Systemd Timer Unit

Replacing Cron with systemd Timers

Systemd timers are a modern alternative to cron that provide better logging, dependency management, and reliability. A timer consists of two files: a .timer unit and a matching .service unit.

The Service Unit (runs the actual task)

[Unit]
Description=Daily Database Backup

[Service]
Type=oneshot
ExecStart=/opt/scripts/backup-database.sh
User=backup
Group=backup
StandardOutput=journal
StandardError=journal

The Timer Unit (schedules when to run)

[Unit]
Description=Run Database Backup Daily at 2 AM

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true
RandomizedDelaySec=300

[Install]
WantedBy=timers.target

OnCalendar Syntax

The OnCalendar= directive uses a flexible format:

Expression	Meaning
*-*-* 02:00:00	Every day at 2:00 AM
Mon *-*-* 09:00:00	Every Monday at 9:00 AM
*-*-01 00:00:00	First day of every month
*-*-* *:00/15:00	Every 15 minutes
weekly	Once a week (Monday 00:00)
daily	Once a day (00:00)
hourly	Once an hour (:00)

You can test expressions with systemd-analyze calendar "Mon *-*-* 09:00:00".

Persistent Timers

Persistent=true is crucial for reliability. If the system was powered off when the timer was supposed to fire, systemd will run the task immediately upon next boot. Without this, missed runs are simply skipped.

RandomizedDelaySec

RandomizedDelaySec=300 adds a random delay of up to 5 minutes. This prevents the "thundering herd" problem when multiple servers run the same timer — they won't all hit the database backup server simultaneously.

Advantages Over Cron

Feature	Cron	Systemd Timers
Logging	Custom (redirect to file)	Automatic (journal)
Missed runs	Lost	Persistent=true catches up
Dependencies	None	After=, Requires=
Resource limits	None	MemoryMax=, CPUQuota=
Calendar expressions	5-field	Human-readable
Monitoring	Check crontab	systemctl list-timers
Email on failure	MAILTO=	OnFailure= integration

Enabling the Timer

sudo systemctl enable --now backup-database.timer
systemctl list-timers    # Verify it's scheduled