man/systemd.timer.xml - systemd-stable - Rivoreo Source Code Repositories

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 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

 <refentry id="systemd.timer">
   <refentryinfo>
     <title>systemd.timer</title>
     <productname>systemd</productname>
   </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>Automatic Dependencies</title>

     <refsect2>
       <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>
     </refsect2>

     <refsect2>
       <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>
     </refsect2>
   </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:</para>

         <table>
           <title>Settings and their starting points</title>

           <tgroup cols='2'>
             <thead>
               <row>
                 <entry>Setting</entry>
                 <entry>Meaning</entry>
               </row>
             </thead>
             <tbody>
               <row>
                 <entry><varname>OnActiveSec=</varname></entry>
                 <entry>Defines a timer relative to the moment the timer unit itself is activated.</entry>
               </row>
               <row>
                 <entry><varname>OnBootSec=</varname></entry>
                 <entry>Defines a timer relative to when the machine was booted up. In containers, for the system manager instance, this is mapped to <varname>OnStartupSec=</varname>, making both equivalent.</entry>
               </row>
               <row>
                 <entry><varname>OnStartupSec=</varname></entry>
                 <entry>Defines a timer relative to when the service manager was first started. For system timer units this is very similar to <varname>OnBootSec=</varname> as the system service manager is generally started very early at boot. It's primarily useful when configured in units running in the per-user service manager, as the user service manager is generally started on first login only, not already during boot.</entry>
               </row>
               <row>
                 <entry><varname>OnUnitActiveSec=</varname></entry>
                 <entry>Defines a timer relative to when the unit the timer unit is activating was last activated.</entry>
               </row>
               <row>
                 <entry><varname>OnUnitInactiveSec=</varname></entry>
                 <entry>Defines a timer relative to when the unit the timer unit is activating was last deactivated.</entry>
               </row>
             </tbody>
           </tgroup>
         </table>

         <para>Multiple directives may be combined of the same and of different types, in which case the timer
         unit will trigger whenever any of the specified timer expressions elapse. 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. Moreover, both
         monotonic time expressions and <varname>OnCalendar=</varname> calendar expressions may be combined in
         the same timer unit.</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 generally pauses, too. Note that if
         <varname>WakeSystem=</varname> is used, a different monotonic clock is selected that continues to
         advance while the system is suspended and thus can be used as the trigger to resume the
         system.</para>

         <para>If the empty string is assigned to any of these options, the list of timers is reset (both
         monotonic timers and <varname>OnCalendar=</varname> timers, see below), 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, in which case the timer unit will trigger whenever any of the
         specified expressions elapse. Moreover calendar timers and monotonic timers (see above) may be
         combined within the same timer unit.</para>

         <para>If the empty string is assigned to any of these options, the list of timers is reset (both
         <varname>OnCalendar=</varname> timers and monotonic timers, see above), and all prior assignments
         will have no effect.</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>

         <para>Note that this setting is primarily a power saving option that allows coalescing CPU
         wake-ups. It should not be confused with <varname>RandomizedDelaySec=</varname> (see below) which
         adds a random value to the time the timer shall elapse next and whose purpose is the opposite: to
         stretch elapsing of timer events over a longer period to reduce workload spikes. For further details
         and explanations and how both settings play together, see below.</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 before each iteration, and the delay will simply
         be added on top of the next determined elapsing time, unless modified with
         <varname>FixedRandomDelay=</varname>, see below.</para>

         <para>This setting is useful to stretch dispatching of similarly configured timer events over a
         certain time interval, to prevent them from firing all at the same time, possibly resulting in
         resource congestion.</para>

         <para>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, while
         this setting does the opposite: it stretches timer events over an interval, 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 1 minute 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, set
         <varname>AccuracySec=1us</varname> and <varname>RandomizedDelaySec=</varname> to some higher value.
         </para></listitem>
       </varlistentry>

       <varlistentry>
         <term><varname>FixedRandomDelay=</varname></term>

         <listitem><para>Takes a boolean argument. When enabled, the randomized offset specified by
         <varname>RandomizedDelaySec=</varname> is reused for all firings of the same timer. For a given timer
         unit, the offset depends on the machine ID, user identifier and timer name, which means that it is
         stable between restarts of the manager. This effectively creates a fixed offset for an individual
         timer, reducing the jitter in firings of this timer, while still avoiding firing at the same time as
         other similarly configured timers.</para>

         <para>This setting has no effect if <varname>RandomizedDelaySec=</varname> is set to 0. Defaults to
         <option>false</option>.</para></listitem>
       </varlistentry>

       <varlistentry>
         <term><varname>OnClockChange=</varname></term>
         <term><varname>OnTimezoneChange=</varname></term>

