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SYSTEMD.TIMER(5) systemd.timer 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).
Unless DefaultDependencies= is set to false, all timer units will implicitly have
dependencies of type 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.unit(5).
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 a randomized
time is added, and the result is then possibly shifted further to coalesce it with
other timer events possibly 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=.
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.
SEE ALSO
systemd(1), systemctl(1), systemd.unit(5), systemd.service(5), systemd.time(7),
systemd.directives(7), systemd-system.conf(5), prctl(2)
systemd 219 SYSTEMD.TIMER(5)
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