Introduction
Cron is a time-based job scheduler in Unix-like operating systems. It enables you to automate tasks by running commands or scripts at specified intervals - hourly, daily, weekly, or any custom schedule you need. Understanding cron expressions is essential for DevOps, system administration, and automated task scheduling in modern applications.
Understanding the Five Fields
A cron expression consists of five fields separated by spaces:
* * * * * │ │ │ │ │ │ │ │ │ └── Day of week (0=Sunday, 1=Monday, ..., 6=Saturday) │ │ │ └─────── Month (1-12) │ │ └──────────── Day of month (1-31) │ └───────────────── Hour (0-23) └────────────────────── Minute (0-59)
Special Characters
* - Asterisk (Any Value)
Matches any possible value. * * * * * runs every minute.
, - Comma (Multiple Values)
Specifies multiple values. 0,15,30,45 * * * * runs at minutes 0, 15, 30, 45.
- - Hyphen (Range)
Defines a range. 0 9-17 * * * runs hourly from 9 AM to 5 PM.
/ - Slash (Step)
Defines intervals. */5 * * * * runs every 5 minutes.
Common Schedules
| Expression | Meaning |
|---|---|
| * * * * * | Every minute |
| 0 * * * * | Every hour |
| 0 0 * * * | Daily at midnight |
| 0 0 * * 0 | Weekly (Sunday midnight) |
| 0 0 1 * * | Monthly (1st day) |
| */5 * * * * | Every 5 minutes |
| 0 9 * * 1-5 | 9 AM weekdays |
| 0 9,17 * * * | 9 AM and 5 PM daily |
| 0 0 1 1 * | Yearly (January 1st) |
Special Shortcuts
Some cron implementations support these shortcuts:
@yearly@monthly@weekly@daily@hourly@rebootRelated Tools
Conclusion
Cron expressions are essential for automating tasks in Unix systems. Understanding the five fields and special characters lets you create any schedule you need. Start with simple patterns like daily (0 0 * * *) or hourly (0 * * * *) and gradually use more complex expressions. Use our Cron Generator to build and verify expressions visually.