CRON Expression Explainer

What is a CRON Expression?

A CRON expression is a compact string of five fields that defines a schedule for automated tasks on Unix and Linux systems. Cron jobs are used everywhere — from running database backups at midnight to sending weekly report emails. The expression format is: minute hour day-of-month month day-of-week.

Each field accepts specific values, ranges (1-5), lists (1,3,5), steps (*/15), and the wildcard * (meaning “any”). Understanding this syntax lets you schedule tasks with precision, from “every 5 minutes” to “at 3:30 AM on the first Monday of every quarter.”

Common CRON Schedules

* * * * * — Every minute. Useful for health checks or queue workers.
0 * * * * — Every hour, on the hour. Good for hourly data syncs.
0 0 * * * — Daily at midnight. The classic schedule for backups and cleanup tasks.
0 9 * * 1-5 — Every weekday at 9:00 AM. Perfect for business-hours notifications.
0 12 * * 0 — Every Sunday at noon. Weekly report or digest schedule.
0 0 1 * * — First day of every month at midnight. Monthly billing or reporting.
*/15 * * * * — Every 15 minutes. Common for monitoring and polling tasks.

Frequently Asked Questions

What is a cron job?

A cron job is a scheduled task on Unix/Linux that runs automatically at times defined by a cron expression. It's the standard way to automate recurring tasks like backups, reports, and maintenance scripts.

What are the 5 fields in a cron expression?

Minute (0–59), Hour (0–23), Day of Month (1–31), Month (1–12), and Day of Week (0–7, where 0 and 7 are both Sunday).

How do I run a cron job every 5 minutes?

Use */5 * * * *. The */5 in the minute field means every 5th minute. The asterisks mean every hour, day, month, and weekday.

What does */15 mean in cron?

*/15 means “every 15th value” of that field. In the minute field, it triggers at :00, :15, :30, and :45 every hour.

Is Sunday 0 or 7 in cron?

Both. Sunday can be 0 or 7 in standard cron. Monday is 1 through Saturday which is 6. This dual representation exists for historical compatibility.