What is the Cron job?
Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time. Cron is one of the most useful tool in Linux or UNIX like operating systems. The cron service (daemon) runs in the background and constantly checks the /etc/crontab file, and /etc/cron.*/ directories. It also checks the /var/spool/cron/ directory.
What is the Crontab?
Each user can have their own crontab file, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly. You need to use crontab command for editing or setting up your own cron jobs.
To see your crontab file, type the following command at the UNIX / Linux shell prompt:
$ crontab -l
In order to edit your crontab file, type the following command at the UNIX / Linux shell prompt:
$ crontab -e