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:
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.
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.
