xenial (5) systemd.timer.5.gz

NAME

       systemd.timer - Timer unit configuration

SYNOPSIS

       timer.timer

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).

AUTOMATIC DEPENDENCIES

       Timer units automatically gain a Before= dependency on the service they are supposed to activate.

       Unless DefaultDependencies= 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 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.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.

       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=.

       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 RemainAfterLeapse= is off, it might be started again if it is already elapsed, and thus
           be triggered multiple times. Defaults to yes.

SEE ALSO

       systemd(1), systemctl(1), systemd.unit(5), systemd.service(5), systemd.time(7), systemd.directives(7),
       systemd-system.conf(5), prctl(2)