systemd/Timers

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Timers are systemd unit files whose name ends in .timer that control .service files or events. Timers can be used as an alternative to cron (read #As a cron replacement). Timers have built-in support for calendar time events, monotonic time events, and can be run asynchronously.

Timer units

Timers are systemd unit files with a suffix of .timer. Timers are like other unit configuration files and are loaded from the same paths but include a [Timer] section. The [Timer] section defines when and how the timer activates. Timers are defined as one of two types:

  • Monotonic timers activate after a time span relative to a varying starting point. There are number of different monotonic timers but all have the form of: OnTypeSec=. OnBootSec and OnActiveSec are common monotonic timers.
  • Realtime timers (a.k.a. wallclock timers) activate on a calendar event (like cronjobs). The option OnCalendar= is used to define them.

For a full explanation of timer options, see the systemd.timer(5)[1] man page. The argument syntax for calendar events and time spans is defined on the systemd.time(7)[2] man page.

Service unit

For each .timer file, a matching .service file exists (e.g. foo.timer and foo.service). The .timer file activates and controls the .service file. The .service does not require an [Install] section as it is the timer units that are enabled. If necessary, it is possible to control a differently-named unit using the Unit= option in the timer's [Timer] section.

Management

To use a timer unit enable and start it like any other unit (remember to add the .timer suffix). To view all started timers, run:

$ systemctl list-timers
NEXT                          LEFT        LAST                          PASSED     UNIT                         ACTIVATES
Thu 2014-07-10 19:37:03 CEST  11h left    Wed 2014-07-09 19:37:03 CEST  12h ago    systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
Fri 2014-07-11 00:00:00 CEST  15h left    Thu 2014-07-10 00:00:13 CEST  8h ago     logrotate.timer              logrotate.service
Note:
  • To list all timers (including inactive), use systemctl list-timers --all.
  • The status of a service started by a timer will likely be inactive unless it is currently being triggered.
  • If a timer gets out of sync, it may help to delete its stamp-* file in /var/lib/systemd/timers. These are zero length files which mark the last time each timer was run. If deleted, they will be reconstructed on the next start of their timer.

Example

No changes to service unit files are needed to schedule them with a timer. The following example schedules foo.service to be run with a corresponding timer called foo.timer.

Monotonic timer

A timer which will start 15 minutes after boot and again every week while the system is running.

/etc/systemd/system/foo.timer
[Unit]
Description=Run foo weekly and on boot

[Timer]
OnBootSec=15min
OnUnitActiveSec=1w 

[Install]
WantedBy=timers.target

Realtime timer

A timer which starts once a week (at 12:00am on Monday). It starts once immediately if it missed the last start time (option Persistent=true), for example due to the system being powered off:

/etc/systemd/system/foo.timer
[Unit]
Description=Run foo weekly

[Timer]
OnCalendar=weekly
Persistent=true     
 
[Install]
WantedBy=timers.target

The format controlling OnCalendar events uses the following format when more specific dates and times are required: DayOfWeek Year-Month-Day Hour:Minute:Second. An asterisk may be used to specify any value and commas may be used to list possible values. In this example the service is run the first four days of each month at 12:00 PM, but only if that day is also on a Monday or a Tuesday. More information is available in man systemd.time.

OnCalendar=Mon,Tue *-*-01,02,03,04 12:00:00
Tip: Special event expressions like daily and weekly refer to specific start times and thus any timers sharing such calendar events will start simultaneously. Timers sharing start events can cause poor system performance if the timers' services compete for system resources. The RandomizedDelaySec option avoids this problem by randomly staggering the start time of each timer. See systemd.timer (5).

Transient .timer units

One can use systemd-run to create transient .timer units. That is, one can set a command to run at a specified time without having a service file. For example the following command touches a file after 30 seconds:

# systemd-run --on-active=30 /bin/touch /tmp/foo

One can also specify a pre-existing service file that does not have a timer file. For example, the following starts the systemd unit named someunit.service after 12.5 hours have elapsed:

# systemd-run --on-active="12h 30m" --unit someunit.service

See man systemd-run for more information and examples.

As a cron replacement

Although cron is arguably the most well-known job scheduler, systemd timers can be an alternative.

Benefits

The main benefits of using timers come from each job having its own systemd service. Some of these benefits are:

  • Jobs can be easily started independently of their timers. This simplifies debugging.
  • Each job can be configured to run in a specific environment (see the systemd.exec(5) man page).
  • Jobs can be attached to cgroups.
  • Jobs can be set up to depend on other systemd units.
  • Jobs are logged in the systemd journal for easy debugging.

Caveats

Some things that are easy to do with cron are difficult to do with timer units alone.

  • Complexity: to set up a timed job with systemd you create two files and run a couple systemctl commands. Compare that to adding a single line to a crontab.
  • Emails: there is no built-in equivalent to cron's MAILTO for sending emails on job failure. See the next section for an example of setting up an equivalent using OnFailure=.

MAILTO

You can set up systemd to send an e-mail when a unit fails - much like Cron does with MAILTO. First you need two files: an executable for sending the mail and a .service for starting the executable. For this example, the executable is just a shell script using sendmail:

/usr/local/bin/systemd-email
#!/bin/bash

/usr/bin/sendmail -t <<ERRMAIL
To: $1
From: systemd <root@$HOSTNAME>
Subject: $2
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=UTF-8

$(systemctl status --full "$2")
ERRMAIL

Whatever executable you use, it should probably take at least two arguments as this shell script does: the address to send to and the unit file to get the status of. The .service we create will pass these arguments:

/etc/systemd/system/status-email-user@.service
[Unit]
Description=status email for %I to user

[Service]
Type=oneshot
ExecStart=/usr/local/bin/systemd-email address %i
User=nobody
Group=systemd-journal

Where user is the user being emailed and address is that user's email address. Although the recipient is hard-coded, the unit file to report on is passed as an instance parameter, so this one service can send email for many other units. At this point you can start status-email-user@dbus.service to verify that you can receive the emails.

Then simply edit the service you want emails for and add OnFailure=status-email-user@%n.service to the [Unit] section. %n passes the unit's name to the template.

Note:
  • If you set up SSMTP security according to SSMTP#Security the user nobody will not have access to /etc/ssmtp/ssmtp.conf, and the systemctl start status-email-user@dbus.service command will fail. One solution is to use root as the User in the status-email-user@.service unit.
  • If you try to use mail -s somelogs address in your email script, mail will fork and systemd will kill the mail process when it sees your script exit. Make the mail non-forking by doing mail -Ssendwait -s somelogs address.

Using a crontab

Several of the caveats can be worked around by installing a package that parses a traditional crontab to configure the timers. systemd-cron-nextAUR and systemd-cronAUR are two such packages. These can provide the missing MAILTO feature.

If you like crontabs just because they provide a unified view of all scheduled jobs, systemctl can provide this. See #Management.

See also

https://github.com/systemd-cron/systemd-cron-next || systemd-cron-nextAUR
  • systemd-cron — provides systemd units to run cron scripts; using systemd-crontab-generator to convert crontabs
https://github.com/systemd-cron/systemd-cron || systemd-cronAUR