1SYSTEMD.TIMER(5) systemd.timer SYSTEMD.TIMER(5)
2
6 systemd.timer - Timer unit configuration
7
9 timer.timer
10
12 A unit configuration file whose name ends in ".timer" encodes
13 information about a timer controlled and supervised by systemd, for
14 timer-based activation.
15 This man page lists the configuration options specific to this unit
17 type. See systemd.unit(5) for the common options of all unit
18 configuration files. The common configuration items are configured in
19 the generic [Unit] and [Install] sections. The timer specific
20 configuration options are configured in the [Timer] section.
21 For each timer file, a matching unit file must exist, describing the
23 unit to activate when the timer elapses. By default, a service by the
24 same name as the timer (except for the suffix) is activated. Example: a
25 timer file foo.timer activates a matching service foo.service. The unit
26 to activate may be controlled by Unit= (see below).
27 Note that in case the unit to activate is already active at the time
29 the timer elapses it is not restarted, but simply left running. There
30 is no concept of spawning new service instances in this case. Due to
31 this, services with RemainAfterExit= set (which stay around
32 continuously even after the service's main process exited) are usually
33 not suitable for activation via repetitive timers, as they will only be
34 activated once, and then stay around forever.
35
37 Implicit Dependencies
38 The following dependencies are implicitly added:
39 • Timer units automatically gain a Before= dependency on the service
41 they are supposed to activate.
42 Default Dependencies
44 The following dependencies are added unless DefaultDependencies=no is
45 set:
46 • Timer units will automatically have dependencies of type Requires=
48 and After= on sysinit.target, a dependency of type Before= on
49 timers.target, as well as Conflicts= and Before= on shutdown.target
50 to ensure that they are stopped cleanly prior to system shutdown.
51 Only timer units involved with early boot or late system shutdown
52 should disable the DefaultDependencies= option.
53 • Timer units with at least one OnCalendar= directive acquire a pair
55 of additional After= dependencies on time-set.target and
56 time-sync.target, in order to avoid being started before the system
57 clock has been correctly set. See systemd.special(7) for details on
58 these two targets.
59
61 Timer unit files may include [Unit] and [Install] sections, which are
62 described in systemd.unit(5).
63 Timer unit files must include a [Timer] section, which carries
65 information about the timer it defines. The options specific to the
66 [Timer] section of timer units are the following:
67 OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec=,
69 OnUnitInactiveSec=
70 Defines monotonic timers relative to different starting points:
71 Table 1. Settings and their starting points
73 ┌───────────────────┬────────────────────────────┐
74 │Setting │ Meaning │
75 ├───────────────────┼────────────────────────────┤
76 │OnActiveSec= │ Defines a timer relative │
77 │ │ to the moment the timer │
78 │ │ unit itself is activated. │
79 ├───────────────────┼────────────────────────────┤
80 │OnBootSec= │ Defines a timer relative │
81 │ │ to when the machine was │
82 │ │ booted up. In containers, │
83 │ │ for the system manager │
84 │ │ instance, this is mapped │
85 │ │ to OnStartupSec=, making │
86 │ │ both equivalent. │
87 ├───────────────────┼────────────────────────────┤
88 │OnStartupSec= │ Defines a timer relative │
89 │ │ to when the service │
90 │ │ manager was first started. │
91 │ │ For system timer units │
92 │ │ this is very similar to │
93 │ │ OnBootSec= as the system │
94 │ │ service manager is │
95 │ │ generally started very │
96 │ │ early at boot. It's │
97 │ │ primarily useful when │
98 │ │ configured in units │
99 │ │ running in the per-user │
100 │ │ service manager, as the │
101 │ │ user service manager is │
102 │ │ generally started on first │
103 │ │ login only, not already │
104 │ │ during boot. │
105 ├───────────────────┼────────────────────────────┤
106 │OnUnitActiveSec= │ Defines a timer relative │
107 │ │ to when the unit the timer │
108 │ │ unit is activating was │
109 │ │ last activated. │
110 ├───────────────────┼────────────────────────────┤
111 │OnUnitInactiveSec= │ Defines a timer relative │
112 │ │ to when the unit the timer │
113 │ │ unit is activating was │
114 │ │ last deactivated. │
115 └───────────────────┴────────────────────────────┘
116 Multiple directives may be combined of the same and of different
117 types, in which case the timer unit will trigger whenever any of
118 the specified timer expressions elapse. For example, by combining
119 OnBootSec= and OnUnitActiveSec=, it is possible to define a timer
120 that elapses in regular intervals and activates a specific service
121 each time. Moreover, both monotonic time expressions and
122 OnCalendar= calendar expressions may be combined in the same timer
123 unit.
