systemd.timer - Timer unit configuration
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.
The following dependencies are implicitly added:
The following dependencies are added unless DefaultDependencies=no is set:
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=
Table 1. Settings and their starting points
|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.
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).
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.
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.
This setting has no effect if RandomizedDelaySec= is set to 0. 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.
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.
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.