If you’ve ever needed to schedule something to run automatically on a server—like sending reports, backing up files, or running a script—you’ve probably heard the term cron job.
At the center of every cron job is something called a cron expression. But what is it exactly? Let’s break it down.
Cron expressions are used mostly on Linux and UNIX systems, and they’re handled by a built-in scheduler called cron. That’s where the name comes from.
A cron expression is a special line of text that tells a computer when to run a task. It’s like setting a reminder, but for your server.
What Does a Cron Expression Look Like?
Here’s a basic example of a cron expression:
0 9 * * *This means: run the task every day at 9:00 AM.
A cron expression usually has five parts (sometimes six, depending on the system), each representing a unit of time:
- Minute (0–59)
- Hour (0–23)
- Day of the month (1–31)
- Month (1–12 or JAN–DEC)
- Day of the week (0–7 or SUN–SAT)
Examples
Here are a few more examples to help you understand how cron expressions work:
* * * * *— Runs every minute0 * * * *— Runs every hour on the hour0 0 * * *— Runs once a day at midnight0 9 * * 1— Runs every Monday at 9 AM0 0 1 * *— Runs on the 1st day of every month at midnight
The asterisk * means “every value” — for example, * in the hour field means “every hour”.
Why Are Cron Expressions Useful?
Cron expressions help automate important tasks. Instead of running a command manually every day or hour, you can set it once and let the system handle it.
They’re often used for things like:
- Scheduled reports
- Database backups
- Cleaning up logs
- Sending reminder emails
- Running scripts or commands at set times
Final Thoughts
A cron expression is just a way to schedule tasks automatically. Once you understand how the parts fit together, you can use them to save time and automate your work.
Whether you’re new to cron or just need a quick way to check your schedule.