124 The arguments to the directives are time spans configured in
126 seconds. Example: "OnBootSec=50" means 50s after boot-up. The
127 argument may also include time units. Example: "OnBootSec=5h 30min"
128 means 5 hours and 30 minutes after boot-up. For details about the
129 syntax of time spans, see systemd.time(7).
130 If a timer configured with OnBootSec= or OnStartupSec= is already
132 in the past when the timer unit is activated, it will immediately
133 elapse and the configured unit is started. This is not the case for
134 timers defined in the other directives.
135 These are monotonic timers, independent of wall-clock time and
137 timezones. If the computer is temporarily suspended, the monotonic
138 clock generally pauses, too. Note that if WakeSystem= is used, a
139 different monotonic clock is selected that continues to advance
140 while the system is suspended and thus can be used as the trigger
141 to resume the system.
142 If the empty string is assigned to any of these options, the list
144 of timers is reset (both monotonic timers and OnCalendar= timers,
145 see below), and all prior assignments will have no effect.
146 Note that timers do not necessarily expire at the precise time
148 configured with these settings, as they are subject to the
149 AccuracySec= setting below.
150 OnCalendar=
152 Defines realtime (i.e. wallclock) timers with calendar event
153 expressions. See systemd.time(7) for more information on the syntax
154 of calendar event expressions. Otherwise, the semantics are similar
155 to OnActiveSec= and related settings.
156 Note that timers do not necessarily expire at the precise time
158 configured with this setting, as it is subject to the AccuracySec=
159 setting below.
160 May be specified more than once, in which case the timer unit will
162 trigger whenever any of the specified expressions elapse. Moreover
163 calendar timers and monotonic timers (see above) may be combined
164 within the same timer unit.
165 If the empty string is assigned to any of these options, the list
167 of timers is reset (both OnCalendar= timers and monotonic timers,
168 see above), and all prior assignments will have no effect.
169 AccuracySec=
171 Specify the accuracy the timer shall elapse with. Defaults to 1min.
172 The timer is scheduled to elapse within a time window starting with
173 the time specified in OnCalendar=, OnActiveSec=, OnBootSec=,
174 OnStartupSec=, OnUnitActiveSec= or OnUnitInactiveSec= and ending
175 the time configured with AccuracySec= later. Within this time
176 window, the expiry time will be placed at a host-specific,
177 randomized, but stable position that is synchronized between all
178 local timer units. This is done in order to optimize power
179 consumption to suppress unnecessary CPU wake-ups. To get best
180 accuracy, set this option to 1us. Note that the timer is still
181 subject to the timer slack configured via systemd-system.conf(5)'s
182 TimerSlackNSec= setting. See prctl(2) for details. To optimize
183 power consumption, make sure to set this value as high as possible
184 and as low as necessary.
185 Note that this setting is primarily a power saving option that
187 allows coalescing CPU wake-ups. It should not be confused with
188 RandomizedDelaySec= (see below) which adds a random value to the
189 time the timer shall elapse next and whose purpose is the opposite:
190 to stretch elapsing of timer events over a longer period to reduce
191 workload spikes. For further details and explanations and how both
192 settings play together, see below.
193 RandomizedDelaySec=
195 Delay the timer by a randomly selected, evenly distributed amount
196 of time between 0 and the specified time value. Defaults to 0,
197 indicating that no randomized delay shall be applied. Each timer
198 unit will determine this delay randomly before each iteration, and
199 the delay will simply be added on top of the next determined
200 elapsing time, unless modified with FixedRandomDelay=, see below.
201 This setting is useful to stretch dispatching of similarly
203 configured timer events over a certain time interval, to prevent
204 them from firing all at the same time, possibly resulting in
205 resource congestion.
