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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.
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 time-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.
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.
May be specified more than once.
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.
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
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COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
systemd 234 SYSTEMD.TIMER(5)
Pages that refer to this page: systemd(1), systemd-run(1), systemd-system.conf(5), systemd.unit(5), daemon(7), systemd.directives(7), systemd.index(7), systemd.special(7), systemd.time(7)