Scheduling in Linux: cron and crontab

cron runs commands on a schedule, and each user edits their own schedule with crontab -e.

Syntax

crontab -e   # minute hour day month weekday command

Common cases

crontab -e                   # open your schedule in an editor
crontab -l                   # show your jobs
# example schedule lines:
0 3 * * *   /path/backup.sh   # every day at 03:00
*/15 * * * * /path/check.sh   # every 15 minutes
0 9 * * 1   /path/report.sh   # Mondays at 09:00

How it works: the five time fields

A schedule line starts with five fields: minute, hour, day of month, month, day of week. A star * means 'any value', */15 means 'every 15'; comma lists and dash ranges work too. cron is a service: it checks the schedules once a minute and runs whatever is due. cron has a very bare environment (a short PATH), so in jobs you write full paths to commands and files. systemd has its own replacement - timers (systemctl list-timers).

In the book

This topic is covered in full in the chapter Scheduling tasks.

See also

Services: systemctl.