206 Note the relation to AccuracySec= above: the latter allows the
208 service manager to coalesce timer events within a specified time
209 range in order to minimize wakeups, while this setting does the
210 opposite: it stretches timer events over an interval, to make it
211 unlikely that they fire simultaneously. If RandomizedDelaySec= and
212 AccuracySec= are used in conjunction, first the randomized delay is
213 added, and then the result is possibly further shifted to coalesce
214 it with other timer events happening on the system. As mentioned
215 above AccuracySec= defaults to 1 minute and RandomizedDelaySec= to
216 0, thus encouraging coalescing of timer events. In order to
217 optimally stretch timer events over a certain range of time, set
218 AccuracySec=1us and RandomizedDelaySec= to some higher value.
219 FixedRandomDelay=
221 Takes a boolean argument. When enabled, the randomized offset
222 specified by RandomizedDelaySec= is reused for all firings of the
223 same timer. For a given timer unit, the offset depends on the
224 machine ID, user identifier and timer name, which means that it is
225 stable between restarts of the manager. This effectively creates a
226 fixed offset for an individual timer, reducing the jitter in
227 firings of this timer, while still avoiding firing at the same time
228 as other similarly configured timers.
229 This setting has no effect if RandomizedDelaySec= is set to 0.
231 Defaults to false.
232 OnClockChange=, OnTimezoneChange=
234 These options take boolean arguments. When true, the service unit
235 will be triggered when the system clock (CLOCK_REALTIME) jumps
236 relative to the monotonic clock (CLOCK_MONOTONIC), or when the
237 local system timezone is modified. These options can be used alone
238 or in combination with other timer expressions (see above) within
239 the same timer unit. These options default to false.
240 Unit=
242 The unit to activate when this timer elapses. The argument is a
243 unit name, whose suffix is not ".timer". If not specified, this
244 value defaults to a service that has the same name as the timer
245 unit, except for the suffix. (See above.) It is recommended that
246 the unit name that is activated and the unit name of the timer unit
247 are named identically, except for the suffix.
248 Persistent=
250 Takes a boolean argument. If true, the time when the service unit
251 was last triggered is stored on disk. When the timer is activated,
252 the service unit is triggered immediately if it would have been
253 triggered at least once during the time when the timer was
254 inactive. Such triggering is nonetheless subject to the delay
255 imposed by RandomizedDelaySec=. This is useful to catch up on
256 missed runs of the service when the system was powered down. Note
257 that this setting only has an effect on timers configured with
258 OnCalendar=. Defaults to false.
259 Use systemctl clean --what=state ... on the timer unit to remove
261 the timestamp file maintained by this option from disk. In
262 particular, use this command before uninstalling a timer unit. See
263 systemctl(1) for details.
264 WakeSystem=
266 Takes a boolean argument. If true, an elapsing timer will cause the
267 system to resume from suspend, should it be suspended and if the
268 system supports this. Note that this option will only make sure the
269 system resumes on the appropriate times, it will not take care of
270 suspending it again after any work that is to be done is finished.
271 Defaults to false.
272 Note that this functionality requires privileges and is thus
274 generally only available in the system service manager.
275 Note that behaviour of monotonic clock timers (as configured with
277 OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec=,
278 OnUnitInactiveSec=, see above) is altered depending on this option.
279 If false, a monotonic clock is used that is paused during system
280 suspend (CLOCK_MONOTONIC), if true a different monotonic clock is
281 used that continues advancing during system suspend
282 (CLOCK_BOOTTIME), see clock_getres(2) for details.
283 RemainAfterElapse=
285 Takes a boolean argument. If true, a timer will stay loaded, and
286 its state remains queryable even after it elapsed and the
287 associated unit (as configured with Unit=, see above) deactivated
288 again. If false, an elapsed timer unit that cannot elapse anymore
289 is unloaded once its associated unit deactivated again. Turning
290 this off is particularly useful for transient timer units. Note
291 that this setting has an effect when repeatedly starting a timer
292 unit: if RemainAfterElapse= is on, starting the timer a second time
293 has no effect. However, if RemainAfterElapse= is off and the timer
294 unit was already unloaded, it can be started again, and thus the
295 service can be triggered multiple times. Defaults to true.
296 Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more
298 settings.
299
301 systemd(1), systemctl(1), systemd.unit(5), systemd.service(5),
302 systemd.time(7), systemd.directives(7), systemd-system.conf(5),
303 prctl(2)
304systemd 251 SYSTEMD.TIMER(5)