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.
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:
OnActiveSecare 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.
.timer file, a matching
.service file exists (e.g.
.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
$ 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
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
A timer which will start 15 minutes after boot and again every week while the system is running.
[Unit] Description=Run foo weekly and on boot [Timer] OnBootSec=15min OnUnitActiveSec=1w [Install] WantedBy=timers.target
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:
[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.
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
- 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.
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
systemctlcommands. Compare that to adding a single line to a crontab.
- Emails: there is no built-in equivalent to cron's
MAILTOfor sending emails on job failure. See the next section for an example of setting up an equivalent using
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
#!/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:
[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 email@example.com
Then simply edit the service you want emails for and add
OnFailure=status-email-user1@%n.service to the
%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.
If you like crontabs just because they provide a unified view of all scheduled jobs,
systemctl can provide this. See #Management.
- 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