SYSTEMD.TIMER(5) systemd.timer SYSTEMD.TIMER(5)
NAME
systemd.timer - Timer unit configuration
SYNOPSIS
timer.timer
DESCRIPTION
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=yes 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. Target units, which by default do
not deactivate on their own, can be activated repeatedly by timers by
setting StopWhenUnneeded=yes on them. This will cause a target unit to
be stopped immediately after its activation, if it is not a dependency
of another running unit.
AUTOMATIC DEPENDENCIES
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
Timer unit files may include [Unit] and [Install] sections, which are
described in systemd.unit(5).
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:
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.
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 systemd-time-wait-sync.service to ensure the clock
is adjusted to a network time source before the timer event is set
up. Timer units with at least one OnCalendar= expression are
automatically ordered after time-sync.target, which
systemd-time-wait-sync.service is ordered before.
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 continuously sleeping the timer will only result in a
single service activation. If WakeSystem= (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).
Added in version 197.
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.
Added in version 209.
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,
unless modified with FixedRandomDelay=, see below. The delay is
added on top of the next determined elapsing time or the service
manager's startup time, whichever is later.
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.
Added in version 229.
FixedRandomDelay=
Takes a boolean argument. When enabled, the randomized delay
specified by RandomizedDelaySec= is chosen deterministically, and
remains stable between all firings of the same timer, even if the
manager is restarted. The delay is derived from the machine ID, the
manager's user identifier, and the timer unit's name. This
effectively creates a unique fixed offset for each timer, reducing
the jitter in firings of an individual timer while still avoiding
firing at the same time as other similarly configured timers.
This setting has an effect only if RandomizedDelaySec= is not 0.
Defaults to false.
Added in version 247.
DeferReactivation=
Takes a boolean argument. When enabled, the timer schedules the next
elapse based on the trigger unit entering inactivity, instead of the
last trigger time. This is most apparent in the case where the
service unit takes longer to run than the timer interval. With this
setting enabled, the timer will schedule the next elapse based on
when the service finishes running, and so it will have to wait until
the next realtime elapse time to trigger. Otherwise, the default
behavior is for the timer unit to immediately trigger again once the
service finishes running. This happens because the timer schedules
the next elapse based on the previous trigger time, and since the
interval is shorter than the service runtime, that elapse will be in
the past, causing it to immediately trigger once done.
This setting has an effect only if a realtime timer has been
specified with OnCalendar=. Defaults to false.
Added in version 257.
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.
Added in version 242.
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.
Added in version 212.
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.
Added in version 212.
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.
Added in version 229.
Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more
settings.
SEE ALSO
Environment variables with details on the trigger will be set for
triggered units. See the "Environment Variables Set or Propagated by the
Service Manager" section in systemd.exec(5) for more details.
systemd(1), systemctl(1), systemd.unit(5), systemd.service(5),
systemd.time(7), systemd.directives(7), systemd-system.conf(5), prctl(2)
systemd 257.9 SYSTEMD.TIMER(5)
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