varname>ExecReload=</varname>,
        <varname>ExecStop=</varname> and <varname>ExecStopPost=</varname>.</para>

        <para>If more than one command is specified, the commands are
        invoked sequentially in the order they appear in the unit
        file. If one of the commands fails (and is not prefixed with
        <literal>-</literal>), other lines are not executed, and the
        unit is considered failed.</para>

        <para>Unless <varname>Type=forking</varname> is set, the
        process started via this command line will be considered the
        main process of the daemon.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExecStartPre=</varname></term>
        <term><varname>ExecStartPost=</varname></term>
        <listitem><para>Additional commands that are executed before
        or after the command in <varname>ExecStart=</varname>,
        respectively. Syntax is the same as for
        <varname>ExecStart=</varname>, except that multiple command
        lines are allowed and the commands are executed one after the
        other, serially.</para>

        <para>If any of those commands (not prefixed with
        <literal>-</literal>) fail, the rest are not executed and the
        unit is considered failed.</para>

        <para><varname>ExecStart=</varname> commands are only run after
        all <varname>ExecStartPre=</varname> commands that were not prefixed
        with a <literal>-</literal> exit successfully.</para>

        <para><varname>ExecStartPost=</varname> commands are only run after the commands specified in
        <varname>ExecStart=</varname> have been invoked successfully, as determined by <varname>Type=</varname>
        (i.e. the process has been started for <varname>Type=simple</varname> or <varname>Type=idle</varname>, the last
        <varname>ExecStart=</varname> process exited successfully for <varname>Type=oneshot</varname>, the initial
        process exited successfully for <varname>Type=forking</varname>, <literal>READY=1</literal> is sent for
        <varname>Type=notify</varname>, or the <varname>BusName=</varname> has been taken for
        <varname>Type=dbus</varname>).</para>

        <para>Note that <varname>ExecStartPre=</varname> may not be
        used to start long-running processes. All processes forked
        off by processes invoked via <varname>ExecStartPre=</varname> will
        be killed before the next service process is run.</para>

        <para>Note that if any of the commands specified in <varname>ExecStartPre=</varname>,
        <varname>ExecStart=</varname>, or <varname>ExecStartPost=</varname> fail (and are not prefixed with
        <literal>-</literal>, see above) or time out before the service is fully up, execution continues with commands
        specified in <varname>ExecStopPost=</varname>, the commands in <varname>ExecStop=</varname> are skipped.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExecReload=</varname></term>
        <listitem><para>Commands to execute to trigger a configuration
        reload in the service. This argument takes multiple command
        lines, following the same scheme as described for
        <varname>ExecStart=</varname> above. Use of this setting is
        optional. Specifier and environment variable substitution is
        supported here following the same scheme as for
        <varname>ExecStart=</varname>.</para>

        <para>One additional, special environment variable is set: if
        known, <varname>$MAINPID</varname> is set to the main process
        of the daemon, and may be used for command lines like the
        following:</para>

        <programlisting>/bin/kill -HUP $MAINPID</programlisting>

        <para>Note however that reloading a daemon by sending a signal
        (as with the example line above) is usually not a good choice,
        because this is an asynchronous operation and hence not
        suitable to order reloads of multiple services against each
        other. It is strongly recommended to set
        <varname>ExecReload=</varname> to a command that not only
        triggers a configuration reload of the daemon, but also
        synchronously waits for it to complete.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExecStop=</varname></term>
        <listitem><para>Commands to execute to stop the service
        started via <varname>ExecStart=</varname>. This argument takes
        multiple command lines, following the same scheme as described
        for <varname>ExecStart=</varname> above. Use of this setting
        is optional. After the commands configured in this option are
        run, it is implied that the service is stopped, and any processes
        remaining for it are terminated
        according to the <varname>KillMode=</varname> setting (see
        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        If this option is not specified, the process is terminated by
        sending the signal specified in <varname>KillSignal=</varname>
        when service stop is requested. Specifier and environment
        variable substitution is supported (including
        <varname>$MAINPID</varname>, see above).</para>

        <para>Note that it is usually not sufficient to specify a command for this setting that only asks the service
        to terminate (for example, by queuing some form of termination signal for it), but does not wait for it to do
        so. Since the remaining processes of the services are killed according to <varname>KillMode=</varname> and
        <varname>KillSignal=</varname> as described above immediately after the command exited, this may not result in
        a clean stop. The specified command should hence be a synchronous operation, not an asynchronous one.</para>

        <para>Note that the commands specified in <varname>ExecStop=</varname> are only executed when the service
        started successfully first. They are not invoked if the service was never started at all, or in case its
        start-up failed, for example because any of the commands specified in <varname>ExecStart=</varname>,
        <varname>ExecStartPre=</varname> or <varname>ExecStartPost=</varname> failed (and weren't prefixed with
        <literal>-</literal>, see above) or timed out. Use <varname>ExecStopPost=</varname> to invoke commands when a
        service failed to start up correctly and is shut down again. Also note that the stop operation is always
        performed if the service started successfully, even if the processes in the service terminated on their
        own or were killed. The stop commands must be prepared to deal with that case. <varname>$MAINPID</varname>
        will be unset if systemd knows that the main process exited by the time the stop commands are called.</para>

        <para>Service restart requests are implemented as stop operations followed by start operations. This
        means that <varname>ExecStop=</varname> and <varname>ExecStopPost=</varname> are executed during a
        service restart operation.</para>

