Simple definition
A systemd timer schedules services or other tasks on Linux systems that use systemd.
Technical definition
A .timer unit activates a corresponding .service unit either on a calendar schedule (OnCalendar) or after a relative interval (for example, OnBootSec or OnUnitActiveSec). The timer must be enabled and started, and its next execution can be checked with systemctl list-timers.
What is it used for ?
Schedule recurring maintenance, backups or periodic application jobs with systemd rather than using cron.
Practical example
For example, backup.timer can trigger backup.service every night. Check the service exit status and journal after the first scheduled run.
Common issues
- Timer enabled but service absent
- Timezone misunderstood
- UR not configured
Related terms
Key takeaway: Exact behavior and commands may vary depending on versions, products and architectures.