377 lines
20 KiB
XML
377 lines
20 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd.timer">
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<refentryinfo>
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<title>systemd.timer</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.timer</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.timer</refname>
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<refpurpose>Timer unit configuration</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename><replaceable>timer</replaceable>.timer</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>A unit configuration file whose name ends in
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<literal>.timer</literal> encodes information about a timer
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controlled and supervised by systemd, for timer-based
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activation.</para>
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<para>This man page lists the configuration options specific to
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this unit type. See
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for the common options of all unit configuration files. The common
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configuration items are configured in the generic [Unit] and
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[Install] sections. The timer specific configuration options are
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configured in the [Timer] section.</para>
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<para>For each timer file, a matching unit file must exist,
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describing the unit to activate when the timer elapses. By
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default, a service by the same name as the timer (except for the
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suffix) is activated. Example: a timer file
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<filename>foo.timer</filename> activates a matching service
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<filename>foo.service</filename>. The unit to activate may be
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controlled by <varname>Unit=</varname> (see below).</para>
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<para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
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but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
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with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process
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exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
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then stay around forever.</para>
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</refsect1>
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<refsect1>
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<title>Automatic Dependencies</title>
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<refsect2>
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<title>Implicit Dependencies</title>
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<para>The following dependencies are implicitly added:</para>
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<itemizedlist>
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<listitem><para>Timer units automatically gain a <varname>Before=</varname>
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dependency on the service they are supposed to activate.</para></listitem>
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</itemizedlist>
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</refsect2>
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<refsect2>
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<title>Default Dependencies</title>
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<para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para>
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<itemizedlist>
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<listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and
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<varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname>
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on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on
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<filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer
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units involved with early boot or late system shutdown should disable the
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<varname>DefaultDependencies=</varname> option.</para></listitem>
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<listitem><para>Timer units with at least one <varname>OnCalendar=</varname> directive acquire a pair
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of additional <varname>After=</varname> dependencies on <filename>time-set.target</filename> and
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<filename>time-sync.target</filename>, in order to avoid being started before the system clock has
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been correctly set. See
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for details on these two targets.</para></listitem>
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</itemizedlist>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<para>Timer files must include a [Timer] section, which carries
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information about the timer it defines. The options specific to
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the [Timer] section of timer units are the following:</para>
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<variablelist class='unit-directives'>
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<varlistentry>
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<term><varname>OnActiveSec=</varname></term>
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<term><varname>OnBootSec=</varname></term>
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<term><varname>OnStartupSec=</varname></term>
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<term><varname>OnUnitActiveSec=</varname></term>
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<term><varname>OnUnitInactiveSec=</varname></term>
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<listitem><para>Defines monotonic timers relative to different
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starting points:</para>
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<table>
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<title>Settings and their starting points</title>
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<tgroup cols='2'>
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<thead>
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<row>
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<entry>Setting</entry>
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<entry>Meaning</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><varname>OnActiveSec=</varname></entry>
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<entry>Defines a timer relative to the moment the timer unit itself is activated.</entry>
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</row>
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<row>
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<entry><varname>OnBootSec=</varname></entry>
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<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>
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</row>
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<row>
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<entry><varname>OnStartupSec=</varname></entry>
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<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>
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</row>
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<row>
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<entry><varname>OnUnitActiveSec=</varname></entry>
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<entry>Defines a timer relative to when the unit the timer unit is activating was last activated.</entry>
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</row>
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<row>
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<entry><varname>OnUnitInactiveSec=</varname></entry>
