man/systemd.timer.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7 -->
   8
   9 <refentry id="systemd.timer">
  10   <refentryinfo>
  11     <title>systemd.timer</title>
  12     <productname>systemd</productname>
  13   </refentryinfo>
  14
  15   <refmeta>
  16     <refentrytitle>systemd.timer</refentrytitle>
  17     <manvolnum>5</manvolnum>
  18   </refmeta>
  19
  20   <refnamediv>
  21     <refname>systemd.timer</refname>
  22     <refpurpose>Timer unit configuration</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <para><filename><replaceable>timer</replaceable>.timer</filename></para>
  27   </refsynopsisdiv>
  28
  29   <refsect1>
  30     <title>Description</title>
  31
  32     <para>A unit configuration file whose name ends in
  33     <literal>.timer</literal> encodes information about a timer
  34     controlled and supervised by systemd, for timer-based
  35     activation.</para>
  36
  37     <para>This man page lists the configuration options specific to
  38     this unit type. See
  39     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  40     for the common options of all unit configuration files. The common
  41     configuration items are configured in the generic [Unit] and
  42     [Install] sections. The timer specific configuration options are
  43     configured in the [Timer] section.</para>
  44
  45     <para>For each timer file, a matching unit file must exist,
  46     describing the unit to activate when the timer elapses. By
  47     default, a service by the same name as the timer (except for the
  48     suffix) is activated. Example: a timer file
  49     <filename>foo.timer</filename> activates a matching service
  50     <filename>foo.service</filename>. The unit to activate may be
  51     controlled by <varname>Unit=</varname> (see below).</para>
  52
  53     <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
  54     but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
  55     with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process
  56     exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
  57     then stay around forever.</para>
  58   </refsect1>
  59
  60   <refsect1>
  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   </refsect1>
  70
  71   <refsect1>
  72     <title>Automatic Dependencies</title>
  73
  74     <refsect2>
  75       <title>Implicit Dependencies</title>
  76
  77       <para>There are no implicit dependencies for timer units.</para>
  78     </refsect2>
  79
  80     <refsect2>
  81       <title>Default Dependencies</title>
  82
  83       <para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para>
  84
  85       <itemizedlist>
  86         <listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and
  87         <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname>
  88         on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on
  89         <filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer
  90         units involved with early boot or late system shutdown should disable the
  91         <varname>DefaultDependencies=</varname> option.</para></listitem>
  92
  93         <listitem><para>Timer units
  94         with at least one <varname>OnCalendar=</varname> directive will have an additional <varname>After=</varname>
  95         dependency on <filename>time-sync.target</filename> to avoid being started before the system clock has been
  96         correctly set.</para></listitem>
  97       </itemizedlist>
  98     </refsect2>
  99   </refsect1>
 100
 101   <refsect1>
 102     <title>Options</title>
 103
 104     <para>Timer files must include a [Timer] section, which carries
 105     information about the timer it defines. The options specific to
 106     the [Timer] section of timer units are the following:</para>
 107
 108     <variablelist class='unit-directives'>
 109       <varlistentry>
 110         <term><varname>OnActiveSec=</varname></term>
 111         <term><varname>OnBootSec=</varname></term>
 112         <term><varname>OnStartupSec=</varname></term>
 113         <term><varname>OnUnitActiveSec=</varname></term>
 114         <term><varname>OnUnitInactiveSec=</varname></term>
 115
 116         <listitem><para>Defines monotonic timers relative to different
 117         starting points: <varname>OnActiveSec=</varname> defines a
 118         timer relative to the moment the timer itself is activated.
 119         <varname>OnBootSec=</varname> defines a timer relative to when
 120         the machine was booted up. <varname>OnStartupSec=</varname>
 121         defines a timer relative to when systemd was first started.
 122         <varname>OnUnitActiveSec=</varname> defines a timer relative
 123         to when the unit the timer is activating was last activated.
