man/systemd.timer.xml

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   8   This file is part of systemd.
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  10   Copyright 2010 Lennart Poettering
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  12
  13 <refentry id="systemd.timer">
  14   <refentryinfo>
  15     <title>systemd.timer</title>
  16     <productname>systemd</productname>
  17
  18     <authorgroup>
  19       <author>
  20         <contrib>Developer</contrib>
  21         <firstname>Lennart</firstname>
  22         <surname>Poettering</surname>
  23         <email>lennart@poettering.net</email>
  24       </author>
  25     </authorgroup>
  26   </refentryinfo>
  27
  28   <refmeta>
  29     <refentrytitle>systemd.timer</refentrytitle>
  30     <manvolnum>5</manvolnum>
  31   </refmeta>
  32
  33   <refnamediv>
  34     <refname>systemd.timer</refname>
  35     <refpurpose>Timer unit configuration</refpurpose>
  36   </refnamediv>
  37
  38   <refsynopsisdiv>
  39     <para><filename><replaceable>timer</replaceable>.timer</filename></para>
  40   </refsynopsisdiv>
  41
  42   <refsect1>
  43     <title>Description</title>
  44
  45     <para>A unit configuration file whose name ends in
  46     <literal>.timer</literal> encodes information about a timer
  47     controlled and supervised by systemd, for timer-based
  48     activation.</para>
  49
  50     <para>This man page lists the configuration options specific to
  51     this unit type. See
  52     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  53     for the common options of all unit configuration files. The common
  54     configuration items are configured in the generic [Unit] and
  55     [Install] sections. The timer specific configuration options are
  56     configured in the [Timer] section.</para>
  57
  58     <para>For each timer file, a matching unit file must exist,
  59     describing the unit to activate when the timer elapses. By
  60     default, a service by the same name as the timer (except for the
  61     suffix) is activated. Example: a timer file
  62     <filename>foo.timer</filename> activates a matching service
  63     <filename>foo.service</filename>. The unit to activate may be
  64     controlled by <varname>Unit=</varname> (see below).</para>
  65
  66     <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
  67     but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
  68     with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process
  69     exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
  70     then stay around forever.</para>
  71   </refsect1>
  72
  73   <refsect1>
  74     <title>Implicit Dependencies</title>
  75
  76     <para>The following dependencies are implicitly added:</para>
  77
  78     <itemizedlist>
  79       <listitem><para>Timer units automatically gain a <varname>Before=</varname>
  80       dependency on the service they are supposed to activate.</para></listitem>
  81     </itemizedlist>
  82   </refsect1>
  83
  84   <refsect1>
  85     <title>Automatic Dependencies</title>
  86
  87     <refsect2>
  88       <title>Implicit Dependencies</title>
  89
  90       <para>There are no implicit dependencies for timer units.</para>
  91     </refsect2>
  92
  93     <refsect2>
  94       <title>Default Dependencies</title>
  95
  96       <para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para>
  97
  98       <itemizedlist>
  99         <listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and
 100         <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname>
 101         on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on
 102         <filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer
 103         units involved with early boot or late system shutdown should disable the
 104         <varname>DefaultDependencies=</varname> option.</para></listitem>
 105
 106         <listitem><para>Timer units
 107         with at least one <varname>OnCalendar=</varname> directive will have an additional <varname>After=</varname>
 108         dependency on <filename>time-sync.target</filename> to avoid being started before the system clock has been
 109         correctly set.</para></listitem>
 110       </itemizedlist>
 111     </refsect2>
 112   </refsect1>
 113
 114   <refsect1>
 115     <title>Options</title>
 116
 117     <para>Timer files must include a [Timer] section, which carries
 118     information about the timer it defines. The options specific to
 119     the [Timer] section of timer units are the following:</para>
 120
 121     <variablelist class='unit-directives'>
 122       <varlistentry>
 123         <term><varname>OnActiveSec=</varname></term>
 124         <term><varname>OnBootSec=</varname></term>
 125         <term><varname>OnStartupSec=</varname></term>
 126         <term><varname>OnUnitActiveSec=</varname></term>
 127         <term><varname>OnUnitInactiveSec=</varname></term>
 128
 129         <listitem><para>Defines monotonic timers relative to different
 130         starting points: <varname>OnActiveSec=</varname> defines a
 131         timer relative to the moment the timer itself is activated.
