NAME
systemd.timer - Timer unit configurationSYNOPSIS
timer.timerDESCRIPTION
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
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
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=
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. |
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 continously 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).
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.
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 on Triggered Units" section in systemd.exec(1) 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 252 |