Timer unit files are a type of systemd file that the systemctl utility uses to schedule tasks, similar to the cron utility that uses crontab and other cron jobs for the same purpose. Note that the cron daemon runs as a service within systemd, so timer units are preferred because they remove a layer of added processing and offer much more utility and more granular configuration than is available in the cron service.

Typically, packages that use specific services to function in the system include their own systemd timer unit files. Thus, when these packages are installed with Oracle Linux, the timer unit files are automatically included. You can display with the timer files in the system with the following command:

systemctl list-unit-files --type=timer

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. The file contains the following settings:

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.