Unix cron

Last verified: 2026-05-15

The five fields

Standard Unix cron is a five-field schedule: minute hour day-of-month month day-of-week. Every field accepts a single value, a range, a list, a step, or the wildcard *. Day-of-week 0 and 7 both mean Sunday. Field order is fixed, whitespace separates fields, and there is no seconds column.

Special strings

GNU cron and most modern crons recognize a small set of aliases that expand to standard five-field expressions:

• @yearly / @annually — same as 0 0 1 1 *
• @monthly — same as 0 0 1 * *
• @weekly — same as 0 0 * * 0
• @daily / @midnight — same as 0 0 * * *
• @hourly — same as 0 * * * *
• @reboot — fires once when cron starts (not portable; not supported here)

Step values

Step notation */n means "every n units". */5 in the minute field fires at 00, 05, 10, …, 55. Steps work on ranges too: 9-17/2 in the hour field fires at 09, 11, 13, 15, 17.

The Linux gotchas

• PATH is minimal. The cron daemon runs jobs with a stripped environment. Use absolute paths or set PATH= at the top of your crontab.
• MAILTO="" suppresses email; without it, any output to stdout or stderr is mailed to the user, which fills mail spools fast.
• Use crontab -e rather than editing /etc/crontab directly so syntax is validated before saving.
• Locking. Long-running jobs can overlap. Wrap with flock (e.g. flock -n /tmp/my.lock my-command) to prevent re-entry.
• Daylight saving. A job scheduled at 02:30 * * * in a DST-aware zone will be skipped on the spring-forward day and run twice on the fall-back day, unless the cron daemon explicitly compensates.

When this dialect is the wrong choice

Unix five-field cron has no ?, no L (last day), no W (nearest weekday), no # (nth weekday of month), no seconds, and no year. If you need any of those, switch to Quartz, AWS EventBridge, or build the logic in application code with a more permissive scheduler.

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