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= 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
Timer units automatically gain a Before= dependency on the service they are supposed to activate. Unless DefaultDependencies= in the "[Unit]" section is set to false, all timer units will implicitly 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. Timer units with at least one OnCalendar= directive will have an additional After= dependency on timer-sync.target to avoid being started before the system clock has been correctly set. Only timer units involved with early boot or late system shutdown should disable the DefaultDependencies= option.
OPTIONS
Timer 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:
OnActiveSec= defines a timer relative to the moment the timer
itself is activated. OnBootSec= defines a timer relative to when
the machine was booted up. OnStartupSec= defines a timer relative
to when systemd was first started. OnUnitActiveSec= defines a
timer relative to when the unit the timer is activating was last
activated. OnUnitInactiveSec= defines a timer relative to when the
unit the timer is activating was last deactivated.
Multiple directives may be combined of the same and of different
types. 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.
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 stops too.
If the empty string is assigned to any of these options, the list
of timers is reset, 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.
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.
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 each time it is started,
and the delay will simply be added on top of the next determined
elapsing time. This is useful to stretch dispatching of similarly
configured timer events over a certain amount time, to avoid that
they all fire 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, the former does
the opposite: it stretches timer events over a time range, 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 1min and
RandomizedDelaySec= to 0, thus encouraging coalescing of timer
events. In order to optimally stretch timer events over a certain
range of time, make sure to set RandomizedDelaySec= to a higher
value, and AccuracySec=1us.
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. This is useful to catch up on missed runs of the service
when the machine was off. Note that this setting only has an effect
on timers configured with OnCalendar=. Defaults to false.
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.
RemainAfterElapse=
Takes a boolean argument. If true, an elapsed timer will stay
loaded, and its state remains queriable. If false, an elapsed timer
unit that cannot elapse anymore is unloaded. Turning this off is
particularly useful for transient timer units that shall disappear
after they first elapse. Note that this setting has an effect on
repeatedly starting a timer unit that only elapses once: if
RemainAfterElapse= is on, it will not be started again, and is
guaranteed to elapse only once. However, if RemainAfterElapse= is
off, it might be started again if it is already elapsed, and thus
be triggered multiple times. Defaults to yes.
SEE ALSO
systemd(1), systemctl(1), systemd.unit(5), systemd.service(5), systemd.time(7), systemd.directives(7), systemd-system.conf(5), prctl(2)
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