man: explicitly distinguish "implicit dependencies" and "default dependencies"

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

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<refentry id="systemd.timer">
  <refentryinfo>
    <title>systemd.timer</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.timer</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.timer</refname>
    <refpurpose>Timer unit configuration</refpurpose>
  </refnamediv>

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

  <refsect1>
    <title>Description</title>

    <para>A unit configuration file whose name ends in
    <literal>.timer</literal> encodes information about a timer
    controlled and supervised by systemd, for timer-based
    activation.</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 [Unit] and
    [Install] sections. The timer specific configuration options are
    configured in the [Timer] section.</para>

    <para>For each timer file, a matching unit file must exist,
    describing the unit to activate when the timer elapses. By
    default, a service by the same name as the timer (except for the
    suffix) is activated. Example: a timer file
    <filename>foo.timer</filename> activates a matching service
    <filename>foo.service</filename>. The unit to activate may be
    controlled by <varname>Unit=</varname> (see below).</para>

    <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
    but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
    with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process
    exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
    then stay around forever.</para>
  </refsect1>

  <refsect1>
    <title>Implicit Dependencies</title>

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

    <itemizedlist>
      <listitem><para>Timer units automatically gain a <varname>Before=</varname>
      dependency on the service they are supposed to activate.</para></listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Default Dependencies</title>

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

    <itemizedlist>
      <listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and
      <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname>
      on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on
      <filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer
      units involved with early boot or late system shutdown should disable the
      <varname>DefaultDependencies=</varname> option.</para></listitem>

      <listitem><para>Timer units
      with at least one <varname>OnCalendar=</varname> directive will have an additional <varname>After=</varname>
      dependency on <filename>time-sync.target</filename> to avoid being started before the system clock has been
      correctly set.</para></listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>Timer files must include a [Timer] section, which carries
    information about the timer it defines. The options specific to
    the [Timer] section of timer units are the following:</para>

    <variablelist class='unit-directives'>
      <varlistentry>
        <term><varname>OnActiveSec=</varname></term>
        <term><varname>OnBootSec=</varname></term>
        <term><varname>OnStartupSec=</varname></term>
        <term><varname>OnUnitActiveSec=</varname></term>
        <term><varname>OnUnitInactiveSec=</varname></term>

        <listitem><para>Defines monotonic timers relative to different
        starting points: <varname>OnActiveSec=</varname> defines a
        timer relative to the moment the timer itself is activated.
        <varname>OnBootSec=</varname> defines a timer relative to when
        the machine was booted up. <varname>OnStartupSec=</varname>
        defines a timer relative to when systemd was first started.
        <varname>OnUnitActiveSec=</varname> defines a timer relative
        to when the unit the timer is activating was last activated.
        <varname>OnUnitInactiveSec=</varname> defines a timer relative
        to when the unit the timer is activating was last
        deactivated.</para>

        <para>Multiple directives may be combined of the same and of
        different types. For example, by combining
        <varname>OnBootSec=</varname> and
        <varname>OnUnitActiveSec=</varname>, it is possible to define
        a timer that elapses in regular intervals and activates a
        specific service each time.</para>

        <para>The arguments to the directives are time spans
        configured in seconds. Example: "OnBootSec=50" means 50s after
        boot-up. The argument may also include time units. Example:
        "OnBootSec=5h 30min" means 5 hours and 30 minutes after
        boot-up. For details about the syntax of time spans, see
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

        <para>If a timer configured with <varname>OnBootSec=</varname>
        or <varname>OnStartupSec=</varname> is already in the past
        when the timer unit is activated, it will immediately elapse
        and the configured unit is started. This is not the case for
        timers defined in the other directives.</para>

        <para>These are monotonic timers, independent of wall-clock
        time and timezones. If the computer is temporarily suspended,
        the monotonic clock stops too.</para>

        <para>If the empty string is assigned to any of these options,
        the list of timers is reset, and all prior assignments will
        have no effect.</para>

        <para>Note that timers do not necessarily expire at the
        precise time configured with these settings, as they are
        subject to the <varname>AccuracySec=</varname> setting
        below.</para></listitem>

      </varlistentry>

      <varlistentry>
        <term><varname>OnCalendar=</varname></term>

        <listitem><para>Defines realtime (i.e. wallclock) timers with
        calendar event expressions. See
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for more information on the syntax of calendar event
        expressions. Otherwise, the semantics are similar to
        <varname>OnActiveSec=</varname> and related settings.</para>

        <para>Note that timers do not necessarily expire at the
        precise time configured with this setting, as it is subject to
        the <varname>AccuracySec=</varname> setting
        below.</para>

