This article explains task scheduling using a program called cron which is used for complex task scheduling. Scheduling of tasks under Linux is an extremely powerful procedure which is used by almost everyone.

The basic advantage of cron over at is that if you want a task to occur at regular intervals or basically more than once, then you don't have to repeatedly enter 'at' commands multiple times. You can use cron and feed in the number of times that you want the task to occur and cron handles the details of executing that task again and again.



Cron basically uses a particular table which has all the information about the task you want to execute and when you want to execute it. The user has to make this table and ask cron to use that table for its scheduling. You can make this table using any text editor such as vi or emacs and store it in any text file. This file is called the crontab file and I shall name mine as 'myjobs'.


Procedure :

Simply make a new text file, and enter this line in that text file. I have named my text file (crontab file) as 'myjobs'

30 04 * 3-5 * backup

Save the file and then run this command at the prompt

$ crontab myjobs

Thats it !! Now you will have your program called backup (in case you really have one) running at the times you have set it to run.To run whichever program you want, you have to make similar entries in the crontab file that you make and simply tell cron which file you have made using the command 'crontab [filename]'


Format of the Crontab File :

The crontab file consists of lines each explaining a task. