6 Working With Systemd Timers

Use timer unit files in systemd to schedule tasks, in a similar way to the cron utility that uses crontab and other cron jobs for the same purpose.

Packages that use specific services to function in the system include their own systemd timer unit files. When those packages are installed on Oracle Linux 8, the timer unit files are automatically included. To display active timer unit files, run the following command:

systemctl list-unit-files --type=timer

Note:

The list of timer files differs depending on where Oracle Linux 8 is running, such as in an Oracle Cloud Infrastructure instance, a physical system, and so on.

Each timer unit file contains parameter settings that manage the schedule of a task. For example, the schedule for running dnf-makecache.service is set in the dnf-makecache.timer file. To review the contents of that file, run the following command:

systemctl cat dnf-makecache.timer

# /usr/lib/systemd/system/dnf-makecache.timer
[Unit]
Description=dnf makecache --timer
ConditionKernelCommandLine=!rd.live.image
# See comment in dnf-makecache.service
ConditionPathExists=!/run/ostree-booted
Wants=network-online.target

[Timer]
OnBootSec=10min
OnUnitInactiveSec=1h
RandomizedDelaySec=60m
Unit=dnf-makecache.service

[Install]
WantedBy=timers.target

The schedule information is specified under the [Timer] section. In the sample configuration, the dnf-makecache.service service is set to automatically run 10 minutes after the system is booted. The service then goes into idle mode for an hour, as specified by the OnUnitInactiveSec parameter. At the end of the hour, the service runs again. This cycle continues every hour indefinitely.

The RandomizedDelaySec setting provides a value limit for how much a run can be delayed beyond its schedule. In the example, the service is allowed to run one minute later than its schedule at the latest. This parameter is useful for preventing too many jobs that start at the same time on a specified schedule, which would otherwise risk overloading the resources.

OnCalendar is another useful parameter for task scheduling. Suppose that the parameter is set as follows:

OnCalendar=*:00/10

The *:00 indicates every hour at the top of the hour, while the /10 setting indicates 10 minutes. Therefore, the job is set to run hourly, at ten minutes past the top of the hour.

For a complete list of systemd timer unit file parameters for scheduling a job, see the systemd.timer(5) manual pages.

For more information about using systemd with Oracle Linux 8 systems, see Oracle Linux 8: Managing the System With systemd.