| SYSTEMD.TIMER(5) | systemd.timer | SYSTEMD.TIMER(5) |
.SH "NAME" systemd.timer - Timer unit configuration
.SH "SYNOPSIS"
.PP timer.timer
.SH "DESCRIPTION"
.PP A unit configuration file whose name ends in .timer encodes information
about a timer controlled and supervised by systemd, for timer-based
activation.
.PP 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.
.PP 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).
.PP 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.
.SH "AUTOMATIC DEPENDENCIES"
.SS "Implicit Dependencies"
.PP The following dependencies are implicitly added:
.SS "Default Dependencies"
.PP The following dependencies are added unless DefaultDependencies=no
is set:
.SH "OPTIONS"
.PP Timer unit files may include [Unit] and [Install] sections, which are
described in systemd.unit(5).
.PP 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:
.PP OnActiveSec=, OnBootSec=, OnStartupSec=,
OnUnitActiveSec=, OnUnitInactiveSec=
Defines monotonic timers relative to different starting points:
.sp
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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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 on the local
system.
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.
.RE
.PP 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.
.RE
.PP RandomizedOffsetSec=
Offsets the timer by a stable, randomly-selected, and evenly distributed
amount of time between 0 and the specified time value. Defaults to 0,
indicating that no such offset shall be applied. The offset is chosen
deterministically, and is derived the same way as FixedRandomDelay=,
see above. The offset is added on top of the next determined elapsing time.
This setting only has an effect on timers configured with
OnCalendar=, and it can be combined with
RandomizedDelaySec=.
Much like RandomizedDelaySec=, this setting is for distributing timer
events to prevent them from firing all at once. However, this setting is
most useful to prevent resource congestion on a remote service, from a fleet
of similarly-configured clients. Unlike RandomizedDelaySec=, this
setting applies its offset with no regard to manager startup time. This
maintains the periodicity of configured OnCalendar= events across
manager restarts.
For example, let's say you're running a backup service and have a fleet of
laptops that wish to make backups weekly. To distribute load on the backup
service, each laptop should randomly pick a weekday to upload its backups.
This could be achieved by setting OnCalendar= to weekly, and then
configuring a RandomizedDelaySec= of 5 days with
FixedRandomDelay= enabled. Let's say that some laptop randomly
chooses a delay of 4 days. If this laptop is restarted more often than that,
then the timer will never fire: on each fresh boot, the 4 day delay is
restarted and will not be finished by the time of the next shutdown.
Instead, you should use RandomizedOffsetSec=, which will maintain the
configured weekly cadence of timer events, even across reboots.
Added in version 258.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP Check systemd.unit(5), systemd.exec(5), and
systemd.kill(5) for more settings.
.SH "SEE ALSO"
.PP 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.
.PP systemd(1), systemctl(1), systemd.unit(5),
systemd.service(5), systemd.time(7),
systemd.directives(7), systemd-system.conf(5),
prctl(2)
| systemd 262.2 |