systemd/Timers - ArchWiki

# systemd/Timers

Related articles

- systemd
- systemd/User
- systemd/Services
- systemd FAQ
- cron

Timers are systemd unit files whose name ends in `.timer` that control `.service` files or events. Timers have the ability to be an alternate 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.

## 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)` 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

Note:

- 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 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

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. Consider manually staggering such timers using specific events e.g. `OnCalendar=Wed, 23:15`. See #Caveats.

## 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.

- Verbosity: 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=`.
- Random delay: there is no built-in equivalent to cron's `RANDOM_DELAY` for randomly spreading timers out across a given interval (see bug report, test results). Services which you do not want to run concurrently must have their timers manually set to minimize overlap.

  Note: The `AccuracySec` option is not useful for randomly staggering timers since it "is synchronized between all local timers units" (`systemd.timer(5)`). In other words, `AccuracySec` shifts all timer activation times by the same amount. For example, all `OnCalendar=daily` timer units with `AccuracySec=15m` will trigger the associated services at the same point in time between 00:00 and 00:15.

### 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-crontab-generator and systemd-cron from the AUR 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-crontab-generator — tool to generate timers/services from crontab and anacrontab files

  https://github.com/kstep/systemd-crontab-generator || systemd-crontab-generator

- systemd-cron — provides systemd units to run cron scripts; using systemd-crontab-generator to convert crontabs

  https://github.com/systemd-cron/systemd-cron || systemd-cron

Categories:

- Daemons and system services
- Boot process