systemd/Timers
Related articles
Timers are systemd unit files whose name ends in .timer
that control .service
files or events. Timers have the ability to be an alternative to cron (read #As a cron replacement). Timers have built-in support for calendar time events, monotonic time events, and have the ability to run asynchronously.
Contents
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
andOnActiveSec
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)
man page. The argument syntax for calendar events and time spans is defined on the systemd.time(7)
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
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
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 or impossible 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 usingOnFailure=
.
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-user1@.service
[Unit] Description=status email for %I to user1 [Service] Type=oneshot ExecStart=/usr/local/bin/systemd-email user1@mailhost %i User=nobody Group=systemd-journal
First notice that the unit to send email about is an instance parameter, so this one service can be used to send email for many other units. However the recipient is hard-coded (since unit templates can only take a single parameter) so you will need to create multiple services if you want to send emails to different sets of recipients. At this point you should test the service to verify that you can receive the emails:
# systemctl start status-email-user1@dbus.service
Then simply add OnFailure=status-email-user1@%n.service
to the [Unit]
section of any unit you want emails for. %n
passes the unit's name to the template.
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
and RANDOM_DELAY
features.
If you like crontabs just because they provide a unified view of all scheduled jobs, systemctl
can provide this. See #Management.
See also
- systemd.timer man page on freedesktop.org
- Fedora Project wiki page on systemd calendar timers
- Gentoo wiki section on systemd timer services
- systemd-cron-next — tool to generate timers/services from crontab and anacrontab files
- systemd-cron — provides systemd units to run cron scripts; using systemd-crontab-generator to convert crontabs