1<?xml version='1.0'?> 2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" 3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> 4<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> 5 6<refentry id="systemd.timer" xmlns:xi="http://www.w3.org/2001/XInclude"> 7 <refentryinfo> 8 <title>systemd.timer</title> 9 <productname>systemd</productname> 10 </refentryinfo> 11 12 <refmeta> 13 <refentrytitle>systemd.timer</refentrytitle> 14 <manvolnum>5</manvolnum> 15 </refmeta> 16 17 <refnamediv> 18 <refname>systemd.timer</refname> 19 <refpurpose>Timer unit configuration</refpurpose> 20 </refnamediv> 21 22 <refsynopsisdiv> 23 <para><filename><replaceable>timer</replaceable>.timer</filename></para> 24 </refsynopsisdiv> 25 26 <refsect1> 27 <title>Description</title> 28 29 <para>A unit configuration file whose name ends in 30 <literal>.timer</literal> encodes information about a timer 31 controlled and supervised by systemd, for timer-based 32 activation.</para> 33 34 <para>This man page lists the configuration options specific to 35 this unit type. See 36 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> 37 for the common options of all unit configuration files. The common 38 configuration items are configured in the generic [Unit] and 39 [Install] sections. The timer specific configuration options are 40 configured in the [Timer] section.</para> 41 42 <para>For each timer file, a matching unit file must exist, 43 describing the unit to activate when the timer elapses. By 44 default, a service by the same name as the timer (except for the 45 suffix) is activated. Example: a timer file 46 <filename>foo.timer</filename> activates a matching service 47 <filename>foo.service</filename>. The unit to activate may be 48 controlled by <varname>Unit=</varname> (see below).</para> 49 50 <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted, 51 but simply left running. There is no concept of spawning new service instances in this case. Due to this, services 52 with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process 53 exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and 54 then stay around forever.</para> 55 </refsect1> 56 57 <refsect1> 58 <title>Automatic Dependencies</title> 59 60 <refsect2> 61 <title>Implicit Dependencies</title> 62 63 <para>The following dependencies are implicitly added:</para> 64 65 <itemizedlist> 66 <listitem><para>Timer units automatically gain a <varname>Before=</varname> 67 dependency on the service they are supposed to activate.</para></listitem> 68 </itemizedlist> 69 </refsect2> 70 71 <refsect2> 72 <title>Default Dependencies</title> 73 74 <para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para> 75 76 <itemizedlist> 77 <listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and 78 <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname> 79 on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on 80 <filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer 81 units involved with early boot or late system shutdown should disable the 82 <varname>DefaultDependencies=</varname> option.</para></listitem> 83 84 <listitem><para>Timer units with at least one <varname>OnCalendar=</varname> directive acquire a pair 85 of additional <varname>After=</varname> dependencies on <filename>time-set.target</filename> and 86 <filename>time-sync.target</filename>, in order to avoid being started before the system clock has 87 been correctly set. See 88 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> 89 for details on these two targets.</para></listitem> 90 </itemizedlist> 91 </refsect2> 92 </refsect1> 93 94 <refsect1> 95 <title>Options</title> 96 97 <para>Timer unit files may include [Unit] and [Install] sections, which are described in 98 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. 99 </para> 100 101 <para>Timer unit files must include a [Timer] section, which carries 102 information about the timer it defines. The options specific to 103 the [Timer] section of timer units are the following:</para> 104 105 <variablelist class='unit-directives'> 106 <varlistentry> 107 <term><varname>OnActiveSec=</varname></term> 108 <term><varname>OnBootSec=</varname></term> 109 <term><varname>OnStartupSec=</varname></term> 110 <term><varname>OnUnitActiveSec=</varname></term> 111 <term><varname>OnUnitInactiveSec=</varname></term> 112 113 <listitem><para>Defines monotonic timers relative to different 114 starting points:</para> 115 116 <table> 117 <title>Settings and their starting points</title> 118 119 <tgroup cols='2'> 120 <thead> 121 <row> 122 <entry>Setting</entry> 123 <entry>Meaning</entry> 124 </row> 125 </thead> 126 <tbody> 127 <row> 128 <entry><varname>OnActiveSec=</varname></entry> 129 <entry>Defines a timer relative to the moment the timer unit itself is activated.