How To Set Up Cron Jobs In Lnix

 CRONTAB

What is Crontab?

Crontab is a file which contains the schedule of cron entries to be run and at specified times.

Crontab Commands

           export EDITOR=vi ;to specify a editor to open crontab file.

1.    crontab -e    Edit crontab file, or create one if it doesn’t already exist.
2.    crontab -l    crontab list of cronjobs , display crontab file contents.
3. crontab -r    Remove your crontab file.
4. crontab -v    Display the last time you edited your crontab file.

Crontab Examples

A line in crontab file like below removes the tmp files from /home/appsolworld/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/appsolworld/tmp/*

Crontab every hour

This is most commonly used for running cron every hour and executing a command after an interval of one hour.

crontab format every hour is simple to have hour field as *  which runs every hour as the clock switches to new hour.  if you want to run it at the beginning of hour the minute filed needs to be 0 or any other minutes when you want to run it at a specific minute of the hour. 

cron every hour to run at the beginning of the hour.00     *     *     *     *         rm /home/appsolworld/tmp/*

 

cron every hour to run at 15 minute of an hour..

15     *     *     *     *         rm /home/someuser/tmp/*

Crontab every minute

To run cron every minute keep the minutes field as * , as minute changes to new minute cron will be executed every minute.  if you want to run it continuously every hour then the hour field also needs to have value of  * .

*     *     *     *     *         rm /home/appsolworld/tmp/*

if you want to run a script every minute at specific hour, change the value of hour field to specific value such as 11th hour.

*     11     *     *     *         rm /home/appsolworld/tmp/*