 132         <varname>OnBootSec=</varname> defines a timer relative to when
 133         the machine was booted up. <varname>OnStartupSec=</varname>
 134         defines a timer relative to when systemd was first started.
 135         <varname>OnUnitActiveSec=</varname> defines a timer relative
 136         to when the unit the timer is activating was last activated.
 137         <varname>OnUnitInactiveSec=</varname> defines a timer relative
 138         to when the unit the timer is activating was last
 139         deactivated.</para>
 140
 141         <para>Multiple directives may be combined of the same and of
 142         different types. For example, by combining
 143         <varname>OnBootSec=</varname> and
 144         <varname>OnUnitActiveSec=</varname>, it is possible to define
 145         a timer that elapses in regular intervals and activates a
 146         specific service each time.</para>
 147
 148         <para>The arguments to the directives are time spans
 149         configured in seconds. Example: "OnBootSec=50" means 50s after
 150         boot-up. The argument may also include time units. Example:
 151         "OnBootSec=5h 30min" means 5 hours and 30 minutes after
 152         boot-up. For details about the syntax of time spans, see
 153         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 154
 155         <para>If a timer configured with <varname>OnBootSec=</varname>
 156         or <varname>OnStartupSec=</varname> is already in the past
 157         when the timer unit is activated, it will immediately elapse
 158         and the configured unit is started. This is not the case for
 159         timers defined in the other directives.</para>
 160
 161         <para>These are monotonic timers, independent of wall-clock
 162         time and timezones. If the computer is temporarily suspended,
 163         the monotonic clock stops too.</para>
 164
 165         <para>If the empty string is assigned to any of these options,
 166         the list of timers is reset, and all prior assignments will
 167         have no effect.</para>
 168
 169         <para>Note that timers do not necessarily expire at the
 170         precise time configured with these settings, as they are
 171         subject to the <varname>AccuracySec=</varname> setting
 172         below.</para></listitem>
 173
 174       </varlistentry>
 175
 176       <varlistentry>
 177         <term><varname>OnCalendar=</varname></term>
 178
 179         <listitem><para>Defines realtime (i.e. wallclock) timers with
 180         calendar event expressions. See
 181         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 182         for more information on the syntax of calendar event
 183         expressions. Otherwise, the semantics are similar to
 184         <varname>OnActiveSec=</varname> and related settings.</para>
 185
 186         <para>Note that timers do not necessarily expire at the
 187         precise time configured with this setting, as it is subject to
 188         the <varname>AccuracySec=</varname> setting
 189         below.</para>
 190
 191        <para>May be specified more than once.</para></listitem>
 192       </varlistentry>
 193
 194       <varlistentry>
 195         <term><varname>AccuracySec=</varname></term>
 196
 197         <listitem><para>Specify the accuracy the timer shall elapse
 198         with. Defaults to 1min. The timer is scheduled to elapse
 199         within a time window starting with the time specified in
 200         <varname>OnCalendar=</varname>,
 201         <varname>OnActiveSec=</varname>,
 202         <varname>OnBootSec=</varname>,
 203         <varname>OnStartupSec=</varname>,
 204         <varname>OnUnitActiveSec=</varname> or
 205         <varname>OnUnitInactiveSec=</varname> and ending the time
 206         configured with <varname>AccuracySec=</varname> later. Within
 207         this time window, the expiry time will be placed at a
 208         host-specific, randomized, but stable position that is
 209         synchronized between all local timer units. This is done in
 210         order to optimize power consumption to suppress unnecessary
 211         CPU wake-ups. To get best accuracy, set this option to
 212         1us. Note that the timer is still subject to the timer slack
 213         configured via
 214         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
 215         <varname>TimerSlackNSec=</varname> setting. See
 216         <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 217         for details. To optimize power consumption, make sure to set
 218         this value as high as possible and as low as
 219         necessary.</para></listitem>
 220       </varlistentry>
 221
 222       <varlistentry>
 223         <term><varname>RandomizedDelaySec=</varname></term>
 224
 225         <listitem><para>Delay the timer by a randomly selected, evenly
 226         distributed amount of time between 0 and the specified time
 227         value. Defaults to 0, indicating that no randomized delay
 228         shall be applied. Each timer unit will determine this delay
 229         randomly each time it is started, and the delay will simply be