        <para>It is recommended to use this setting for commands that communicate with the service requesting
        clean termination. For post-mortem clean-up steps use <varname>ExecStopPost=</varname> instead.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ExecStopPost=</varname></term>
        <listitem><para>Additional commands that are executed after the service is stopped. This includes cases where
        the commands configured in <varname>ExecStop=</varname> were used, where the service does not have any
        <varname>ExecStop=</varname> defined, or where the service exited unexpectedly. This argument takes multiple
        command lines, following the same scheme as described for <varname>ExecStart=</varname>. Use of these settings
        is optional. Specifier and environment variable substitution is supported. Note that – unlike
        <varname>ExecStop=</varname> – commands specified with this setting are invoked when a service failed to start
        up correctly and is shut down again.</para>

        <para>It is recommended to use this setting for clean-up operations that shall be executed even when the
        service failed to start up correctly. Commands configured with this setting need to be able to operate even if
        the service failed starting up half-way and left incompletely initialized data around. As the service's
        processes have been terminated already when the commands specified with this setting are executed they should
        not attempt to communicate with them.</para>

        <para>Note that all commands that are configured with this setting are invoked with the result code of the
        service, as well as the main process' exit code and status, set in the <varname>$SERVICE_RESULT</varname>,
        <varname>$EXIT_CODE</varname> and <varname>$EXIT_STATUS</varname> environment variables, see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestartSec=</varname></term>
        <listitem><para>Configures the time to sleep before restarting
        a service (as configured with <varname>Restart=</varname>).
        Takes a unit-less value in seconds, or a time span value such
        as "5min 20s". Defaults to 100ms.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutStartSec=</varname></term>
        <listitem><para>Configures the time to wait for start-up. If a
        daemon service does not signal start-up completion within the
        configured time, the service will be considered failed and
        will be shut down again. Takes a unit-less value in seconds,
        or a time span value such as "5min 20s". Pass
        <literal>infinity</literal> to disable the timeout logic. Defaults to
        <varname>DefaultTimeoutStartSec=</varname> from the manager
        configuration file, except when
        <varname>Type=oneshot</varname> is used, in which case the
        timeout is disabled by default (see
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        </para>

        <para>If a service of <varname>Type=notify</varname> sends <literal>EXTEND_TIMEOUT_USEC=…</literal>, this may cause
        the start time to be extended beyond <varname>TimeoutStartSec=</varname>. The first receipt of this message
        must occur before <varname>TimeoutStartSec=</varname> is exceeded, and once the start time has exended beyond
        <varname>TimeoutStartSec=</varname>, the service manager will allow the service to continue to start, provided
        the service repeats <literal>EXTEND_TIMEOUT_USEC=…</literal> within the interval specified until the service
        startup status is finished by <literal>READY=1</literal>. (see
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutStopSec=</varname></term>
        <listitem><para>This option serves two purposes. First, it configures the time to wait for each
        <constant>ExecStop=</constant> command. If any of them times out, subsequent <constant>ExecStop=</constant> commands
        are skipped and the service will be terminated by <constant>SIGTERM</constant>. If no <constant>ExecStop=</constant>
        commands are specified, the service gets the <constant>SIGTERM</constant> immediately. Second, it configures the time
        to wait for the service itself to stop. If it doesn't terminate in the specified time, it will be forcibly terminated
        by <constant>SIGKILL</constant> (see <varname>KillMode=</varname> in
        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        Takes a unit-less value in seconds, or a time span value such
        as "5min 20s". Pass <literal>infinity</literal> to disable the
        timeout logic. Defaults to
        <varname>DefaultTimeoutStopSec=</varname> from the manager
        configuration file (see
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        </para>

        <para>If a service of <varname>Type=notify</varname> sends <literal>EXTEND_TIMEOUT_USEC=…</literal>, this may cause
        the stop time to be extended beyond <varname>TimeoutStopSec=</varname>. The first receipt of this message
        must occur before <varname>TimeoutStopSec=</varname> is exceeded, and once the stop time has exended beyond
        <varname>TimeoutStopSec=</varname>, the service manager will allow the service to continue to stop, provided
        the service repeats <literal>EXTEND_TIMEOUT_USEC=…</literal> within the interval specified, or terminates itself
        (see <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutAbortSec=</varname></term>
        <listitem><para>This option configures the time to wait for the service to terminate when it was aborted due to a
        watchdog timeout (see <varname>WatchdogSec=</varname>). If the service has a short <varname>TimeoutStopSec=</varname>
        this option can be used to give the system more time to write a core dump of the service. Upon expiration the service
        will be forcibly terminated by <constant>SIGKILL</constant> (see <varname>KillMode=</varname> in
        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>). The core file will
        be truncated in this case. Use <varname>TimeoutAbortSec=</varname> to set a sensible timeout for the core dumping per
        service that is large enough to write all expected data while also being short enough to handle the service failure
        in due time.
        </para>

        <para>Takes a unit-less value in seconds, or a time span value such as "5min 20s". Pass an empty value to skip
        the dedicated watchdog abort timeout handling and fall back <varname>TimeoutStopSec=</varname>. Pass
        <literal>infinity</literal> to disable the timeout logic. Defaults to <varname>DefaultTimeoutAbortSec=</varname> from
        the manager configuration file (see
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        </para>

