systemd/Timers

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 which defines when and how the timer activates. Timers are defined as one of two types:

Realtime timers (a.k.a. wallclock timers) activate on a calendar event, the same way that cronjobs do. The option OnCalendar= is used to define them.
Monotonic timers activate after a time span relative to a varying starting point. They stop if the computer is temporarily suspended or shut down. There are number of different monotonic timers but all have the form: OnTypeSec=. Common monotonic timers include OnBootSec and .

For a full explanation of timer options, see the . The argument syntax for calendar events and time spans is defined in .

Service units

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 section as it is the timer units that are enabled. If necessary, it is possible to control a differently-named unit using the 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:

Examples

A service unit file can be scheduled with a timer out-of-the-box. The following examples schedule 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.

Realtime timer

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

When more specific dates and times are required, events uses the following format:

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. Two values separated by .. indicate a contiguous range.

In the below example the service is run the first four days of each month at 12:00 PM, but only if that day is a Monday or a Tuesday.

OnCalendar=Mon,Tue *-*-01..04 12:00:00

To run a service on the first Saturday of every month, use:

OnCalendar=Sat *-*-1..7 18:00:00

When using the part, at least one weekday has to be specified. If you want something to run every day at 4am, use:

OnCalendar=*-*-* 4:00:00

To run a service at different times, may be specified more than once. In the example below, the service runs at 22:30 on weekdays and at 20:00 on weekends.

OnCalendar=Mon..Fri 22:30
OnCalendar=Sat,Sun 20:00

More information is available in .

Tip:

OnCalendar time specifications can be tested in order to verify their validity and to calculate the next time the condition would elapse when used on a timer unit file with the calendar option of the systemd-analyze utility. For example, one can use systemd-analyze calendar weekly or systemd-analyze calendar "Mon,Tue *-*-01..04 12:00:00".
The faketime command is especially useful to test various scenarios with the above command; it comes with the libfaketime package.
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 in the [Timer] section avoids this problem by randomly staggering the start time of each timer. See systemd.timer(5).
Add the option AccuracySec=1us to the [Timer] section, to avoid the inaccuracy of the 1m default value of AccuracySec. Also see systemd.timer(5).

Transient timer units

One can use 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 after 12.5 hours have elapsed:

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

See 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 ).
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:

Creation: to set up a timed job with systemd you need to create two files and run commands, compared to adding a single line to a crontab.
Emails: there is no built-in equivalent to cron's for sending emails on job failure. See the next section for an example of setting up a similar functionality using .

Also note that user timer units will only run during an active user login session by default. However, lingering can enable services to run at boot even when the user has no active login session.

MAILTO

You can set up systemd to send an e-mail when a unit fails. Cron sends mail to if the job outputs to stdout or stderr, but many jobs are setup to only output on error. 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 , which is in packages that provide smtp-forwarder.

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:

Where is the user being emailed and 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 to the section. 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. and systemd-cron^AUR are two such packages. These can provide the missing feature.

Also, like with crontabs, a unified view of all scheduled jobs can be obtained with . See #Management.

gollark: Most systems don't have that much.

gollark: But mostly stuff is too big.

gollark: Mostly miniature Linux distros.

gollark: There are a *few* OSes which run entirely in RAM.

gollark: OSes already do tons of in-memory caching.