</entry> 130 </row> 131 <row> 132 <entry><varname>OnBootSec=</varname></entry> 133 <entry>Defines a timer relative to when the machine was booted up. In containers, for the system manager instance, this is mapped to <varname>OnStartupSec=</varname>, making both equivalent.</entry> 134 </row> 135 <row> 136 <entry><varname>OnStartupSec=</varname></entry> 137 <entry>Defines a timer relative to when the service manager was first started. For system timer units this is very similar to <varname>OnBootSec=</varname> 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.</entry> 138 </row> 139 <row> 140 <entry><varname>OnUnitActiveSec=</varname></entry> 141 <entry>Defines a timer relative to when the unit the timer unit is activating was last activated.</entry> 142 </row> 143 <row> 144 <entry><varname>OnUnitInactiveSec=</varname></entry> 145 <entry>Defines a timer relative to when the unit the timer unit is activating was last deactivated.</entry> 146 </row> 147 </tbody> 148 </tgroup> 149 </table> 150 151 <para>Multiple directives may be combined of the same and of different types, in which case the timer 152 unit will trigger whenever any of the specified timer expressions elapse. For example, by combining 153 <varname>OnBootSec=</varname> and <varname>OnUnitActiveSec=</varname>, it is possible to define a 154 timer that elapses in regular intervals and activates a specific service each time. Moreover, both 155 monotonic time expressions and <varname>OnCalendar=</varname> calendar expressions may be combined in 156 the same timer unit.</para> 157 158 <para>The arguments to the directives are time spans 159 configured in seconds. Example: "OnBootSec=50" means 50s after 160 boot-up. The argument may also include time units. Example: 161 "OnBootSec=5h 30min" means 5 hours and 30 minutes after 162 boot-up. For details about the syntax of time spans, see 163 <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para> 164 165 <para>If a timer configured with <varname>OnBootSec=</varname> 166 or <varname>OnStartupSec=</varname> is already in the past 167 when the timer unit is activated, it will immediately elapse 168 and the configured unit is started. This is not the case for 169 timers defined in the other directives.</para> 170 171 <para>These are monotonic timers, independent of wall-clock time and timezones. If the computer is 172 temporarily suspended, the monotonic clock generally pauses, too. Note that if 173 <varname>WakeSystem=</varname> is used, a different monotonic clock is selected that continues to 174 advance while the system is suspended and thus can be used as the trigger to resume the 175 system.</para> 176 177 <para>If the empty string is assigned to any of these options, the list of timers is reset (both 178 monotonic timers and <varname>OnCalendar=</varname> timers, see below), and all prior assignments 179 will have no effect.</para> 180 181 <para>Note that timers do not necessarily expire at the 182 precise time configured with these settings, as they are 183 subject to the <varname>AccuracySec=</varname> setting 184 below.</para></listitem> 185 </varlistentry> 186 187 <varlistentry> 188 <term><varname>OnCalendar=</varname></term> 189 190 <listitem><para>Defines realtime (i.e. wallclock) timers with 191 calendar event expressions. See 192 <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> 193 for more information on the syntax of calendar event 194 expressions. Otherwise, the semantics are similar to 195 <varname>OnActiveSec=</varname> and related settings.</para> 196 197 <para>Note that timers do not necessarily expire at the 198 precise time configured with this setting, as it is subject to 199 the <varname>AccuracySec=</varname> setting 200 below.</para> 201 202 <para>May be specified more than once, in which case the timer unit will trigger whenever any of the 203 specified expressions elapse. Moreover calendar timers and monotonic timers (see above) may be 204 combined within the same timer unit.</para> 205 206 <para>If the empty string is assigned to any of these options, the list of timers is reset (both 207 <varname>OnCalendar=</varname> timers and monotonic timers, see above), and all prior assignments 208 will have no effect.