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<entry>Defines a timer relative to when the unit the timer unit is activating was last deactivated.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>Multiple directives may be combined of the same and of different types, in which case the timer
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unit will trigger whenever any of the specified timer expressions elapse. For example, by combining
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<varname>OnBootSec=</varname> and <varname>OnUnitActiveSec=</varname>, it is possible to define a
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timer that elapses in regular intervals and activates a specific service each time. Moreover, both
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monotonic time expressions and <varname>OnCalendar=</varname> calendar expressions may be combined in
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the same timer unit.</para>
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<para>The arguments to the directives are time spans
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configured in seconds. Example: "OnBootSec=50" means 50s after
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boot-up. The argument may also include time units. Example:
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"OnBootSec=5h 30min" means 5 hours and 30 minutes after
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boot-up. For details about the syntax of time spans, see
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<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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<para>If a timer configured with <varname>OnBootSec=</varname>
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or <varname>OnStartupSec=</varname> is already in the past
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when the timer unit is activated, it will immediately elapse
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and the configured unit is started. This is not the case for
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timers defined in the other directives.</para>
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<para>These are monotonic timers, independent of wall-clock time and timezones. If the computer is
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temporarily suspended, the monotonic clock generally pauses, too. Note that if
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<varname>WakeSystem=</varname> is used, a different monotonic clock is selected that continues to
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advance while the system is suspended and thus can be used as the trigger to resume the
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system.</para>
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<para>If the empty string is assigned to any of these options, the list of timers is reset (both
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monotonic timers and <varname>OnCalendar=</varname> timers, see below), and all prior assignments
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will have no effect.</para>
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<para>Note that timers do not necessarily expire at the
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precise time configured with these settings, as they are
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subject to the <varname>AccuracySec=</varname> setting
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below.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>OnCalendar=</varname></term>
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<listitem><para>Defines realtime (i.e. wallclock) timers with
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calendar event expressions. See
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<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for more information on the syntax of calendar event
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expressions. Otherwise, the semantics are similar to
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<varname>OnActiveSec=</varname> and related settings.</para>
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<para>Note that timers do not necessarily expire at the
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precise time configured with this setting, as it is subject to
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the <varname>AccuracySec=</varname> setting
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below.</para>
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<para>May be specified more than once, in which case the timer unit will trigger whenever any of the
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specified expressions elapse. Moreover calendar timers and monotonic timers (see above) may be
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combined within the same timer unit.</para>
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<para>If the empty string is assigned to any of these options, the list of timers is reset (both
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<varname>OnCalendar=</varname> timers and monotonic timers, see above), and all prior assignments
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will have no effect.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>AccuracySec=</varname></term>
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<listitem><para>Specify the accuracy the timer shall elapse
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with. Defaults to 1min. The timer is scheduled to elapse
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within a time window starting with the time specified in
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<varname>OnCalendar=</varname>,
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<varname>OnActiveSec=</varname>,
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<varname>OnBootSec=</varname>,
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<varname>OnStartupSec=</varname>,
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<varname>OnUnitActiveSec=</varname> or
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<varname>OnUnitInactiveSec=</varname> and ending the time
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configured with <varname>AccuracySec=</varname> later. Within
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this time window, the expiry time will be placed at a
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host-specific, randomized, but stable position that is
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synchronized between all local timer units. This is done in
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order to optimize power consumption to suppress unnecessary
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CPU wake-ups. To get best accuracy, set this option to
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1us. Note that the timer is still subject to the timer slack
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configured via
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<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
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<varname>TimerSlackNSec=</varname> setting. See
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<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for details. To optimize power consumption, make sure to set
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this value as high as possible and as low as
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necessary.</para>
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<para>Note that this setting is primarily a power saving option that allows coalescing CPU
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wake-ups. It should not be confused with <varname>RandomizedDelaySec=</varname> (see below) which
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adds a random value to the time the timer shall elapse next and whose purpose is the opposite: to
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stretch elapsing of timer events over a longer period to reduce workload spikes. For further details
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and explanations and how both settings play together, see below.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>RandomizedDelaySec=</varname></term>
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<listitem><para>Delay the timer by a randomly selected, evenly distributed amount of time between 0
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and the specified time value. Defaults to 0, indicating that no randomized delay shall be applied.
