Provided by: systemd_257.4-1ubuntu3.2_amd64 
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, 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.
Added in version 229.
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.
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 no effect if a realtime timer has not 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 257.4 SYSTEMD.TIMER(5)