        <para>If a service of <varname>Type=notify</varname> handles <constant>SIGABRT</constant> itself (instead of relying
        on the kernel to write a core dump) it can send <literal>EXTEND_TIMEOUT_USEC=…</literal> to
        extended the abort time beyond <varname>TimeoutAbortSec=</varname>. The first receipt of this message
        must occur before <varname>TimeoutAbortSec=</varname> is exceeded, and once the abort time has exended beyond
        <varname>TimeoutAbortSec=</varname>, the service manager will allow the service to continue to abort, provided
        the service repeats <literal>EXTEND_TIMEOUT_USEC=…</literal> within the interval specified, or terminates itself
        (see <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutSec=</varname></term>
        <listitem><para>A shorthand for configuring both
        <varname>TimeoutStartSec=</varname> and
        <varname>TimeoutStopSec=</varname> to the specified value.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutCleanSec=</varname></term>
        <listitem><para>Configures a timeout on the clean-up operation requested through <command>systemctl
        clean …</command>, see
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
        details. Takes the usual time values and defaults to <constant>infinity</constant>, i.e. by default
        no time-out is applied. If a time-out is configured the clean operation will be aborted forcibly when
        the time-out is reached, potentially leaving resources on disk.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RuntimeMaxSec=</varname></term>

        <listitem><para>Configures a maximum time for the service to run. If this is used and the service has been
        active for longer than the specified time it is terminated and put into a failure state. Note that this setting
        does not have any effect on <varname>Type=oneshot</varname> services, as they terminate immediately after
        activation completed. Pass <literal>infinity</literal> (the default) to configure no runtime
        limit.</para>

        <para>If a service of <varname>Type=notify</varname> sends <literal>EXTEND_TIMEOUT_USEC=…</literal>, this may cause
        the runtime to be extended beyond <varname>RuntimeMaxSec=</varname>. The first receipt of this message
        must occur before <varname>RuntimeMaxSec=</varname> is exceeded, and once the runtime has exended beyond
        <varname>RuntimeMaxSec=</varname>, the service manager will allow the service to continue to run, provided
        the service repeats <literal>EXTEND_TIMEOUT_USEC=…</literal> within the interval specified until the service
        shutdown is achieved by <literal>STOPPING=1</literal> (or termination). (see
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>WatchdogSec=</varname></term>
        <listitem><para>Configures the watchdog timeout for a service.
        The watchdog is activated when the start-up is completed. The
        service must call
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        regularly with <literal>WATCHDOG=1</literal> (i.e. the
        "keep-alive ping"). If the time between two such calls is
        larger than the configured time, then the service is placed in
        a failed state and it will be terminated with
        <constant>SIGABRT</constant> (or the signal specified by
        <varname>WatchdogSignal=</varname>). By setting
        <varname>Restart=</varname> to <option>on-failure</option>,
        <option>on-watchdog</option>, <option>on-abnormal</option> or
        <option>always</option>, the service will be automatically
        restarted. The time configured here will be passed to the
        executed service process in the
        <varname>WATCHDOG_USEC=</varname> environment variable. This
        allows daemons to automatically enable the keep-alive pinging
        logic if watchdog support is enabled for the service. If this
        option is used, <varname>NotifyAccess=</varname> (see below)
        should be set to open access to the notification socket
        provided by systemd. If <varname>NotifyAccess=</varname> is
        not set, it will be implicitly set to <option>main</option>.
        Defaults to 0, which disables this feature. The service can
        check whether the service manager expects watchdog keep-alive
        notifications. See
        <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for details.
        <citerefentry><refentrytitle>sd_event_set_watchdog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        may be used to enable automatic watchdog notification support.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Restart=</varname></term>
        <listitem><para>Configures whether the service shall be
        restarted when the service process exits, is killed, or a
        timeout is reached. The service process may be the main
        service process, but it may also be one of the processes
        specified with <varname>ExecStartPre=</varname>,
        <varname>ExecStartPost=</varname>,
        <varname>ExecStop=</varname>,
        <varname>ExecStopPost=</varname>, or
        <varname>ExecReload=</varname>. When the death of the process
        is a result of systemd operation (e.g. service stop or
        restart), the service will not be restarted. Timeouts include
        missing the watchdog "keep-alive ping" deadline and a service
        start, reload, and stop operation timeouts.</para>

        <para>Takes one of
        <option>no</option>,
        <option>on-success</option>,
        <option>on-failure</option>,
        <option>on-abnormal</option>,
        <option>on-watchdog</option>,
        <option>on-abort</option>, or
        <option>always</option>.
        If set to <option>no</option> (the default), the service will
        not be restarted. If set to <option>on-success</option>, it
        will be restarted only when the service process exits cleanly.
        In this context, a clean exit means an exit code of 0, or one
        of the signals
        <constant>SIGHUP</constant>,
        <constant>SIGINT</constant>,
        <constant>SIGTERM</constant> or
        <constant>SIGPIPE</constant>, and
        additionally, exit statuses and signals specified in
        <varname>SuccessExitStatus=</varname>. If set to
        <option>on-failure</option>, the service will be restarted
        when the process exits with a non-zero exit code, is
        terminated by a signal (including on core dump, but excluding
        the aforementioned four signals), when an operation (such as
        service reload) times out, and when the configured watchdog
        timeout is triggered. If set to <option>on-abnormal</option>,
        the service will be restarted when the process is terminated
        by a signal (including on core dump, excluding the
        aforementioned four signals), when an operation times out, or
        when the watchdog timeout is triggered. If set to
        <option>on-abort</option>, the service will be restarted only
        if the service process exits due to an uncaught signal not
        specified as a clean exit status. If set to
        <option>on-watchdog</option>, the service will be restarted
        only if the watchdog timeout for the service expires. If set
        to <option>always</option>, the service will be restarted
        regardless of whether it exited cleanly or not, got terminated
        abnormally by a signal, or hit a timeout.</para>

