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NAME | SYNOPSIS | DESCRIPTION | AUTOMATIC DEPENDENCIES | OPTIONS | SEE ALSO | COLOPHON |
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SYSTEMD.TIMER(5) systemd.timer SYSTEMD.TIMER(5)
systemd.timer - Timer unit configuration
timer.timer
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.
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.
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.
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)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2025-02-02.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~devel SYSTEMD.TIMER(5)
Pages that refer to this page: systemctl(1), systemd(1), systemd-analyze(1), systemd-run(1), systemd-system.conf(5), systemd.unit(5), daemon(7), systemd.directives(7), systemd.index(7), systemd.special(7), systemd.syntax(7), systemd.time(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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