</para></listitem> 209 </varlistentry> 210 211 <varlistentry> 212 <term><varname>AccuracySec=</varname></term> 213 214 <listitem><para>Specify the accuracy the timer shall elapse 215 with. Defaults to 1min. The timer is scheduled to elapse 216 within a time window starting with the time specified in 217 <varname>OnCalendar=</varname>, 218 <varname>OnActiveSec=</varname>, 219 <varname>OnBootSec=</varname>, 220 <varname>OnStartupSec=</varname>, 221 <varname>OnUnitActiveSec=</varname> or 222 <varname>OnUnitInactiveSec=</varname> and ending the time 223 configured with <varname>AccuracySec=</varname> later. Within 224 this time window, the expiry time will be placed at a 225 host-specific, randomized, but stable position that is 226 synchronized between all local timer units. This is done in 227 order to optimize power consumption to suppress unnecessary 228 CPU wake-ups. To get best accuracy, set this option to 229 1us. Note that the timer is still subject to the timer slack 230 configured via 231 <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s 232 <varname>TimerSlackNSec=</varname> setting. See 233 <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> 234 for details. To optimize power consumption, make sure to set 235 this value as high as possible and as low as 236 necessary.</para> 237 238 <para>Note that this setting is primarily a power saving option that allows coalescing CPU 239 wake-ups. It should not be confused with <varname>RandomizedDelaySec=</varname> (see below) which 240 adds a random value to the time the timer shall elapse next and whose purpose is the opposite: to 241 stretch elapsing of timer events over a longer period to reduce workload spikes. For further details 242 and explanations and how both settings play together, see below.</para></listitem> 243 </varlistentry> 244 245 <varlistentry> 246 <term><varname>RandomizedDelaySec=</varname></term> 247 248 <listitem><para>Delay the timer by a randomly selected, evenly distributed amount of time between 0 249 and the specified time value. Defaults to 0, indicating that no randomized delay shall be applied. 250 Each timer unit will determine this delay randomly before each iteration, and the delay will simply 251 be added on top of the next determined elapsing time, unless modified with 252 <varname>FixedRandomDelay=</varname>, see below.</para> 253 254 <para>This setting is useful to stretch dispatching of similarly configured timer events over a 255 certain time interval, to prevent them from firing all at the same time, possibly resulting in 256 resource congestion.</para> 257 258 <para>Note the relation to <varname>AccuracySec=</varname> above: the latter allows the service 259 manager to coalesce timer events within a specified time range in order to minimize wakeups, while 260 this setting does the opposite: it stretches timer events over an interval, to make it unlikely that 261 they fire simultaneously. If <varname>RandomizedDelaySec=</varname> and 262 <varname>AccuracySec=</varname> are used in conjunction, first the randomized delay is added, and 263 then the result is possibly further shifted to coalesce it with other timer events happening on the 264 system. As mentioned above <varname>AccuracySec=</varname> defaults to 1 minute and 265 <varname>RandomizedDelaySec=</varname> to 0, thus encouraging coalescing of timer events. In order to 266 optimally stretch timer events over a certain range of time, set 267 <varname>AccuracySec=1us</varname> and <varname>RandomizedDelaySec=</varname> to some higher value. 268 </para></listitem> 269 </varlistentry> 270 271 <varlistentry> 272 <term><varname>FixedRandomDelay=</varname></term> 273 274 <listitem><para>Takes a boolean argument. When enabled, the randomized offset specified by 275 <varname>RandomizedDelaySec=</varname> is reused for all firings of the same timer. For a given timer 276 unit, the offset depends on the machine ID, user identifier and timer name, which means that it is 277 stable between restarts of the manager. This effectively creates a fixed offset for an individual 278 timer, reducing the jitter in firings of this timer, while still avoiding firing at the same time as 279 other similarly configured timers.</para> 280 281 <para>This setting has no effect if <varname>RandomizedDelaySec=</varname> is set to 0. Defaults to 282 <option>false</option>.</para></listitem> 283 </varlistentry> 284 285 <varlistentry> 286 <term><varname>OnClockChange=</varname></term> 287 <term><varname>OnTimezoneChange=</varname></term> 288 289 <listitem><para>These options take boolean arguments. When true, the service unit will be triggered 290 when the system clock (<constant>CLOCK_REALTIME</constant>) jumps relative to the monotonic clock 291 (<constant>CLOCK_MONOTONIC</constant>), or when the local system timezone is modified. These options 292 can be used alone or in combination with other timer expressions (see above) within the same timer 293 unit. These options default to <option>false</option>.