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.