       <para>May be specified more than once.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>AccuracySec=</varname></term>

        <listitem><para>Specify the accuracy the timer shall elapse
        with. Defaults to 1min. The timer is scheduled to elapse
        within a time window starting with the time specified in
        <varname>OnCalendar=</varname>,
        <varname>OnActiveSec=</varname>,
        <varname>OnBootSec=</varname>,
        <varname>OnStartupSec=</varname>,
        <varname>OnUnitActiveSec=</varname> or
        <varname>OnUnitInactiveSec=</varname> and ending the time
        configured with <varname>AccuracySec=</varname> later. Within
        this time window, the expiry time will be placed at a
        host-specific, randomized, but stable position that is
        synchronized between all local timer units. This is done in
        order to optimize power consumption to suppress unnecessary
        CPU wake-ups. To get best accuracy, set this option to
        1us. Note that the timer is still subject to the timer slack
        configured via
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
        <varname>TimerSlackNSec=</varname> setting. See
        <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        for details. To optimize power consumption, make sure to set
        this value as high as possible and as low as
        necessary.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RandomizedDelaySec=</varname></term>

        <listitem><para>Delay the timer by a randomly selected, evenly
        distributed amount of time between 0 and the specified time
        value. Defaults to 0, indicating that no randomized delay
        shall be applied. Each timer unit will determine this delay
        randomly each time it is started, and the delay will simply be
        added on top of the next determined elapsing time. This is
        useful to stretch dispatching of similarly configured timer
        events over a certain amount time, to avoid that they all fire
        at the same time, possibly resulting in resource
        congestion. Note the relation to
        <varname>AccuracySec=</varname> above: the latter allows the
        service manager to coalesce timer events within a specified
        time range in order to minimize wakeups, the former does the
        opposite: it stretches timer events over a time range, to make
        it unlikely that they fire simultaneously. If
        <varname>RandomizedDelaySec=</varname> and
        <varname>AccuracySec=</varname> are used in conjunction, first
        the randomized delay is added, and then the result is
        possibly further shifted to coalesce it with other timer
        events happening on the system. As mentioned above
        <varname>AccuracySec=</varname> defaults to 1min and
        <varname>RandomizedDelaySec=</varname> to 0, thus encouraging
        coalescing of timer events. In order to optimally stretch
        timer events over a certain range of time, make sure to set
        <varname>RandomizedDelaySec=</varname> to a higher value, and
        <varname>AccuracySec=1us</varname>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Unit=</varname></term>

        <listitem><para>The unit to activate when this timer elapses.
        The argument is a unit name, whose suffix is not
        <literal>.timer</literal>. If not specified, this value
        defaults to a service that has the same name as the timer
        unit, except for the suffix. (See above.) It is recommended
        that the unit name that is activated and the unit name of the
        timer unit are named identically, except for the
        suffix.</para></listitem>
      </varlistentry>


      <varlistentry>
        <term><varname>Persistent=</varname></term>

        <listitem><para>Takes a boolean argument. If true, the time
        when the service unit was last triggered is stored on disk.
        When the timer is activated, the service unit is triggered
        immediately if it would have been triggered at least once
        during the time when the timer was inactive. This is useful to
        catch up on missed runs of the service when the machine was
        off. Note that this setting only has an effect on timers
        configured with <varname>OnCalendar=</varname>. Defaults
        to <varname>false</varname>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>WakeSystem=</varname></term>

        <listitem><para>Takes a boolean argument. If true, an elapsing
        timer will cause the system to resume from suspend, should it
        be suspended and if the system supports this. Note that this
        option will only make sure the system resumes on the
        appropriate times, it will not take care of suspending it
        again after any work that is to be done is finished. Defaults
        to <varname>false</varname>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>RemainAfterElapse=</varname></term>

        <listitem><para>Takes a boolean argument. If true, an elapsed
        timer will stay loaded, and its state remains queriable. If
        false, an elapsed timer unit that cannot elapse anymore is
        unloaded. Turning this off is particularly useful for
        transient timer units that shall disappear after they first
        elapse. Note that this setting has an effect on repeatedly
        starting a timer unit that only elapses once: if
        <varname>RemainAfterElapse=</varname> is on, it will not be
        started again, and is guaranteed to elapse only once. However,
        if <varname>RemainAfterElapse=</varname> is off, it might be
        started again if it is already elapsed, and thus be triggered
        multiple times. Defaults to
        <varname>yes</varname>.</para></listitem>
      </varlistentry>
    </variablelist>
  </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.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>