pid1: remove unnecessary counter

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

<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2010 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

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

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

  <refmeta>
    <refentrytitle>systemd.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
    <filename>.service</filename> 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="http://www.freedesktop.org/wiki/Software/systemd/Incompatibilities">Incompatibilities
    with SysV</ulink> document.</para>
  </refsect1>

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

    <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>

    <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>

    <para>Unless <varname>DefaultDependencies=</varname> in the <literal>[Unit]</literal> is set to
    <option>false</option>, service units will implicitly 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>

    <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>

    <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>
  </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>
    and
    <citerefentry><refentrytitle>systemd.kill</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>forking</option>,
        <option>oneshot</option>,
        <option>dbus</option>,
        <option>notify</option> or
        <option>idle</option>.</para>

        <para>If set to <option>simple</option> (the default if
        neither <varname>Type=</varname> nor
        <varname>BusName=</varname>, but <varname>ExecStart=</varname>
        are specified), 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 daemon is started up (e.g.
        sockets set up by systemd, via socket activation), as systemd
        will immediately proceed starting follow-up units.</para>

        <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 daemon process. This is the behavior of
        traditional UNIX daemons. If this setting is used, it is
        recommended to also use the <varname>PIDFile=</varname>
        option, so that systemd can identify the main process of the
        daemon. systemd will proceed with starting follow-up units as
        soon as the parent process exits.</para>

        <para>Behavior of <option>oneshot</option> is similar to
        <option>simple</option>; however, it is expected that the
        process has to exit before systemd starts follow-up units.
        <varname>RemainAfterExit=</varname> is particularly useful for
        this type of service. This is the implied default if neither
        <varname>Type=</varname> or <varname>ExecStart=</varname> are
        specified.</para>

        <para>Behavior of <option>dbus</option> is similar to
        <option>simple</option>; however, it is expected that the
        daemon 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>

        <para>Behavior of <option>notify</option> is similar to
        <option>simple</option>; however, it is expected that the
        daemon 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>

        <para>Behavior of <option>idle</option> is very similar to <option>simple</option>; however, actual execution
        of the service binary 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 time-out, after which the service binary is invoked anyway.</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 an absolute file name pointing to the
        PID file of this daemon. Use of this option is recommended for
        services where <varname>Type=</varname> is set to
        <option>forking</option>. systemd will read the PID of the
        main process of the daemon after start-up of the service.
        systemd will not write to the file configured here, although
        it will remove the file after the service has shut down if it
        still exists.
        </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> set.</para>

        <para>For each of the specified commands, the first argument must be an absolute path to an
        executable. Optionally, if this file name is prefixed with <literal>@</literal>, the second token will be
        passed as <literal>argv[0]</literal> to the executed process, followed by the further arguments specified.  If
        the absolute filename 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 ignored and considered success.  If the
        absolute path is prefixed with <literal>+</literal> then it is executed with full
        privileges. <literal>@</literal>, <literal>-</literal>, and <literal>+</literal> may be used together and they
        can appear in any order.</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 service has started 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 process exits successfully for
        <varname>Type=oneshot</varname>, the initial process exits 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, all processes remaining for a service 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 using
        <constant>SIGKILL</constant> immediately after the command
        exited, this would 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.</para>

        <para>It is recommended to use this setting for commands that communicate with the service requesting clean
        termination. When the commands specified with this option are executed it should be assumed that the service is
        still fully up and is able to react correctly to all commands. 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></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>TimeoutStopSec=</varname></term>
        <listitem><para>Configures the time to wait for stop. If a
        service is asked to stop, but does not terminate in the
        specified time, it will be terminated forcibly via
        <constant>SIGTERM</constant>, and after another timeout of
        equal duration with <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></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>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></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>. 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).
        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.</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>Note that if a process has a signal handler installed
        and exits by calling
        <citerefentry><refentrytitle>_exit</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        in response to a signal, the information about the signal is
        lost. Programs should instead perform cleanup and kill
        themselves with the same signal instead. See
        <ulink url="http://www.cons.org/cracauer/sigint.html">Proper
        handling of SIGINT/SIGQUIT — How to be a proper
        program</ulink>.</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></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>PermissionsStartOnly=</varname></term>
        <listitem><para>Takes a boolean argument. If true, the
        permission-related execution options, as configured with
        <varname>User=</varname> and similar options (see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for more information), are 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>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, and stderr) 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>.
        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 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></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. If the empty string is
        assigned to this option, the list of sockets is reset, and all
        prior uses of this setting will have no
        effect.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>FailureAction=</varname></term>
        <listitem><para>Configure the action to take when the service enters a failed state. Takes the same values as
        the unit setting <varname>StartLimitAction=</varname> and executes the same actions (see
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Defaults to
        <option>none</option>. </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 service restart schemes where the state is
        serialized to <filename>/run</filename> and the file
        descriptors passed to the service manager, to allow restarts
        without losing state. 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 POLLHUP or
        POLLERR is seen on them, or when the service is fully stopped
        and no job is queued or being executed for it.</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>

    </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, in which case everything until the next matching quote
    becomes part of the same argument. C-style escapes are also
    supported. The table below contains the list of allowed escape
    patterns. Only patterns which match the syntax in the table are
    allowed; others will result in an error, and must be escaped by
    doubling the backslash. Quotes themselves are removed after
    parsing and escape sequences substituted. In addition, a trailing
    backslash (<literal>\</literal>) may be used to merge lines.
    </para>

    <para>This syntax is intended to be very similar to shell syntax,
    but only the meta-characters and expansions described in the
    following paragraphs are understood. 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 must be an absolute path name. It 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>.
    Note that the first argument of the command line (i.e. the program
    to execute) may not include specifiers.</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>Example:</para>

    <programlisting>Environment="ONE=one" 'TWO=two two'
ExecStart=/bin/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>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=/bin/sh -c 'dmesg | tac'</programlisting>

    <para>Example:</para>

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

    <para>This will execute <command>/bin/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=/bin/echo / &gt;/dev/null &amp; \; \
/bin/ls</programlisting>

    <para>This will execute <command>/bin/echo</command>
    with five arguments: <literal>/</literal>,
    <literal>&gt;/dev/null</literal>,
    <literal>&amp;</literal>, <literal>;</literal>, and
    <literal>/bin/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.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>
      </para>
  </refsect1>

</refentry>