 230         added on top of the next determined elapsing time. This is
 231         useful to stretch dispatching of similarly configured timer
 232         events over a certain amount time, to avoid that they all fire
 233         at the same time, possibly resulting in resource
 234         congestion. Note the relation to
 235         <varname>AccuracySec=</varname> above: the latter allows the
 236         service manager to coalesce timer events within a specified
 237         time range in order to minimize wakeups, the former does the
 238         opposite: it stretches timer events over a time range, to make
 239         it unlikely that they fire simultaneously. If
 240         <varname>RandomizedDelaySec=</varname> and
 241         <varname>AccuracySec=</varname> are used in conjunction, first
 242         the randomized delay is added, and then the result is
 243         possibly further shifted to coalesce it with other timer
 244         events happening on the system. As mentioned above
 245         <varname>AccuracySec=</varname> defaults to 1min and
 246         <varname>RandomizedDelaySec=</varname> to 0, thus encouraging
 247         coalescing of timer events. In order to optimally stretch
 248         timer events over a certain range of time, make sure to set
 249         <varname>RandomizedDelaySec=</varname> to a higher value, and
 250         <varname>AccuracySec=1us</varname>.</para></listitem>
 251       </varlistentry>
 252
 253       <varlistentry>
 254         <term><varname>Unit=</varname></term>
 255
 256         <listitem><para>The unit to activate when this timer elapses.
 257         The argument is a unit name, whose suffix is not
 258         <literal>.timer</literal>. If not specified, this value
 259         defaults to a service that has the same name as the timer
 260         unit, except for the suffix. (See above.) It is recommended
 261         that the unit name that is activated and the unit name of the
 262         timer unit are named identically, except for the
 263         suffix.</para></listitem>
 264       </varlistentry>
 265
 266       <varlistentry>
 267         <term><varname>Persistent=</varname></term>
 268
 269         <listitem><para>Takes a boolean argument. If true, the time
 270         when the service unit was last triggered is stored on disk.
 271         When the timer is activated, the service unit is triggered
 272         immediately if it would have been triggered at least once
 273         during the time when the timer was inactive. This is useful to
 274         catch up on missed runs of the service when the machine was
 275         off. Note that this setting only has an effect on timers
 276         configured with <varname>OnCalendar=</varname>. Defaults
 277         to <varname>false</varname>.
 278         </para></listitem>
 279       </varlistentry>
 280
 281       <varlistentry>
 282         <term><varname>WakeSystem=</varname></term>
 283
 284         <listitem><para>Takes a boolean argument. If true, an elapsing
 285         timer will cause the system to resume from suspend, should it
 286         be suspended and if the system supports this. Note that this
 287         option will only make sure the system resumes on the
 288         appropriate times, it will not take care of suspending it
 289         again after any work that is to be done is finished. Defaults
 290         to <varname>false</varname>.</para></listitem>
 291       </varlistentry>
 292
 293       <varlistentry>
 294         <term><varname>RemainAfterElapse=</varname></term>
 295
 296         <listitem><para>Takes a boolean argument. If true, an elapsed
 297         timer will stay loaded, and its state remains queriable. If
 298         false, an elapsed timer unit that cannot elapse anymore is
 299         unloaded. Turning this off is particularly useful for
 300         transient timer units that shall disappear after they first
 301         elapse. Note that this setting has an effect on repeatedly
 302         starting a timer unit that only elapses once: if
 303         <varname>RemainAfterElapse=</varname> is on, it will not be
 304         started again, and is guaranteed to elapse only once. However,
 305         if <varname>RemainAfterElapse=</varname> is off, it might be
 306         started again if it is already elapsed, and thus be triggered
 307         multiple times. Defaults to
 308         <varname>yes</varname>.</para></listitem>
 309       </varlistentry>
 310     </variablelist>
 311   </refsect1>
 312
 313   <refsect1>
 314       <title>See Also</title>
 315       <para>
 316         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 317         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 318         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 319         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 320         <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 321         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 322         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 323         <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 324       </para>
 325   </refsect1>
 326
 327 </refentry>