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<refentry id="systemd.timer">

<refentryinfo>

<title>systemd.timer</title>

<productname>systemd</productname>

<authorgroup>

<author>

<contrib>Developer</contrib>

<firstname>Lennart</firstname>

<surname>Poettering</surname>

<email>lennart@poettering.net</email>

</author>

</authorgroup>

</refentryinfo>

<refmeta>

<refentrytitle>systemd.timer</refentrytitle>

<manvolnum>5</manvolnum>

</refmeta>

<refnamediv>

<refname>systemd.timer</refname>

<refpurpose>Timer unit configuration</refpurpose>

</refnamediv>

<refsynopsisdiv>

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

</refsynopsisdiv>

<refsect1>

<title>Description</title>

<para>A unit configuration file whose name ends in

<literal>.timer</literal> encodes information about a timer

controlled and supervised by systemd, for timer-based

activation.</para>

<para>This man page lists the configuration options specific to

this unit type. See

<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>

for the common options of all unit configuration files. The common

configuration items are configured in the generic [Unit] and

[Install] sections. The timer specific configuration options are

configured in the [Timer] section.</para>

<para>For each timer file, a matching unit file must exist,

describing the unit to activate when the timer elapses. By

default, a service by the same name as the timer (except for the

suffix) is activated. Example: a timer file

<filename>foo.timer</filename> activates a matching service

<filename>foo.service</filename>. The unit to activate may be

controlled by <varname>Unit=</varname> (see below).</para>

</refsect1>

<refsect1>

<title>Automatic Dependencies</title>

<para>Timer units automatically gain a <varname>Before=</varname>

dependency on the service they are supposed to activate.</para>

<para>Unless <varname>DefaultDependencies=</varname> is set to

<option>false</option>, all timer units will implicitly 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. Timer units with at

least one <varname>OnCalendar=</varname> directive will have an

additional <varname>After=</varname> dependency on

<filename>timer-sync.target</filename> to avoid being started

before the system clock has been correctly set. Only timer units

involved with early boot or late system shutdown should disable

the <varname>DefaultDependencies=</varname> option.</para>

</refsect1>

<refsect1>

<title>Options</title>

<para>Timer files must include a [Timer] section, which carries

information about the timer it defines. The options specific to

the [Timer] section of timer units are the following:</para>

<variablelist class='unit-directives'>

<varlistentry>

<term><varname>OnActiveSec=</varname></term>

<term><varname>OnBootSec=</varname></term>

<term><varname>OnStartupSec=</varname></term>

<term><varname>OnUnitActiveSec=</varname></term>

<term><varname>OnUnitInactiveSec=</varname></term>

<listitem><para>Defines monotonic timers relative to different

starting points: <varname>OnActiveSec=</varname> defines a

timer relative to the moment the timer itself is activated.

<varname>OnBootSec=</varname> defines a timer relative to when

the machine was booted up. <varname>OnStartupSec=</varname>

defines a timer relative to when systemd was first started.

<varname>OnUnitActiveSec=</varname> defines a timer relative

to when the unit the timer is activating was last activated.

<varname>OnUnitInactiveSec=</varname> defines a timer relative

to when the unit the timer is activating was last

deactivated.</para>

<para>Multiple directives may be combined of the same and of

different types. For example, by combining

<varname>OnBootSec=</varname> and

<varname>OnUnitActiveSec=</varname>, it is possible to define

a timer that elapses in regular intervals and activates a

specific service each time.</para>

<para>The arguments to the directives are time spans

configured in seconds. Example: "OnBootSec=50" means 50s after

boot-up. The argument may also include time units. Example:

"OnBootSec=5h 30min" means 5 hours and 30 minutes after

boot-up. For details about the syntax of time spans, see

<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

<para>If a timer configured with <varname>OnBootSec=</varname>

or <varname>OnStartupSec=</varname> is already in the past

when the timer unit is activated, it will immediately elapse

and the configured unit is started. This is not the case for

timers defined in the other directives.</para>

<para>These are monotonic timers, independent of wall-clock

time and timezones. If the computer is temporarily suspended,

the monotonic clock stops too.</para>

<para>If the empty string is assigned to any of these options,

the list of timers is reset, and all prior assignments will

have no effect.</para>

<para>Note that timers do not necessarily expire at the

precise time configured with these settings, as they are

subject to the <varname>AccuracySec=</varname> setting

below.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Defines realtime (i.e. wallclock) timers with