         <listitem><para>These options take boolean arguments. When true, the service unit will be triggered
         when the system clock (<constant>CLOCK_REALTIME</constant>) jumps relative to the monotonic clock
         (<constant>CLOCK_MONOTONIC</constant>), or when the local system timezone is modified. These options
         can be used alone or in combination with other timer expressions (see above) within the same timer
         unit. These options default to <option>false</option>.</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. Such triggering
         is nonetheless subject to the delay imposed by <varname>RandomizedDelaySec=</varname>.
         This is useful to catch up on missed runs of the service when the system was powered down. Note that
         this setting only has an effect on timers configured with <varname>OnCalendar=</varname>. Defaults to
         <option>false</option>.</para>

         <para>Use <command>systemctl clean --what=state …</command> on the timer unit to remove the timestamp
         file maintained by this option from disk. In particular, use this command before uninstalling a timer
         unit. See
         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
         details.</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
         <option>false</option>.</para>

         <para>Note that this functionality requires privileges and is thus generally only available in the
         system service manager.</para>

         <para>Note that behaviour of monotonic clock timers (as configured with
         <varname>OnActiveSec=</varname>, <varname>OnBootSec=</varname>, <varname>OnStartupSec=</varname>,
         <varname>OnUnitActiveSec=</varname>, <varname>OnUnitInactiveSec=</varname>, see above) is altered
         depending on this option. If false, a monotonic clock is used that is paused during system suspend
         (<constant>CLOCK_MONOTONIC</constant>), if true a different monotonic clock is used that continues
         advancing during system suspend (<constant>CLOCK_BOOTTIME</constant>), see
         <citerefentry><refentrytitle>clock_getres</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
         details.</para></listitem>
       </varlistentry>

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

         <listitem><para>Takes a boolean argument. If true, a timer will stay loaded, and its state remains
         queryable even after it elapsed and the associated unit (as configured with <varname>Unit=</varname>,
         see above) deactivated again. If false, an elapsed timer unit that cannot elapse anymore is unloaded
         once its associated unit deactivated again. Turning this off is particularly useful for transient
         timer units. Note that this setting has an effect when repeatedly starting a timer unit: if
         <varname>RemainAfterElapse=</varname> is on, starting the timer a second time has no effect. However,
         if <varname>RemainAfterElapse=</varname> is off and the timer unit was already unloaded, it can be
         started again, and thus the service can be triggered multiple times. Defaults to
         <option>true</option>.</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>
	<?xml version='1.0'?>
	<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
	<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

	<refentry id="systemd.timer">
	<refentryinfo>
	<title>systemd.timer</title>
	<productname>systemd</productname>
	</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>Automatic Dependencies</title>

	<refsect2>
	<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>
	</refsect2>

	<refsect2>
	<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>
	</refsect2>
	</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:</para>

	<table>
	<title>Settings and their starting points</title>

	<tgroup cols='2'>
	<thead>
	<row>
	<entry>Setting</entry>
	<entry>Meaning</entry>
	</row>
	</thead>
	<tbody>
	<row>
	<entry><varname>OnActiveSec=</varname></entry>
	<entry>Defines a timer relative to the moment the timer unit itself is activated.</entry>
	</row>
	<row>
	<entry><varname>OnBootSec=</varname></entry>
	<entry>Defines a timer relative to when the machine was booted up. In containers, for the system manager instance, this is mapped to <varname>OnStartupSec=</varname>, making both equivalent.</entry>
	</row>
	<row>
	<entry><varname>OnStartupSec=</varname></entry>
	<entry>Defines a timer relative to when the service manager was first started. For system timer units this is very similar to <varname>OnBootSec=</varname> as the system service manager is generally started very early at boot. It's primarily useful when configured in units running in the per-user service manager, as the user service manager is generally started on first login only, not already during boot.</entry>
	</row>
	<row>
	<entry><varname>OnUnitActiveSec=</varname></entry>
	<entry>Defines a timer relative to when the unit the timer unit is activating was last activated.</entry>
	</row>
	<row>
	<entry><varname>OnUnitInactiveSec=</varname></entry>
	<entry>Defines a timer relative to when the unit the timer unit is activating was last deactivated.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<para>Multiple directives may be combined of the same and of different types, in which case the timer
	unit will trigger whenever any of the specified timer expressions elapse. 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. Moreover, both
	monotonic time expressions and <varname>OnCalendar=</varname> calendar expressions may be combined in
	the same timer unit.</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 generally pauses, too. Note that if
	<varname>WakeSystem=</varname> is used, a different monotonic clock is selected that continues to
	advance while the system is suspended and thus can be used as the trigger to resume the
	system.</para>

