Crontab in Linux: Examples and Useful Commands

Crontab is used in Linux to schedule tasks that run periodically at specified times or intervals. This tool is essential for automating repetitive tasks like backups, updates, and system monitoring.

Crontab Syntax

The general format for a crontab entry:

* * * * * command_to_be_executed
- - - - -
| | | | +----- Day of the week (0-7, both 0 and 7 represent Sunday)
| | | +------- Month (1-12)
| | +--------- Day of the month (1-31)
| +----------- Hour (0-23)
+------------- Minute (0-59)

Crontab Commands

# Viewing and Editing Crontab
crontab -l             # View current crontab
crontab -e             # Edit crontab in default editor

# Removing Crontab
crontab -r             # Remove entire crontab (delete all cron jobs)

# Crontab for Another User
sudo crontab -u username -e    # Edit crontab for a specific user
sudo crontab -u username -l    # View crontab for a specific user

Crontab Examples

Every minute:

* * * * * /path/to/command

Every hour at 0 minutes:

0 * * * * /path/to/command

Every day at midnight:

0 0 * * * /path/to/command

Every Sunday at 5:00 AM:

0 5 * * 0 /path/to/command

First day of every month at midnight:

0 0 1 * * /path/to/command

Every weekday at 3:00 PM:

0 15 * * 1-5 /path/to/command

Every 15 minutes:

*/15 * * * * /path/to/command

On the 1st and 15th of every month at 2:30 AM:

30 2 1,15 * * /path/to/command

Every 5 minutes during business hours (9 AM – 6 PM):

*/5 9-18 * * 1-5 /path/to/command

Backup every Sunday at midnight:

0 0 * * 0 /path/to/backup.sh

Advanced Crontab Usage

Redirect output to a file:

* * * * * /path/to/command >> /path/to/logfile.log 2>&1

Set environment variables:

PATH=/usr/bin:/bin:/usr/sbin:/sbin
SHELL=/bin/bash

Run a command as a specific user:

* * * * * sudo -u username /path/to/command

One-time cron job:

echo "0 6 10 1 * /path/to/command" | crontab -

Cron Logs

Check cron logs with:

cat /var/log/syslog | grep cron

Conclusion

Crontab is an essential tool in Linux for automating tasks that need to run on a recurring schedule. Understanding how to use cron expressions effectively is key to leveraging its full potential. This guide covers the basics and advanced techniques, including practical examples to help you automate a wide range of tasks on your Linux system.

By mastering crontab, you can significantly reduce manual tasks, improve system efficiency, and ensure important processes run on time without fail.