CRON Basics

CRON is a time-based job scheduler in Unix-like OS. It allows users to run scripts or commands automatically at specified times and intervals.

CRON Expression Format

Each CRON job uses a 5-field format:

* * * * * command\_to\_run
| | | | |
| | | | └── Day of the week (0-7) (Sunday = 0 or 7)
| | | └──── Month (1-12)
| | └────── Day of the month (1-31)
| └──────── Hour (0-23)
└────────── Minute (0-59)

Examples

Schedule	CRON Expression	Description
Every minute	`* * * * *`	Runs every minute
Every day at 7 AM	`0 7 * * *`	Runs daily at 07:00
Every Monday at 9 AM	`0 9 * * 1`	Runs every Monday at 09:00
First day of month	`0 0 1 * *`	Runs monthly at midnight on the 1st
Every 15 minutes	`/15 * * *`	Runs every 15 minutes

Special Strings

CRON also supports special time strings:

String	Equivalent
`@reboot`	Run once at startup
`@daily`	`0 0 * * *`
`@weekly`	`0 0 * * 0`
`@monthly`	`0 0 1 * *`
`@yearly`	`0 0 1 1 *`

Notes

Use absolute paths in the commands (e.g., /usr/bin/python3 /home/user/script.py)
CRON uses the system’s timezone, verify it with timedatectl
Test schedule using crontab.guru