	<para>If the empty string is assigned to any of these options, the list of timers is reset (both
	monotonic timers and <varname>OnCalendar=</varname> timers, see below), 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, in which case the timer unit will trigger whenever any of the
	specified expressions elapse. Moreover calendar timers and monotonic timers (see above) may be
	combined within the same timer unit.</para>

	<para>If the empty string is assigned to any of these options, the list of timers is reset (both
	<varname>OnCalendar=</varname> timers and monotonic timers, see above), and all prior assignments
	will have no effect.</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>

	<para>Note that this setting is primarily a power saving option that allows coalescing CPU
	wake-ups. It should not be confused with <varname>RandomizedDelaySec=</varname> (see below) which
	adds a random value to the time the timer shall elapse next and whose purpose is the opposite: to
	stretch elapsing of timer events over a longer period to reduce workload spikes. For further details
	and explanations and how both settings play together, see below.</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 before each iteration, and the delay will simply
	be added on top of the next determined elapsing time, unless modified with
	<varname>FixedRandomDelay=</varname>, see below.</para>

	<para>This setting is useful to stretch dispatching of similarly configured timer events over a
	certain time interval, to prevent them from firing all at the same time, possibly resulting in
	resource congestion.</para>

	<para>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, while
	this setting does the opposite: it stretches timer events over an interval, 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 1 minute 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, set
	<varname>AccuracySec=1us</varname> and <varname>RandomizedDelaySec=</varname> to some higher value.
	</para></listitem>
	</varlistentry>

	<varlistentry>
	<term><varname>FixedRandomDelay=</varname></term>

	<listitem><para>Takes a boolean argument. When enabled, the randomized offset specified by
	<varname>RandomizedDelaySec=</varname> is reused for all firings of the same timer. For a given timer
	unit, the offset depends on the machine ID, user identifier and timer name, which means that it is
	stable between restarts of the manager. This effectively creates a fixed offset for an individual
	timer, reducing the jitter in firings of this timer, while still avoiding firing at the same time as
	other similarly configured timers.</para>

	<para>This setting has no effect if <varname>RandomizedDelaySec=</varname> is set to 0. Defaults to
	<option>false</option>.</para></listitem>
	</varlistentry>

	<varlistentry>
	<term><varname>OnClockChange=</varname></term>
	<term><varname>OnTimezoneChange=</varname></term>

	<listitem><para>These options take boolean arguments. When true, the service unit will be triggered
	when the system clock (<constant>CLOCK_REALTIME</constant>) jumps relative to the monotonic clock
	(<constant>CLOCK_MONOTONIC</constant>), or when the local system timezone is modified. These options
	can be used alone or in combination with other timer expressions (see above) within the same timer
	unit. These options default to <option>false</option>.</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. Such triggering
	is nonetheless subject to the delay imposed by <varname>RandomizedDelaySec=</varname>.
	This is useful to catch up on missed runs of the service when the system was powered down. Note that
	this setting only has an effect on timers configured with <varname>OnCalendar=</varname>. Defaults to
	<option>false</option>.</para>

	<para>Use <command>systemctl clean --what=state …</command> on the timer unit to remove the timestamp
	file maintained by this option from disk. In particular, use this command before uninstalling a timer
	unit. See
	<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
	details.</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
	<option>false</option>.</para>

	<para>Note that this functionality requires privileges and is thus generally only available in the
	system service manager.</para>

	<para>Note that behaviour of monotonic clock timers (as configured with
	<varname>OnActiveSec=</varname>, <varname>OnBootSec=</varname>, <varname>OnStartupSec=</varname>,
	<varname>OnUnitActiveSec=</varname>, <varname>OnUnitInactiveSec=</varname>, see above) is altered
	depending on this option. If false, a monotonic clock is used that is paused during system suspend
	(<constant>CLOCK_MONOTONIC</constant>), if true a different monotonic clock is used that continues
	advancing during system suspend (<constant>CLOCK_BOOTTIME</constant>), see
	<citerefentry><refentrytitle>clock_getres</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
	details.</para></listitem>
	</varlistentry>

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

	<listitem><para>Takes a boolean argument. If true, a timer will stay loaded, and its state remains
	queryable even after it elapsed and the associated unit (as configured with <varname>Unit=</varname>,
	see above) deactivated again. If false, an elapsed timer unit that cannot elapse anymore is unloaded
	once its associated unit deactivated again. Turning this off is particularly useful for transient
	timer units. Note that this setting has an effect when repeatedly starting a timer unit: if
	<varname>RemainAfterElapse=</varname> is on, starting the timer a second time has no effect. However,
	if <varname>RemainAfterElapse=</varname> is off and the timer unit was already unloaded, it can be
	started again, and thus the service can be triggered multiple times. Defaults to
	<option>true</option>.</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>