</para></listitem> 294 </varlistentry> 295 296 <varlistentry> 297 <term><varname>Unit=</varname></term> 298 299 <listitem><para>The unit to activate when this timer elapses. 300 The argument is a unit name, whose suffix is not 301 <literal>.timer</literal>. If not specified, this value 302 defaults to a service that has the same name as the timer 303 unit, except for the suffix. (See above.) It is recommended 304 that the unit name that is activated and the unit name of the 305 timer unit are named identically, except for the 306 suffix.</para></listitem> 307 </varlistentry> 308 309 <varlistentry> 310 <term><varname>Persistent=</varname></term> 311 312 <listitem><para>Takes a boolean argument. If true, the time when the service unit was last triggered 313 is stored on disk. When the timer is activated, the service unit is triggered immediately if it 314 would have been triggered at least once during the time when the timer was inactive. Such triggering 315 is nonetheless subject to the delay imposed by <varname>RandomizedDelaySec=</varname>. 316 This is useful to catch up on missed runs of the service when the system was powered down. Note that 317 this setting only has an effect on timers configured with <varname>OnCalendar=</varname>. Defaults to 318 <option>false</option>.</para> 319 320 <para>Use <command>systemctl clean --what=state …</command> on the timer unit to remove the timestamp 321 file maintained by this option from disk. In particular, use this command before uninstalling a timer 322 unit. See 323 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for 324 details.</para></listitem> 325 </varlistentry> 326 327 <varlistentry> 328 <term><varname>WakeSystem=</varname></term> 329 330 <listitem><para>Takes a boolean argument. If true, an elapsing timer will cause the system to resume 331 from suspend, should it be suspended and if the system supports this. Note that this option will only 332 make sure the system resumes on the appropriate times, it will not take care of suspending it again 333 after any work that is to be done is finished. Defaults to 334 <option>false</option>.</para> 335 336 <para>Note that this functionality requires privileges and is thus generally only available in the 337 system service manager.</para> 338 339 <para>Note that behaviour of monotonic clock timers (as configured with 340 <varname>OnActiveSec=</varname>, <varname>OnBootSec=</varname>, <varname>OnStartupSec=</varname>, 341 <varname>OnUnitActiveSec=</varname>, <varname>OnUnitInactiveSec=</varname>, see above) is altered 342 depending on this option. If false, a monotonic clock is used that is paused during system suspend 343 (<constant>CLOCK_MONOTONIC</constant>), if true a different monotonic clock is used that continues 344 advancing during system suspend (<constant>CLOCK_BOOTTIME</constant>), see 345 <citerefentry><refentrytitle>clock_getres</refentrytitle><manvolnum>2</manvolnum></citerefentry> for 346 details.</para></listitem> 347 </varlistentry> 348 349 <varlistentry> 350 <term><varname>RemainAfterElapse=</varname></term> 351 352 <listitem><para>Takes a boolean argument. If true, a timer will stay loaded, and its state remains 353 queryable even after it elapsed and the associated unit (as configured with <varname>Unit=</varname>, 354 see above) deactivated again. If false, an elapsed timer unit that cannot elapse anymore is unloaded 355 once its associated unit deactivated again. Turning this off is particularly useful for transient 356 timer units. Note that this setting has an effect when repeatedly starting a timer unit: if 357 <varname>RemainAfterElapse=</varname> is on, starting the timer a second time has no effect. However, 358 if <varname>RemainAfterElapse=</varname> is off and the timer unit was already unloaded, it can be 359 started again, and thus the service can be triggered multiple times. Defaults to 360 <option>true</option>.</para></listitem> 361 </varlistentry> 362 </variablelist> 363 364 <xi:include href="systemd.service.xml" xpointer="shared-unit-options" /> 365 </refsect1> 366 367 <refsect1> 368 <title>See Also</title> 369 <para> 370 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, 371 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, 372 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>, 373 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>, 374 <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>, 375 <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>, 376 <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, 377 <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> 378 </para> 379 </refsect1> 380 381</refentry> 382