Ravindra BagaleCourses & study guides

Chapter 4: Linux Advanced Commands

4.9 Scheduling tasks: cron

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In short: cron runs commands automatically on a schedule.

cron runs commands automatically on a schedule. Edit your table with crontab -e, list with crontab -l.

 ┌──────── minute (0-59)
 │ ┌────── hour (0-23)
 │ │ ┌──── day of month (1-31)
 │ │ │ ┌── month (1-12)
 │ │ │ │ ┌ day of week (0-7, 0 and 7 = Sunday)
 │ │ │ │ │
 * * * * *  command
Schedule Meaning
*/5 * * * * Every 5 minutes
0 2 * * * Every day at 02:00
30 9 * * 1-5 09:30 Monday to Friday
0 0 1 * * Midnight on the 1st of every month
@reboot Once at every boot

Now open your terminal and try this with me. Example — back up the website every night at 1:30 AM. Run crontab -e and add this line:

30 1 * * * tar -czf $HOME/backups/site-$(date +\%F).tar.gz /var/www/html

(% has a special meaning in crontab, so it is written as \%. Create the folder first with mkdir -p ~/backups.)

cron on Amazon Linux 2023

AL2023 does not ship cron by default. Install it: sudo yum install -y cronie, then sudo service crond start and sudo systemctl enable crond. (systemd timers are the modern alternative.) The server clock is UTC by default — set India time with sudo timedatectl set-timezone Asia/Kolkata.

Ravindra Bagale's Tip

Before scheduling any cron job, run the exact command manually first and check it works. Cron runs with a very small PATH and no interactive shell, so always use full paths (/usr/bin/tar, /home/ubuntu/backup.sh) and redirect output to a log file: >> /home/ubuntu/backup.log 2>&1. Keep this in mind!