        <table>
          <title>Exit causes and the effect of the <varname>Restart=</varname> settings on them</title>

          <tgroup cols='2'>
            <colspec colname='path' />
            <colspec colname='expl' />
            <thead>
              <row>
                <entry>Restart settings/Exit causes</entry>
                <entry><option>no</option></entry>
                <entry><option>always</option></entry>
                <entry><option>on-success</option></entry>
                <entry><option>on-failure</option></entry>
                <entry><option>on-abnormal</option></entry>
                <entry><option>on-abort</option></entry>
                <entry><option>on-watchdog</option></entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>Clean exit code or signal</entry>
                <entry/>
                <entry>X</entry>
                <entry>X</entry>
                <entry/>
                <entry/>
                <entry/>
                <entry/>
              </row>
              <row>
                <entry>Unclean exit code</entry>
                <entry/>
                <entry>X</entry>
                <entry/>
                <entry>X</entry>
                <entry/>
                <entry/>
                <entry/>
              </row>
              <row>
                <entry>Unclean signal</entry>
                <entry/>
                <entry>X</entry>
                <entry/>
                <entry>X</entry>
                <entry>X</entry>
                <entry>X</entry>
                <entry/>
              </row>
              <row>
                <entry>Timeout</entry>
                <entry/>
                <entry>X</entry>
                <entry/>
                <entry>X</entry>
                <entry>X</entry>
                <entry/>
                <entry/>
              </row>
              <row>
                <entry>Watchdog</entry>
                <entry/>
                <entry>X</entry>
                <entry/>
                <entry>X</entry>
                <entry>X</entry>
                <entry/>
                <entry>X</entry>
              </row>
            </tbody>
          </tgroup>
        </table>

        <para>As exceptions to the setting above, the service will not
        be restarted if the exit code or signal is specified in
        <varname>RestartPreventExitStatus=</varname> (see below) or
        the service is stopped with <command>systemctl stop</command>
        or an equivalent operation. Also, the services will always be
        restarted if the exit code or signal is specified in
        <varname>RestartForceExitStatus=</varname> (see below).</para>

        <para>Note that service restart is subject to unit start rate
        limiting configured with <varname>StartLimitIntervalSec=</varname>
        and <varname>StartLimitBurst=</varname>, see
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details.  A restarted service enters the failed state only
        after the start limits are reached.</para>

        <para>Setting this to <option>on-failure</option> is the
        recommended choice for long-running services, in order to
        increase reliability by attempting automatic recovery from
        errors. For services that shall be able to terminate on their
        own choice (and avoid immediate restarting),
        <option>on-abnormal</option> is an alternative choice.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SuccessExitStatus=</varname></term>
        <listitem><para>Takes a list of exit status definitions that,
        when returned by the main service process, will be considered
        successful termination, in addition to the normal successful
        exit code 0 and the signals <constant>SIGHUP</constant>,
        <constant>SIGINT</constant>, <constant>SIGTERM</constant>, and
        <constant>SIGPIPE</constant>. Exit status definitions can
        either be numeric exit codes or termination signal names,
        separated by spaces. For example:

        <programlisting>SuccessExitStatus=1 2 8 SIGKILL</programlisting>

        ensures that exit codes 1, 2, 8 and
        the termination signal <constant>SIGKILL</constant> are
        considered clean service terminations.
        </para>

        <para>This option may appear more than once, in which case the
        list of successful exit statuses is merged. If the empty
        string is assigned to this option, the list is reset, all
        prior assignments of this option will have no
        effect.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestartPreventExitStatus=</varname></term>

        <listitem><para>Takes a list of exit status definitions that, when returned by the main service
        process, will prevent automatic service restarts, regardless of the restart setting configured with
        <varname>Restart=</varname>. Exit status definitions can either be numeric exit codes or termination
        signal names, and are separated by spaces. Defaults to the empty list, so that, by default, no exit
        status is excluded from the configured restart logic. For example:

        <programlisting>RestartPreventExitStatus=1 6 SIGABRT</programlisting>

        ensures that exit codes 1 and 6 and the termination signal <constant>SIGABRT</constant> will not
        result in automatic service restarting. This option may appear more than once, in which case the list
        of restart-preventing statuses is merged. If the empty string is assigned to this option, the list is
        reset and all prior assignments of this option will have no effect.</para>

