What is cron on a Linux or Unix-like systems?

Can you explain what is Cron? Why do I need to cron service on a Linux, OS X, and Unix-like systems?

Cron is UNIX/Linux/BSD service or daemon to execute commands or scripts at given time and date. It is also known as the clock daemon that executes commands at specified dates and times according to instructions in a file.[donotprint][/donotprint]

Generally, crontab uses a daemon, crond, which runs constantly in the background and checks once a minute to see if any of the scheduled jobs need to be executed. If so, it executes them. These jobs are generally referred to as cron jobs.

Cron is controlled by a set of files called “crontabs”. There is the master file in /etc/crontab. Each users cronjob is stored in /var/spool/cron/username directory. To see master file type the following cat command:
$ cat /etc/crontab
Sample outputs:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m h dom mon dow user	command
17 *	* * *	root    cd / && run-parts --report /etc/cron.hourly
25 6	* * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6	* * 7	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6	1 * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

Editing Crontab or setting up your own job

Just type following command:
$ crontab -e
To list your own jobs type the following command:
$ crontab -l

How do I configure and install my own cron jobs?

See our tutorial on how to automatically run given commands or scripts at a specified time and date:

• How do I add jobs to cron under Linux or UNIX oses?