What is cron job scheduler in linux?

Cron is a system daemon used to execute desired tasks (in the background) at designated times.A crontab is a simple text file with a list of commands meant to be run at specified times. It is edited with a command-line utility. These commands (and their run times) are then controlled by the cron daemon, which executes them in the system background. Each user has a crontab file which specifies the actions and times at which they should be executed, these jobs will run regardless of whether the user is actually logged into the system. There is also a root crontab for tasks requiring administrative privileges. This system crontab allows scheduling of systemwide tasks

Some below quick reference of general uses commands

# crontab -l (To listing existing crontab entries)

# crontab -e (editing crontab by existing user)

# crontab -u -e UserName (editing crontab by desired user)

cron job scheduler is used to schedule tasks regularly.

crontab -e is used to edit and add crontab jobs.

crontab -l to list the jobs written as a specific user.

crontab -r to remove a job.

The five stars at the beginning specifies the time

***** /COMMANDTORUN

1st star Min(0-)

2nd star Hour(0-)

3rd Star DAY of the month(1-)

4th Star Month(1-or Jan-Dec)

5th star DAY of week(0-6 or Sun-Sat)

For a range use '-' eg:2-3

For running the command every x minutes use */x ****

For running every alternate days **1-/2 **