        <para>Note that this setting has no effect on processes configured via
        <varname>ExecStartPre=</varname>, <varname>ExecStartPost=</varname>, <varname>ExecStop=</varname>,
        <varname>ExecStopPost=</varname> or <varname>ExecReload=</varname>, but only on the main service
        process, i.e. either the one invoked by <varname>ExecStart=</varname> or (depending on
        <varname>Type=</varname>, <varname>PIDFile=</varname>, …) the otherwise configured main
        process.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RestartForceExitStatus=</varname></term>
        <listitem><para>Takes a list of exit status definitions that,
        when returned by the main service process, will force automatic
        service restarts, regardless of the restart setting configured
        with <varname>Restart=</varname>. The argument format is
        similar to
        <varname>RestartPreventExitStatus=</varname>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RootDirectoryStartOnly=</varname></term>
        <listitem><para>Takes a boolean argument. If true, the root
        directory, as configured with the
        <varname>RootDirectory=</varname> option (see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for more information), is only applied to the process started
        with <varname>ExecStart=</varname>, and not to the various
        other <varname>ExecStartPre=</varname>,
        <varname>ExecStartPost=</varname>,
        <varname>ExecReload=</varname>, <varname>ExecStop=</varname>,
        and <varname>ExecStopPost=</varname> commands. If false, the
        setting is applied to all configured commands the same way.
        Defaults to false.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>NonBlocking=</varname></term>
        <listitem><para>Set the <constant>O_NONBLOCK</constant> flag for all file descriptors passed via socket-based
        activation. If true, all file descriptors >= 3 (i.e. all except stdin, stdout, stderr), excluding those passed
        in via the file descriptor storage logic (see <varname>FileDescriptorStoreMax=</varname> for details), will
        have the <constant>O_NONBLOCK</constant> flag set and hence are in non-blocking mode. This option is only
        useful in conjunction with a socket unit, as described in
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> and has no
        effect on file descriptors which were previously saved in the file-descriptor store for example.  Defaults to
        false.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>NotifyAccess=</varname></term>
        <listitem><para>Controls access to the service status notification socket, as accessible via the
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> call. Takes one
        of <option>none</option> (the default), <option>main</option>, <option>exec</option> or
        <option>all</option>. If <option>none</option>, no daemon status updates are accepted from the service
        processes, all status update messages are ignored. If <option>main</option>, only service updates sent from the
        main process of the service are accepted. If <option>exec</option>, only service updates sent from any of the
        main or control processes originating from one of the <varname>Exec*=</varname> commands are accepted. If
        <option>all</option>, all services updates from all members of the service's control group are accepted. This
        option should be set to open access to the notification socket when using <varname>Type=notify</varname> or
        <varname>WatchdogSec=</varname> (see above). If those options are used but <varname>NotifyAccess=</varname> is
        not configured, it will be implicitly set to <option>main</option>.</para>

        <para>Note that <function>sd_notify()</function> notifications may be attributed to units correctly only if
        either the sending process is still around at the time PID 1 processes the message, or if the sending process
        is explicitly runtime-tracked by the service manager. The latter is the case if the service manager originally
        forked off the process, i.e. on all processes that match <option>main</option> or
        <option>exec</option>. Conversely, if an auxiliary process of the unit sends an
        <function>sd_notify()</function> message and immediately exits, the service manager might not be able to
        properly attribute the message to the unit, and thus will ignore it, even if
        <varname>NotifyAccess=</varname><option>all</option> is set for it.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Sockets=</varname></term>
        <listitem><para>Specifies the name of the socket units this
        service shall inherit socket file descriptors from when the
        service is started. Normally, it should not be necessary to use
        this setting, as all socket file descriptors whose unit shares
        the same name as the service (subject to the different unit
        name suffix of course) are passed to the spawned
        process.</para>

        <para>Note that the same socket file descriptors may be passed
        to multiple processes simultaneously. Also note that a
        different service may be activated on incoming socket traffic
        than the one which is ultimately configured to inherit the
        socket file descriptors. Or, in other words: the
        <varname>Service=</varname> setting of
        <filename>.socket</filename> units does not have to match the
        inverse of the <varname>Sockets=</varname> setting of the
        <filename>.service</filename> it refers to.</para>

        <para>This option may appear more than once, in which case the list of socket units is merged. Note
        that once set, clearing the list of sockets again (for example, by assigning the empty string to this
        option) is not supported.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>FileDescriptorStoreMax=</varname></term>
        <listitem><para>Configure how many file descriptors may be stored in the service manager for the service using
        <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
        <literal>FDSTORE=1</literal> messages. This is useful for implementing services that can restart after an
        explicit request or a crash without losing state. Any open sockets and other file descriptors which should not
        be closed during the restart may be stored this way. Application state can either be serialized to a file in
        <filename>/run</filename>, or better, stored in a
        <citerefentry><refentrytitle>memfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry> memory file
        descriptor. Defaults to 0, i.e. no file descriptors may be stored in the service manager. All file descriptors
        passed to the service manager from a specific service are passed back to the service's main process on the next
        service restart. Any file descriptors passed to the service manager are automatically closed when
        <constant>POLLHUP</constant> or <constant>POLLERR</constant> is seen on them, or when the service is fully
        stopped and no job is queued or being executed for it. If this option is used, <varname>NotifyAccess=</varname>
        (see above) should be set to open access to the notification socket provided by systemd. If
        <varname>NotifyAccess=</varname> is not set, it will be implicitly set to
        <option>main</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>USBFunctionDescriptors=</varname></term>
        <listitem><para>Configure the location of a file containing
        <ulink
        url="https://www.kernel.org/doc/Documentation/usb/functionfs.txt">USB
        FunctionFS</ulink> descriptors, for implementation of USB
        gadget functions. This is used only in conjunction with a
        socket unit with <varname>ListenUSBFunction=</varname>
        configured. The contents of this file are written to the
        <filename>ep0</filename> file after it is
        opened.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>USBFunctionStrings=</varname></term>
        <listitem><para>Configure the location of a file containing
        USB FunctionFS strings.  Behavior is similar to
        <varname>USBFunctionDescriptors=</varname>
        above.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>OOMPolicy=</varname></term>