calendar event expressions. See

<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>

for more information on the syntax of calendar event

expressions. Otherwise, the semantics are similar to

<varname>OnActiveSec=</varname> and related settings.</para>

<para>Note that timers do not necessarily expire at the

precise time configured with this setting, as it is subject to

the <varname>AccuracySec=</varname> setting

below.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Specify the accuracy the timer shall elapse

with. Defaults to 1min. The timer is scheduled to elapse

within a time window starting with the time specified in

<varname>OnCalendar=</varname>,

<varname>OnActiveSec=</varname>,

<varname>OnBootSec=</varname>,

<varname>OnStartupSec=</varname>,

<varname>OnUnitActiveSec=</varname> or

<varname>OnUnitInactiveSec=</varname> and ending the time

configured with <varname>AccuracySec=</varname> later. Within

this time window, the expiry time will be placed at a

host-specific, randomized, but stable position that is

synchronized between all local timer units. This is done in

order to optimize power consumption to suppress unnecessary

CPU wake-ups. To get best accuracy, set this option to

1us. Note that the timer is still subject to the timer slack

configured via

<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s

<varname>TimerSlackNSec=</varname> setting. See

<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>

for details. To optimize power consumption, make sure to set

this value as high as possible and as low as

necessary.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Delay the timer by a randomly selected, evenly

distributed amount of time between 0 and the specified time

value. Defaults to 0, indicating that no randomized delay

shall be applied. Each timer unit will determine this delay

randomly each time it is started, and the delay will simply be

added on top of the next determined elapsing time. This is

useful to stretch dispatching of similarly configured timer

events over a certain amount time, to avoid that they all fire

at the same time, possibly resulting in resource

congestion. Note the relation to

<varname>AccuracySec=</varname> above: the latter allows the

service manager to coalesce timer events within a specified

time range in order to minimize wakeups, the former does the

opposite: it stretches timer events over a time range, to make

it unlikely that they fire simultaneously. If

<varname>RandomizedDelaySec=</varname> and

<varname>AccuracySec=</varname> are used in conjunction, first

the randomized delay is added, and then the result is

possibly further shifted to coalesce it with other timer

events happening on the system. As mentioned above

<varname>AccuracySec=</varname> defaults to 1min and

<varname>RandomizedDelaySec=</varname> to 0, thus encouraging

coalescing of timer events. In order to optimally stretch

timer events over a certain range of time, make sure to set

<varname>RandomizedDelaySec=</varname> to a higher value, and

<varname>AccuracySec=1us</varname>.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>The unit to activate when this timer elapses.

The argument is a unit name, whose suffix is not

<literal>.timer</literal>. If not specified, this value

defaults to a service that has the same name as the timer

unit, except for the suffix. (See above.) It is recommended

that the unit name that is activated and the unit name of the

timer unit are named identically, except for the

suffix.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Takes a boolean argument. If true, the time

when the service unit was last triggered is stored on disk.

When the timer is activated, the service unit is triggered

immediately if it would have been triggered at least once

during the time when the timer was inactive. This is useful to

catch up on missed runs of the service when the machine was

off. Note that this setting only has an effect on timers

configured with <varname>OnCalendar=</varname>.

</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Takes a boolean argument. If true, an elapsing

timer will cause the system to resume from suspend, should it

be suspended and if the system supports this. Note that this

option will only make sure the system resumes on the

appropriate times, it will not take care of suspending it

again after any work that is to be done is finished. Defaults

to <varname>false</varname>.</para></listitem>

</varlistentry>

<varlistentry>

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

<listitem><para>Takes a boolean argument. If true, an elapsed

timer will stay loaded, and its state remains queriable. If

false, an elapsed timer unit that cannot elapse anymore is

unloaded. Turning this off is particularly useful for

transient timer units that shall disappear after they first

elapse. Note that this setting has an effect on repeatedly

starting a timer unit that only elapses once: if

<varname>RemainAfterElapse=</varname> is on, it will not be

started again, and is guaranteed to elapse only once. However,

if <varname>RemainAfterLeapse=</varname> is off, it might be

started again if it is already elapsed, and thus be triggered

multiple times. Defaults to

<varname>yes</varname>.</para></listitem>

</varlistentry>

</variablelist>

</refsect1>

<refsect1>

<title>See Also</title>

<para>

<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,

<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,

<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>

</para>

</refsect1>

</refentry>