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A cron expression is a compact string of five fields that tells Unix-like systems when to run a job: minute, hour, day-of-month, month, and day-of-week. It powers crontab on Linux servers, CI pipelines, Kubernetes CronJobs, and countless schedulers.
| Expression | Meaning |
|---|---|
* * * * * | Every minute |
*/5 * * * * | Every 5 minutes |
0 * * * * | Every hour, on the hour |
0 9 * * * | Daily at 09:00 |
0 9 * * 1-5 | Weekdays at 09:00 |
30 8 1 * * | 08:30 on the 1st of every month |
0 0 * * 0 | Midnight every Sunday |
Day-of-month vs weekday: if both are restricted (not *), most cron implementations run the job when either matches — a classic surprise. Timezone: cron runs in the server's timezone; a 9 AM job on a UTC server fires at 5 PM in Shanghai. Missed runs: cron does not catch up on jobs missed during downtime — use anacron or a scheduler with persistence for that.