Understanding Cron Expressions
Cron Field Positions
The cron daemon, originally from UNIX, uses a simple 5-field scheduling syntax defined in POSIX 1003.2. Each field represents a unit of time: minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (0–7, where both 0 and 7 represent Sunday). The scheduler checks all running jobs every minute and executes those whose fields match the current time.
Cron Expression Examples
| Expression | Description |
|---|---|
| 0 0 * * * | Run at midnight every day |
| */10 * * * * | Run every 10 minutes |
| 0 9-17 * * 1-5 | Run hourly during business hours (Mon–Fri) |
| 0 0 15,30 * * | Run at midnight on the 15th and 30th of each month |
| 30 4 1 * * | Run at 4:30 AM on the 1st of every month |
| 0 0 * * 1 | Run at midnight every Monday |
| @reboot | Run at system startup (not supported everywhere) |
| @daily | Equivalent to 0 0 * * * |
| @weekly | Equivalent to 0 0 * * 0 |
| @monthly | Equivalent to 0 0 1 * * |