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+<?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" xmlns:xi="http://www.w3.org/2001/XInclude">
+ <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 acquire a pair
+ of additional <varname>After=</varname> dependencies on <filename>time-set.target</filename> and
+ <filename>time-sync.target</filename>, in order to avoid being started before the system clock has
+ been correctly set. See
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ for details on these two targets.</para></listitem>
+ </itemizedlist>
+ </refsect2>
+ </refsect1>
+
+ <refsect1>
+ <title>Options</title>
+
+ <para>Timer unit files may include [Unit] and [Install] sections, which are described in
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ </para>
+
+ <para>Timer unit 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>
+
+ <para>Note that calendar timers might be triggered at unexpected times if the system's realtime clock
+ is not set correctly. Specifically, on systems that lack a battery-buffered Realtime Clock (RTC) it
+ might be wise to enable <filename>systemd-time-wait-sync.service</filename> to ensure the clock is
+ adjusted to a network time source <emphasis>before</emphasis> the timer event is set up. Timer units
+ with at least one <varname>OnCalendar=</varname> expression are automatically ordered after
+ <filename>time-sync.target</filename>, which <filename>systemd-time-wait-sync.service</filename> is
+ ordered before.</para>
+
+ <para>When a system is temporarily put to sleep (i.e. system suspend or hibernation) the realtime
+ clock does not pause. When a calendar timer elapses while the system is sleeping it will not be acted
+ on immediately, but once the system is later resumed it will catch up and process all timers that
+ triggered while the system was sleeping. Note that if a calendar timer elapsed more than once while
+ the system was continously sleeping the timer will only result in a single service activation. If
+ <varname>WakeSystem=</varname> (see below) is enabled a calendar time event elapsing while the system
+ is suspended will cause the system to wake up (under the condition the system's hardware supports
+ time-triggered wake-up functionality).</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>
+
+ <xi:include href="systemd.service.xml" xpointer="shared-unit-options" />
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>Environment variables with details on the trigger will be set for triggered units. See the
+ <literal>Environment Variables Set on Triggered Units</literal> section in
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ for more details.</para>
+ <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>