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Each timer unit will determine this delay randomly before each iteration, and the delay will simply
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be added on top of the next determined elapsing time, unless modified with
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<varname>FixedRandomDelay=</varname>, see below.</para>
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<para>This setting is useful to stretch dispatching of similarly configured timer events over a
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certain time interval, to prevent them from firing all at the same time, possibly resulting in
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resource congestion.</para>
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<para>Note the relation to <varname>AccuracySec=</varname> above: the latter allows the service
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manager to coalesce timer events within a specified time range in order to minimize wakeups, while
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this setting does the opposite: it stretches timer events over an interval, to make it unlikely that
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they fire simultaneously. If <varname>RandomizedDelaySec=</varname> and
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<varname>AccuracySec=</varname> are used in conjunction, first the randomized delay is added, and
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then the result is possibly further shifted to coalesce it with other timer events happening on the
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system. As mentioned above <varname>AccuracySec=</varname> defaults to 1 minute and
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<varname>RandomizedDelaySec=</varname> to 0, thus encouraging coalescing of timer events. In order to
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optimally stretch timer events over a certain range of time, set
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<varname>AccuracySec=1us</varname> and <varname>RandomizedDelaySec=</varname> to some higher value.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>FixedRandomDelay=</varname></term>
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<listitem><para>Takes a boolean argument. When enabled, the randomized offset specified by
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<varname>RandomizedDelaySec=</varname> is reused for all firings of the same timer. For a given timer
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unit, the offset depends on the machine ID, user identifier and timer name, which means that it is
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stable between restarts of the manager. This effectively creates a fixed offset for an individual
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timer, reducing the jitter in firings of this timer, while still avoiding firing at the same time as
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other similarly configured timers.</para>
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<para>This setting has no effect if <varname>RandomizedDelaySec=</varname> is set to 0. Defaults to
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<option>false</option>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>OnClockChange=</varname></term>
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<term><varname>OnTimezoneChange=</varname></term>
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<listitem><para>These options take boolean arguments. When true, the service unit will be triggered
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when the system clock (<constant>CLOCK_REALTIME</constant>) jumps relative to the monotonic clock
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(<constant>CLOCK_MONOTONIC</constant>), or when the local system timezone is modified. These options
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can be used alone or in combination with other timer expressions (see above) within the same timer
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unit. These options default to <option>false</option>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Unit=</varname></term>
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<listitem><para>The unit to activate when this timer elapses.
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The argument is a unit name, whose suffix is not
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<literal>.timer</literal>. If not specified, this value
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defaults to a service that has the same name as the timer
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unit, except for the suffix. (See above.) It is recommended
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that the unit name that is activated and the unit name of the
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timer unit are named identically, except for the
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suffix.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Persistent=</varname></term>
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<listitem><para>Takes a boolean argument. If true, the time when the service unit was last triggered
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is stored on disk. When the timer is activated, the service unit is triggered immediately if it
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would have been triggered at least once during the time when the timer was inactive. Such triggering
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is nonetheless subject to the delay imposed by <varname>RandomizedDelaySec=</varname>.
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This is useful to catch up on missed runs of the service when the system was powered down. Note that
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this setting only has an effect on timers configured with <varname>OnCalendar=</varname>. Defaults to
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<option>false</option>.</para>
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<para>Use <command>systemctl clean --what=state …</command> on the timer unit to remove the timestamp
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file maintained by this option from disk. In particular, use this command before uninstalling a timer
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unit. See
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
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details.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>WakeSystem=</varname></term>
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<listitem><para>Takes a boolean argument. If true, an elapsing timer will cause the system to resume
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from suspend, should it be suspended and if the system supports this. Note that this option will only
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make sure the system resumes on the appropriate times, it will not take care of suspending it again
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after any work that is to be done is finished. Defaults to
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<option>false</option>.</para>
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<para>Note that this functionality requires privileges and is thus generally only available in the
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system service manager.</para>
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<para>Note that behaviour of monotonic clock timers (as configured with
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<varname>OnActiveSec=</varname>, <varname>OnBootSec=</varname>, <varname>OnStartupSec=</varname>,
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<varname>OnUnitActiveSec=</varname>, <varname>OnUnitInactiveSec=</varname>, see above) is altered
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depending on this option. If false, a monotonic clock is used that is paused during system suspend
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(<constant>CLOCK_MONOTONIC</constant>), if true a different monotonic clock is used that continues
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advancing during system suspend (<constant>CLOCK_BOOTTIME</constant>), see
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<citerefentry><refentrytitle>clock_getres</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
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details.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>RemainAfterElapse=</varname></term>
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<listitem><para>Takes a boolean argument. If true, a timer will stay loaded, and its state remains
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queryable even after it elapsed and the associated unit (as configured with <varname>Unit=</varname>,
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see above) deactivated again. If false, an elapsed timer unit that cannot elapse anymore is unloaded
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once its associated unit deactivated again. Turning this off is particularly useful for transient
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timer units. Note that this setting has an effect when repeatedly starting a timer unit: if
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<varname>RemainAfterElapse=</varname> is on, starting the timer a second time has no effect. However,
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if <varname>RemainAfterElapse=</varname> is off and the timer unit was already unloaded, it can be
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started again, and thus the service can be triggered multiple times. Defaults to
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<option>true</option>.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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