Textbook of Linux

crontab manages a user's cron table: the list of scheduled commands that run at specified times. Cron has been the standard Unix scheduler since the 1970s, though systemd timers are displacing it on modern Linux.

crontab -e                       # edit your crontab
crontab -l                       # list current entries
crontab -r                       # remove your crontab
sudo crontab -u alice -e         # edit alice's crontab

Each line is minute hour day month weekday command:

0 3 * * *      /usr/local/bin/backup.sh        # daily at 03:00
*/15 * * * *   /usr/bin/check-queue             # every 15 minutes
0 0 * * 0      /usr/bin/weekly-report           # Sundays at midnight
@reboot        /opt/app/start.sh                # at boot

System-wide scheduled jobs live in /etc/cron.d/, /etc/cron.daily/, etc. Output from cron jobs is mailed to the user by default; capture it to a file with >> to avoid a mailbox full of cron noise.