 124         <varname>OnUnitInactiveSec=</varname> defines a timer relative
 125         to when the unit the timer is activating was last
 126         deactivated.</para>
 127
 128         <para>Multiple directives may be combined of the same and of
 129         different types. For example, by combining
 130         <varname>OnBootSec=</varname> and
 131         <varname>OnUnitActiveSec=</varname>, it is possible to define
 132         a timer that elapses in regular intervals and activates a
 133         specific service each time.</para>
 134
 135         <para>The arguments to the directives are time spans
 136         configured in seconds. Example: "OnBootSec=50" means 50s after
 137         boot-up. The argument may also include time units. Example:
 138         "OnBootSec=5h 30min" means 5 hours and 30 minutes after
 139         boot-up. For details about the syntax of time spans, see
 140         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 141
 142         <para>If a timer configured with <varname>OnBootSec=</varname>
 143         or <varname>OnStartupSec=</varname> is already in the past
 144         when the timer unit is activated, it will immediately elapse
 145         and the configured unit is started. This is not the case for
 146         timers defined in the other directives.</para>
 147
 148         <para>These are monotonic timers, independent of wall-clock
 149         time and timezones. If the computer is temporarily suspended,
 150         the monotonic clock stops too.</para>
 151
 152         <para>If the empty string is assigned to any of these options,
 153         the list of timers is reset, and all prior assignments will
 154         have no effect.</para>
 155
 156         <para>Note that timers do not necessarily expire at the
 157         precise time configured with these settings, as they are
 158         subject to the <varname>AccuracySec=</varname> setting
 159         below.</para></listitem>
 160
 161       </varlistentry>
 162
 163       <varlistentry>
 164         <term><varname>OnCalendar=</varname></term>
 165
 166         <listitem><para>Defines realtime (i.e. wallclock) timers with
 167         calendar event expressions. See
 168         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 169         for more information on the syntax of calendar event
 170         expressions. Otherwise, the semantics are similar to
 171         <varname>OnActiveSec=</varname> and related settings.</para>
 172
 173         <para>Note that timers do not necessarily expire at the
 174         precise time configured with this setting, as it is subject to
 175         the <varname>AccuracySec=</varname> setting
 176         below.</para>
 177
 178        <para>May be specified more than once.</para></listitem>
 179       </varlistentry>
 180
 181       <varlistentry>
 182         <term><varname>AccuracySec=</varname></term>
 183
 184         <listitem><para>Specify the accuracy the timer shall elapse
 185         with. Defaults to 1min. The timer is scheduled to elapse
 186         within a time window starting with the time specified in
 187         <varname>OnCalendar=</varname>,
 188         <varname>OnActiveSec=</varname>,
 189         <varname>OnBootSec=</varname>,
 190         <varname>OnStartupSec=</varname>,
 191         <varname>OnUnitActiveSec=</varname> or
 192         <varname>OnUnitInactiveSec=</varname> and ending the time
 193         configured with <varname>AccuracySec=</varname> later. Within
 194         this time window, the expiry time will be placed at a
 195         host-specific, randomized, but stable position that is
 196         synchronized between all local timer units. This is done in
 197         order to optimize power consumption to suppress unnecessary
 198         CPU wake-ups. To get best accuracy, set this option to
 199         1us. Note that the timer is still subject to the timer slack
 200         configured via
 201         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
 202         <varname>TimerSlackNSec=</varname> setting. See
 203         <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 204         for details. To optimize power consumption, make sure to set
 205         this value as high as possible and as low as
 206         necessary.</para></listitem>
 207       </varlistentry>
 208
 209       <varlistentry>
 210         <term><varname>RandomizedDelaySec=</varname></term>
 211
 212         <listitem><para>Delay the timer by a randomly selected, evenly
 213         distributed amount of time between 0 and the specified time
 214         value. Defaults to 0, indicating that no randomized delay
 215         shall be applied. Each timer unit will determine this delay
 216         randomly each time it is started, and the delay will simply be
 217         added on top of the next determined elapsing time. This is
 218         useful to stretch dispatching of similarly configured timer
 219         events over a certain amount time, to avoid that they all fire