        <listitem><para>Configure the Out-Of-Memory (OOM) killer policy. On Linux, when memory becomes scarce
        the kernel might decide to kill a running process in order to free up memory and reduce memory
        pressure. This setting takes one of <constant>continue</constant>, <constant>stop</constant> or
        <constant>kill</constant>. If set to <constant>continue</constant> and a process of the service is
        killed by the kernel's OOM killer this is logged but the service continues running. If set to
        <constant>stop</constant> the event is logged but the service is terminated cleanly by the service
        manager. If set to <constant>kill</constant> and one of the service's processes is killed by the OOM
        killer the kernel is instructed to kill all remaining processes of the service, too. Defaults to the
        setting <varname>DefaultOOMPolicy=</varname> in
        <citerefentry><refentrytitle>system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> is
        set to, except for services where <varname>Delegate=</varname> is turned on, where it defaults to
        <constant>continue</constant>.</para>

        <para>Use the <varname>OOMScoreAdjust=</varname> setting to configure whether processes of the unit
        shall be considered preferred or less preferred candidates for process termination by the Linux OOM
        killer logic. See
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
        details.</para></listitem>
      </varlistentry>

    </variablelist>

    <para>Check
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    and
    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for more settings.</para>

  </refsect1>

  <refsect1>
    <title>Command lines</title>

    <para>This section describes command line parsing and
    variable and specifier substitutions for
    <varname>ExecStart=</varname>,
    <varname>ExecStartPre=</varname>,
    <varname>ExecStartPost=</varname>,
    <varname>ExecReload=</varname>,
    <varname>ExecStop=</varname>, and
    <varname>ExecStopPost=</varname> options.</para>

    <para>Multiple command lines may be concatenated in a single
    directive by separating them with semicolons (these semicolons
    must be passed as separate words). Lone semicolons may be escaped
    as <literal>\;</literal>.</para>

    <para>Each command line is split on whitespace, with the first item being the command to
    execute, and the subsequent items being the arguments. Double quotes ("…") and single quotes
    ('…') may be used to wrap a whole item (the opening quote may appear only at the beginning or
    after whitespace that is not quoted, and the closing quote must be followed by whitespace or the
    end of line), in which case everything until the next matching quote becomes part of the same
    argument. Quotes themselves are removed. C-style escapes are also supported. The table below
    contains the list of known escape patterns. Only escape patterns which match the syntax in the
    table are allowed; other patterns may be added in the future and unknown patterns will result in
    a warning. In particular, any backslashes should be doubled. Finally, a trailing backslash
    (<literal>\</literal>) may be used to merge lines.</para>

    <para>This syntax is inspired by shell syntax, but only the meta-characters and expansions
    described in the following paragraphs are understood, and the expansion of variables is
    different. Specifically, redirection using
    <literal>&lt;</literal>,
    <literal>&lt;&lt;</literal>,
    <literal>&gt;</literal>, and
    <literal>&gt;&gt;</literal>, pipes using
    <literal>|</literal>, running programs in the background using
    <literal>&amp;</literal>, and <emphasis>other elements of shell
    syntax are not supported</emphasis>.</para>

    <para>The command to execute may contain spaces, but control characters are not allowed.</para>

    <para>The command line accepts <literal>%</literal> specifiers as described in
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

    <para>Basic environment variable substitution is supported. Use
    <literal>${FOO}</literal> as part of a word, or as a word of its
    own, on the command line, in which case it will be replaced by the
    value of the environment variable including all whitespace it
    contains, resulting in a single argument. Use
    <literal>$FOO</literal> as a separate word on the command line, in
    which case it will be replaced by the value of the environment
    variable split at whitespace, resulting in zero or more arguments.
    For this type of expansion, quotes are respected when splitting
    into words, and afterwards removed.</para>

    <para>If the command is not a full (absolute) path, it will be resolved to a full path using a
    fixed search path determinted at compilation time. Searched directories include
    <filename>/usr/local/bin/</filename>, <filename>/usr/bin/</filename>, <filename>/bin/</filename>
    on systems using split <filename>/usr/bin/</filename> and <filename>/bin/</filename>
    directories, and their <filename>sbin/</filename> counterparts on systems using split
    <filename>bin/</filename> and <filename>sbin/</filename>. It is thus safe to use just the
    executable name in case of executables located in any of the "standard" directories, and an
    absolute path must be used in other cases. Using an absolute path is recommended to avoid
    ambiguity. Hint: this search path may be queried using
    <command>systemd-path search-binaries-default</command>.</para>

    <para>Example:</para>

    <programlisting>Environment="ONE=one" 'TWO=two two'
ExecStart=echo $ONE $TWO ${TWO}</programlisting>

    <para>This will execute <command>/bin/echo</command> with four
    arguments: <literal>one</literal>, <literal>two</literal>,
    <literal>two</literal>, and <literal>two two</literal>.</para>

    <para>Example:</para>
    <programlisting>Environment=ONE='one' "TWO='two two' too" THREE=
ExecStart=/bin/echo ${ONE} ${TWO} ${THREE}
ExecStart=/bin/echo $ONE $TWO $THREE</programlisting>
    <para>This results in <filename>/bin/echo</filename> being
    called twice, the first time with arguments
    <literal>'one'</literal>,
    <literal>'two two' too</literal>, <literal></literal>,
    and the second time with arguments
    <literal>one</literal>, <literal>two two</literal>,
    <literal>too</literal>.
    </para>

    <para>To pass a literal dollar sign, use <literal>$$</literal>.
    Variables whose value is not known at expansion time are treated
    as empty strings. Note that the first argument (i.e. the program
    to execute) may not be a variable.</para>

