systemd.timer(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | AUTOMATIC DEPENDENCIES | OPTIONS | SEE ALSO | COLOPHON

SYSTEMD.TIMER(5)                systemd.timer               SYSTEMD.TIMER(5)

NAME         top

       systemd.timer - Timer unit configuration

SYNOPSIS         top

       timer.timer

DESCRIPTION         top

       A unit configuration file whose name ends in ".timer" encodes
       information about a timer controlled and supervised by systemd, for
       timer-based activation.

       This man page lists the configuration options specific to this unit
       type. See systemd.unit(5) 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.

       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 foo.timer activates a matching service foo.service. The
       unit to activate may be controlled by Unit= (see below).

       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 RemainAfterExit= 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.

AUTOMATIC DEPENDENCIES         top

   Implicit Dependencies
       The following dependencies are implicitly added:

       ·   Timer units automatically gain a Before= dependency on the
           service they are supposed to activate.

   Default Dependencies
       The following dependencies are added unless DefaultDependencies=no is
       set:

       ·   Timer units will automatically have dependencies of type
           Requires= and After= on sysinit.target, a dependency of type
           Before= on timers.target, as well as Conflicts= and Before= on
           shutdown.target 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 DefaultDependencies=
           option.

       ·   Timer units with at least one OnCalendar= directive will have an
           additional After= dependency on time-sync.target to avoid being
           started before the system clock has been correctly set.

OPTIONS         top

       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:

       OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec=,
       OnUnitInactiveSec=
           Defines monotonic timers relative to different starting points:

           Table 1. Settings and their starting points
           ┌───────────────────┬───────────────────────────┐
           │Setting            Meaning                   │
           ├───────────────────┼───────────────────────────┤
           │OnActiveSec=       │ Defines a timer relative  │
           │                   │ to the moment the timer   │
           │                   │ unit itself is activated. │
           ├───────────────────┼───────────────────────────┤
           │OnBootSec=         │ Defines a timer relative  │
           │                   │ to when the machine was   │
           │                   │ booted up. In containers, │
           │                   │ for the system manager    │
           │                   │ instance, this is mapped  │
           │                   │ to OnStartupSec=, making  │
           │                   │ both equivalent.          │
           ├───────────────────┼───────────────────────────┤
           │OnStartupSec=      │ Defines a timer relative  │
           │                   │ to when the service       │
           │                   │ manager was first         │
           │                   │ started. For system timer │
           │                   │ units this is very        │
           │                   │ similar to OnBootSec= 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.                     │
           ├───────────────────┼───────────────────────────┤
           │OnUnitActiveSec=   │ Defines a timer relative  │
           │                   │ to when the unit the      │
           │                   │ timer unit is activating  │
           │                   │ was last activated.       │
           ├───────────────────┼───────────────────────────┤
           │OnUnitInactiveSec= │ Defines a timer relative  │
           │                   │ to when the unit the      │
           │                   │ timer unit is activating  │
           │                   │ was last deactivated.     │
           └───────────────────┴───────────────────────────┘
           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
           OnBootSec= and OnUnitActiveSec=, 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
           OnCalendar= calendar expressions may be combined in the same
           timer unit.

           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 systemd.time(7).

           If a timer configured with OnBootSec= or OnStartupSec= 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.

           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 WakeSystem=
           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.

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

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

       OnCalendar=
           Defines realtime (i.e. wallclock) timers with calendar event
           expressions. See systemd.time(7) for more information on the
           syntax of calendar event expressions. Otherwise, the semantics
           are similar to OnActiveSec= and related settings.

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

           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.

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

       AccuracySec=
           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 OnCalendar=, OnActiveSec=,
           OnBootSec=, OnStartupSec=, OnUnitActiveSec= or OnUnitInactiveSec=
           and ending the time configured with AccuracySec= 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
           systemd-system.conf(5)'s TimerSlackNSec= setting. See prctl(2)
           for details. To optimize power consumption, make sure to set this
           value as high as possible and as low as necessary.

           Note that this setting is primarily a power saving option that
           allows coalescing CPU wake-ups. It should not be confused with
           RandomizedDelaySec= (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.

       RandomizedDelaySec=
           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. 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 AccuracySec= 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
           RandomizedDelaySec= and AccuracySec= 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 AccuracySec= defaults
           to 1min and RandomizedDelaySec= 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 RandomizedDelaySec= to
           a higher value, and AccuracySec=1us.

       OnClockChange=, OnTimezoneChange=
           These options take boolean arguments. When true, the service unit
           will be triggered when the system clock (CLOCK_REALTIME) jumps
           relative to the monotonic clock (CLOCK_MONOTONIC), 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 false.

       Unit=
           The unit to activate when this timer elapses. The argument is a
           unit name, whose suffix is not ".timer". 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.

       Persistent=
           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 system was powered down. Note that this setting
           only has an effect on timers configured with OnCalendar=.
           Defaults to false.

           Use systemctl clean --what=state ...  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 systemctl(1) for details.

       WakeSystem=
           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 false.

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

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

       RemainAfterElapse=
           Takes a boolean argument. If true, an elapsed timer will stay
           loaded, and its state remains queryable. 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 RemainAfterElapse= is on, it will not be started again,
           and is guaranteed to elapse only once. However, if
           RemainAfterElapse= is off, it might be started again if it is
           already elapsed, and thus be triggered multiple times. Defaults
           to yes.

SEE ALSO         top

       systemd(1), systemctl(1), systemd.unit(5), systemd.service(5),
       systemd.time(7), systemd.directives(7), systemd-system.conf(5),
       prctl(2)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2020-08-13.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-08-11.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 246                                                 SYSTEMD.TIMER(5)

Pages that refer to this page: init(1)systemctl(1)systemd(1)systemd-analyze(1)systemd-run(1)system.conf.d(5)systemd-system.conf(5)systemd.unit(5)systemd-user.conf(5)user.conf.d(5)30-systemd-environment-d-generator(7)daemon(7)systemd.directives(7)systemd.index(7)systemd.special(7)systemd.syntax(7)systemd.time(7)