 220         at the same time, possibly resulting in resource
 221         congestion. Note the relation to
 222         <varname>AccuracySec=</varname> above: the latter allows the
 223         service manager to coalesce timer events within a specified
 224         time range in order to minimize wakeups, the former does the
 225         opposite: it stretches timer events over a time range, to make
 226         it unlikely that they fire simultaneously. If
 227         <varname>RandomizedDelaySec=</varname> and
 228         <varname>AccuracySec=</varname> are used in conjunction, first
 229         the randomized delay is added, and then the result is
 230         possibly further shifted to coalesce it with other timer
 231         events happening on the system. As mentioned above
 232         <varname>AccuracySec=</varname> defaults to 1min and
 233         <varname>RandomizedDelaySec=</varname> to 0, thus encouraging
 234         coalescing of timer events. In order to optimally stretch
 235         timer events over a certain range of time, make sure to set
 236         <varname>RandomizedDelaySec=</varname> to a higher value, and
 237         <varname>AccuracySec=1us</varname>.</para></listitem>
 238       </varlistentry>
 239
 240       <varlistentry>
 241         <term><varname>Unit=</varname></term>
 242
 243         <listitem><para>The unit to activate when this timer elapses.
 244         The argument is a unit name, whose suffix is not
 245         <literal>.timer</literal>. If not specified, this value
 246         defaults to a service that has the same name as the timer
 247         unit, except for the suffix. (See above.) It is recommended
 248         that the unit name that is activated and the unit name of the
 249         timer unit are named identically, except for the
 250         suffix.</para></listitem>
 251       </varlistentry>
 252
 253       <varlistentry>
 254         <term><varname>Persistent=</varname></term>
 255
 256         <listitem><para>Takes a boolean argument. If true, the time
 257         when the service unit was last triggered is stored on disk.
 258         When the timer is activated, the service unit is triggered
 259         immediately if it would have been triggered at least once
 260         during the time when the timer was inactive. This is useful to
 261         catch up on missed runs of the service when the machine was
 262         off. Note that this setting only has an effect on timers
 263         configured with <varname>OnCalendar=</varname>. Defaults
 264         to <varname>false</varname>.
 265         </para></listitem>
 266       </varlistentry>
 267
 268       <varlistentry>
 269         <term><varname>WakeSystem=</varname></term>
 270
 271         <listitem><para>Takes a boolean argument. If true, an elapsing
 272         timer will cause the system to resume from suspend, should it
 273         be suspended and if the system supports this. Note that this
 274         option will only make sure the system resumes on the
 275         appropriate times, it will not take care of suspending it
 276         again after any work that is to be done is finished. Defaults
 277         to <varname>false</varname>.</para></listitem>
 278       </varlistentry>
 279
 280       <varlistentry>
 281         <term><varname>RemainAfterElapse=</varname></term>
 282
 283         <listitem><para>Takes a boolean argument. If true, an elapsed
 284         timer will stay loaded, and its state remains queriable. If
 285         false, an elapsed timer unit that cannot elapse anymore is
 286         unloaded. Turning this off is particularly useful for
 287         transient timer units that shall disappear after they first
 288         elapse. Note that this setting has an effect on repeatedly
 289         starting a timer unit that only elapses once: if
 290         <varname>RemainAfterElapse=</varname> is on, it will not be
 291         started again, and is guaranteed to elapse only once. However,
 292         if <varname>RemainAfterElapse=</varname> is off, it might be
 293         started again if it is already elapsed, and thus be triggered
 294         multiple times. Defaults to
 295         <varname>yes</varname>.</para></listitem>
 296       </varlistentry>
 297     </variablelist>
 298   </refsect1>
 299
 300   <refsect1>
 301       <title>See Also</title>
 302       <para>
 303         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 304         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 305         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 306         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 307         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 308         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 309         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 310         <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 311       </para>
 312   </refsect1>
 313
 314 </refentry>