    <para>Variables to be used in this fashion may be defined through
    <varname>Environment=</varname> and
    <varname>EnvironmentFile=</varname>. In addition, variables listed
    in the section "Environment variables in spawned processes" in
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    which are considered "static configuration", may be used (this
    includes e.g. <varname>$USER</varname>, but not
    <varname>$TERM</varname>).</para>

    <para>Note that shell command lines are not directly supported. If
    shell command lines are to be used, they need to be passed
    explicitly to a shell implementation of some kind. Example:</para>
    <programlisting>ExecStart=sh -c 'dmesg | tac'</programlisting>

    <para>Example:</para>

    <programlisting>ExecStart=echo one ; echo "two two"</programlisting>

    <para>This will execute <command>echo</command> two times,
    each time with one argument: <literal>one</literal> and
    <literal>two two</literal>, respectively. Because two commands are
    specified, <varname>Type=oneshot</varname> must be used.</para>

    <para>Example:</para>

    <programlisting>ExecStart=echo / &gt;/dev/null &amp; \; \
ls</programlisting>

    <para>This will execute <command>echo</command>
    with five arguments: <literal>/</literal>,
    <literal>&gt;/dev/null</literal>,
    <literal>&amp;</literal>, <literal>;</literal>, and
    <literal>ls</literal>.</para>

    <table>
      <title>C escapes supported in command lines and environment variables</title>
      <tgroup cols='2'>
        <colspec colname='escape' />
        <colspec colname='meaning' />
        <thead>
          <row>
            <entry>Literal</entry>
            <entry>Actual value</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry><literal>\a</literal></entry>
            <entry>bell</entry>
          </row>
          <row>
            <entry><literal>\b</literal></entry>
            <entry>backspace</entry>
          </row>
          <row>
            <entry><literal>\f</literal></entry>
            <entry>form feed</entry>
          </row>
          <row>
            <entry><literal>\n</literal></entry>
            <entry>newline</entry>
          </row>
          <row>
            <entry><literal>\r</literal></entry>
            <entry>carriage return</entry>
          </row>
          <row>
            <entry><literal>\t</literal></entry>
            <entry>tab</entry>
          </row>
          <row>
            <entry><literal>\v</literal></entry>
            <entry>vertical tab</entry>
          </row>
          <row>
            <entry><literal>\\</literal></entry>
            <entry>backslash</entry>
          </row>
          <row>
            <entry><literal>\"</literal></entry>
            <entry>double quotation mark</entry>
          </row>
          <row>
            <entry><literal>\'</literal></entry>
            <entry>single quotation mark</entry>
          </row>
          <row>
            <entry><literal>\s</literal></entry>
            <entry>space</entry>
          </row>
          <row>
            <entry><literal>\x<replaceable>xx</replaceable></literal></entry>
            <entry>character number <replaceable>xx</replaceable> in hexadecimal encoding</entry>
          </row>
          <row>
            <entry><literal>\<replaceable>nnn</replaceable></literal></entry>
            <entry>character number <replaceable>nnn</replaceable> in octal encoding</entry>
          </row>
        </tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Simple service</title>

      <para>The following unit file creates a service that will
      execute <filename>/usr/sbin/foo-daemon</filename>. Since no
      <varname>Type=</varname> is specified, the default
      <varname>Type=</varname><option>simple</option> will be assumed.
      systemd will assume the unit to be started immediately after the
      program has begun executing.</para>

      <programlisting>[Unit]
Description=Foo

[Service]
ExecStart=/usr/sbin/foo-daemon

[Install]
WantedBy=multi-user.target</programlisting>

      <para>Note that systemd assumes here that the process started by
      systemd will continue running until the service terminates. If
      the program daemonizes itself (i.e. forks), please use
      <varname>Type=</varname><option>forking</option> instead.</para>

      <para>Since no <varname>ExecStop=</varname> was specified,
      systemd will send SIGTERM to all processes started from this
      service, and after a timeout also SIGKILL. This behavior can be
      modified, see
      <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      for details.</para>

      <para>Note that this unit type does not include any type of
      notification when a service has completed initialization. For
      this, you should use other unit types, such as
      <varname>Type=</varname><option>notify</option> if the service
      understands systemd's notification protocol,
      <varname>Type=</varname><option>forking</option> if the service
      can background itself or
      <varname>Type=</varname><option>dbus</option> if the unit
      acquires a DBus name once initialization is complete. See
      below.</para>
    </example>

    <example>
      <title>Oneshot service</title>

      <para>Sometimes, units should just execute an action without
      keeping active processes, such as a filesystem check or a
      cleanup action on boot. For this,
      <varname>Type=</varname><option>oneshot</option> exists. Units
      of this type will wait until the process specified terminates
      and then fall back to being inactive. The following unit will
      perform a cleanup action:</para>

      <programlisting>[Unit]
Description=Cleanup old Foo data

[Service]
Type=oneshot
ExecStart=/usr/sbin/foo-cleanup

[Install]
WantedBy=multi-user.target</programlisting>

      <para>Note that systemd will consider the unit to be in the
      state "starting" until the program has terminated, so ordered
      dependencies will wait for the program to finish before starting
      themselves. The unit will revert to the "inactive" state after
      the execution is done, never reaching the "active" state. That
      means another request to start the unit will perform the action
      again.</para>

      <para><varname>Type=</varname><option>oneshot</option> are the
      only service units that may have more than one
      <varname>ExecStart=</varname> specified. They will be executed
      in order until either they are all successful or one of them
      fails.</para>
    </example>

