systemd timers replacing cron - the modern scheduler

systemd timers are the modern replacement for cron. They integrate with systemd’s logging, dependencies, and resource control. This lesson covers timer syntax and migration from cron.

A timer unit

# /etc/systemd/system/backup.service
[Unit]
Description=Nightly backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

# /etc/systemd/system/backup.timer
[Unit]
Description=Run backup nightly at 02:30

[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true
Unit=backup.service

[Install]
WantedBy=timers.target

Enable and start:

sudo systemctl daemon-reload
sudo systemctl enable --now backup.timer

The timer fires the service at the specified time.

OnCalendar syntax

OnCalendar= uses a calendar expression:

OnCalendar=Mon..Fri 09:00           # weekdays at 09:00
OnCalendar=*-*-* 02:30:00            # every day at 02:30
OnCalendar=*:0/15                    # every 15 minutes
OnCalendar=Sun 03:00:00              # Sunday at 03:00
OnCalendar=2026-12-31 23:59:00       # specific date and time

man systemd.time has the full syntax. Test with systemd-analyze calendar "Mon..Fri 09:00".

Monotonic timers

For “every X minutes after boot” or “1 hour after the service started”:

[Timer]
OnBootSec=5min
OnUnitActiveSec=1h
Unit=myservice.service

OnBootSec=5min triggers 5 minutes after boot. OnUnitActiveSec=1h triggers 1 hour after the unit was last active. Useful for “restart every hour”.

Persistent=true

With Persistent=true, the timer catches up on missed runs:

# If the machine was off at 02:30, run at next boot
Persistent=true

Without it, missed runs are dropped. Use Persistent=true for jobs that must run daily.

Advantages over cron

• Logging: timer fires are logged in journald. cron only emails.
• Dependencies: timer units can depend on other units (e.g. require network-online.target).
• Resource control: services inherit systemd’s resource limits.
• Catchup: Persistent=true catches missed runs.
• Failure handling: failures trigger dependencies, not silent retries.

Migrate from cron

For each cron job:

1. Create a .service unit that runs the command.
2. Create a .timer unit that schedules the service.
3. systemctl daemon-reload
4. systemctl enable --now <name>.timer
5. Remove from crontab (crontab -e, delete the line).

Test in staging first.

Knowledge check