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NAME | SYNOPSIS | DESCRIPTION | AUTOMATIC DEPENDENCIES | OPTIONS | SEE ALSO | COLOPHON |
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= 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.
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 will have an
additional After= dependency on time-sync.target to avoid being
started before the system clock has been correctly set.
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. 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.
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. 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, an elapsed timer will stay
loaded, and its state remains queryable. 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.
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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) 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 246 SYSTEMD.TIMER(5)
Pages that refer to this page: init(1) , systemctl(1) , systemd(1) , systemd-analyze(1) , systemd-run(1) , system.conf.d(5) , systemd-system.conf(5) , systemd.unit(5) , systemd-user.conf(5) , user.conf.d(5) , 30-systemd-environment-d-generator(7) , daemon(7) , systemd.directives(7) , systemd.index(7) , systemd.special(7) , systemd.syntax(7) , systemd.time(7)