    <example>
      <title>Stoppable oneshot service</title>

      <para>Similarly to the oneshot services, there are sometimes
      units that need to execute a program to set up something and
      then execute another to shut it down, but no process remains
      active while they are considered "started". Network
      configuration can sometimes fall into this category. Another use
      case is if a oneshot service shall not be executed each time
      when they are pulled in as a dependency, but only the first
      time.</para>

      <para>For this, systemd knows the setting
      <varname>RemainAfterExit=</varname><option>yes</option>, which
      causes systemd to consider the unit to be active if the start
      action exited successfully. This directive can be used with all
      types, but is most useful with
      <varname>Type=</varname><option>oneshot</option> and
      <varname>Type=</varname><option>simple</option>. With
      <varname>Type=</varname><option>oneshot</option>, systemd waits
      until the start action has completed before it considers the
      unit to be active, so dependencies start only after the start
      action has succeeded. With
      <varname>Type=</varname><option>simple</option>, dependencies
      will start immediately after the start action has been
      dispatched. The following unit provides an example for a simple
      static firewall.</para>

      <programlisting>[Unit]
Description=Simple firewall

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/sbin/simple-firewall-start
ExecStop=/usr/local/sbin/simple-firewall-stop

[Install]
WantedBy=multi-user.target</programlisting>

      <para>Since the unit is considered to be running after the start
      action has exited, invoking <command>systemctl start</command>
      on that unit again will cause no action to be taken.</para>
    </example>

    <example>
      <title>Traditional forking services</title>

      <para>Many traditional daemons/services background (i.e. fork,
      daemonize) themselves when starting. Set
      <varname>Type=</varname><option>forking</option> in the
      service's unit file to support this mode of operation. systemd
      will consider the service to be in the process of initialization
      while the original program is still running. Once it exits
      successfully and at least a process remains (and
      <varname>RemainAfterExit=</varname><option>no</option>), the
      service is considered started.</para>

      <para>Often, a traditional daemon only consists of one process.
      Therefore, if only one process is left after the original
      process terminates, systemd will consider that process the main
      process of the service. In that case, the
      <varname>$MAINPID</varname> variable will be available in
      <varname>ExecReload=</varname>, <varname>ExecStop=</varname>,
      etc.</para>

      <para>In case more than one process remains, systemd will be
      unable to determine the main process, so it will not assume
      there is one. In that case, <varname>$MAINPID</varname> will not
      expand to anything. However, if the process decides to write a
      traditional PID file, systemd will be able to read the main PID
      from there. Please set <varname>PIDFile=</varname> accordingly.
      Note that the daemon should write that file before finishing
      with its initialization. Otherwise, systemd might try to read the
      file before it exists.</para>

      <para>The following example shows a simple daemon that forks and
      just starts one process in the background:</para>

      <programlisting>[Unit]
Description=Some simple daemon

[Service]
Type=forking
ExecStart=/usr/sbin/my-simple-daemon -d

[Install]
WantedBy=multi-user.target</programlisting>

      <para>Please see
      <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      for details on how you can influence the way systemd terminates
      the service.</para>
    </example>

    <example>
      <title>DBus services</title>

      <para>For services that acquire a name on the DBus system bus,
      use <varname>Type=</varname><option>dbus</option> and set
      <varname>BusName=</varname> accordingly. The service should not
      fork (daemonize). systemd will consider the service to be
      initialized once the name has been acquired on the system bus.
      The following example shows a typical DBus service:</para>

      <programlisting>[Unit]
Description=Simple DBus service

[Service]
Type=dbus
BusName=org.example.simple-dbus-service
ExecStart=/usr/sbin/simple-dbus-service

[Install]
WantedBy=multi-user.target</programlisting>

      <para>For <emphasis>bus-activatable</emphasis> services, do not
      include a <literal>[Install]</literal> section in the systemd
      service file, but use the <varname>SystemdService=</varname>
      option in the corresponding DBus service file, for example
      (<filename>/usr/share/dbus-1/system-services/org.example.simple-dbus-service.service</filename>):</para>

      <programlisting>[D-BUS Service]
Name=org.example.simple-dbus-service
Exec=/usr/sbin/simple-dbus-service
User=root
SystemdService=simple-dbus-service.service</programlisting>

      <para>Please see
      <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      for details on how you can influence the way systemd terminates
      the service.</para>
    </example>

    <example>
      <title>Services that notify systemd about their initialization</title>

      <para><varname>Type=</varname><option>simple</option> services
      are really easy to write, but have the major disadvantage of
      systemd not being able to tell when initialization of the given
      service is complete. For this reason, systemd supports a simple
      notification protocol that allows daemons to make systemd aware
      that they are done initializing. Use
      <varname>Type=</varname><option>notify</option> for this. A
      typical service file for such a daemon would look like
      this:</para>

      <programlisting>[Unit]
Description=Simple notifying service

[Service]
Type=notify
ExecStart=/usr/sbin/simple-notifying-service

[Install]
WantedBy=multi-user.target</programlisting>

      <para>Note that the daemon has to support systemd's notification
      protocol, else systemd will think the service has not started yet
      and kill it after a timeout. For an example of how to update
      daemons to support this protocol transparently, take a look at
      <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
      systemd will consider the unit to be in the 'starting' state
      until a readiness notification has arrived.</para>

      <para>Please see
      <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      for details on how you can influence the way systemd terminates
      the service.</para>
    </example>
  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para>
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-run</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>