CRONTAB Basics Disclosed Here...

A crontab is an unix program.
It is used as a scheduler.
Scheduling is achieved by puting corntab entries into crontab.

Few of its commands are below:

$>crontab -l

The above command displayes the entries in crontab if any.

$>crontab -e

The above command opens crontab for adding/modifying crontab entries ( ordinary file save, exit - vi commands are used to save the cron entries in cron)


Crontab entry dedais are as below:

It will have six fields, starting 5 describe schedule informations and last field , the action as in sequence show here:

* * * * * {command}
Minute(0-59) Hour(0-23) DayofMonth(1-31) Month(1-12) WeekDay(0-6) {command}

In the weekday field 0 equals sunday and in the hour field the hours are in military 24 hour time format and that an asterisk [*] is "wildcard" meaning it will match any value

Few egs to clarify:

0 * * * * cd /home/username/script.sh
cron will run this script on 0th minute of every hour, every days of the month, every month of the year and every 7 days of the week

0 0 * * * cd /home/username/script.sh
cron will run this script on 0th minute of mightnight at every days of the month, every month of the year and every 7 days of the week

0 13 * * 0 cd /home/username/script.sh
cron will run this script on 0th minute of 1 PM at every days of the month, every month of the year and every Sunday of the week

0 0 15 * * cd /home/username/script.sh
cron will run this script on 0th minute of midnight on 15th day of the month , every month of the every year 15th day of teh month can be any of the 7 days of a week.