Overview¶

Cron is a time-based job scheduler in Linux used to automate repetitive tasks such as:

Running scripts
System maintenance
Backups
Log rotation

It allows commands or scripts to run automatically at specified times or intervals.

Cron Job Format¶

A cron job consists of 6 fields:

* * * * * command_to_be_executed

Structure¶

*   *   *   *   *   command
|   |   |   |   |
|   |   |   |   +---- Day of week (0–6) (Sunday=0)
|   |   |   +-------- Month (1–12)
|   |   +------------ Day of month (1–31)
|   +---------------- Hour (0–23)
+-------------------- Minute (0–59)

Field Explanation¶

Field	Description
Minute	0–59
Hour	0–23
Day of Month	1–31
Month	1–12
Day of Week	0–6

Special Characters¶

Symbol	Meaning
`*`	Any value
`,`	Multiple values
`-`	Range
`/`	Step values

Examples¶

Run Every Minute¶

* * * * * /path/to/script.sh

Run Every 5 Minutes¶

*/5 * * * * /path/to/script.sh

Run Every Hour¶

0 * * * * /path/to/script.sh

Run Daily at Midnight¶

0 0 * * * /path/to/script.sh

Run at 2 AM Daily¶

0 2 * * * /path/to/script.sh

Run Every Weekday at 5 PM¶

0 17 * * 1-5 /path/to/script.sh

Run Every Sunday at 9:30 AM¶

30 9 * * 0 /path/to/script.sh

Run First Day of Every Month at 8 AM¶

0 8 1 * * /path/to/script.sh

Special Strings¶

Instead of writing full cron expressions, you can use shortcuts:

String	Equivalent
`@reboot`	Run at startup
`@hourly`	Every hour
`@daily`	Every day
`@weekly`	Every week
`@monthly`	Every month
`@yearly`	Every year

Examples¶

Run Script at Startup¶

@reboot /path/to/script.sh

Run Every Hour¶

@hourly /path/to/script.sh

Run Daily¶

@daily /path/to/script.sh

Managing Cron Jobs¶

Edit Cron Jobs¶

crontab -e

Explanation

Opens crontab editor
Add or modify scheduled tasks

List Cron Jobs¶

crontab -l

Remove All Cron Jobs¶

crontab -r

Warning

Deletes all scheduled jobs permanently

Practical Examples¶

Backup Script Daily¶

30 2 * * * /home/user/backup.sh

Clean Logs Every Week¶

0 0 * * 0 /home/user/cleanup.sh

Run Script Every 15 Minutes¶

*/15 * * * * /home/user/script.sh

Output Handling¶

Redirect Output to File¶

* * * * * /script.sh >> /var/log/script.log 2>&1

Discard Output¶

* * * * * /script.sh > /dev/null 2>&1

Important Notes¶

Always use absolute paths in cron jobs
Cron runs with limited environment variables
Test scripts before scheduling
Monitor logs for failures
Be careful with overlapping jobs

Best Practices¶

Use comments in crontab
Schedule heavy jobs during low usage
Backup crontab entries
Use logging for debugging
Avoid running multiple instances of the same job

Summary Table¶

Command	Purpose
`crontab -e`	Edit cron jobs
`crontab -l`	List cron jobs
`crontab -r`	Remove all jobs
`* * * * *`	Schedule format
`@daily`	Run daily
`@reboot`	Run at startup

Conclusion¶

Cron is a powerful automation tool in Linux that helps:

Schedule repetitive tasks
Automate system maintenance
Improve efficiency

Understanding cron syntax and best practices allows you to build reliable and automated workflows in Linux environments.