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 acquire a
           pair of additional After= dependencies on time-set.target and
           time-sync.target, in order to avoid being started before the
           system clock has been correctly set. See systemd.special(7)
           for details on these two targets.

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, unless modified
           with FixedRandomDelay=, see below.

           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.

           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, while this
           setting does the opposite: it stretches timer events over an
           interval, 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 1 minute and
           RandomizedDelaySec= to 0, thus encouraging coalescing of
           timer events. In order to optimally stretch timer events over
           a certain range of time, set AccuracySec=1us and
           RandomizedDelaySec= to some higher value.

       FixedRandomDelay=
           Takes a boolean argument. When enabled, the randomized offset
           specified by RandomizedDelaySec= 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.

           This setting has no effect if RandomizedDelaySec= is set to
           0. Defaults to false.

       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. Such triggering is nonetheless
           subject to the delay imposed by RandomizedDelaySec=. 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, a timer will stay loaded,
           and its state remains queryable even after it elapsed and the
           associated unit (as configured with Unit=, 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
           RemainAfterElapse= is on, starting the timer a second time
           has no effect. However, if RemainAfterElapse= 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
           true.

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 2021-04-01.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-04-01.)  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 248                                             SYSTEMD.TIMER(5)

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