Run a Job Every 45 Seconds (Cron Can't

Run a Job Every 45 Seconds

Standard cron cannot schedule sub-minute intervals — its finest resolution is one minute. To run every 45 seconds, use a short shell loop or a systemd timer with OnUnitActiveSec=45s.

Cron expression (minute hour day-of-month month day-of-week)

Why cron can't do it

The smallest cron field is minutes, so the fastest schedule is * * * * * (once a minute). There is no seconds field in standard cron.

Workaround 1: a self-scheduling loop

* * * * * /path/run.sh
* * * * * sleep 45; /path/run.sh

The second line waits 45 s, so together you get runs near 0 s and 45 s each minute — crude but works.

Workaround 2: systemd timer (recommended)

[Timer]
OnBootSec=0
OnUnitActiveSec=45s

This fires the service every 45 seconds precisely and survives reboots.

FAQ

Can cron run every 45 seconds?

No. Cron's minimum interval is one minute. Use a systemd timer with OnUnitActiveSec=45s or a sleep-based loop instead.

What is the fastest cron interval?

Once per minute, using * * * * *. Cron has no seconds field.

How do I run every 45 seconds reliably?

Use a systemd timer: OnUnitActiveSec=45s fires the unit every 45 seconds and restarts after reboot.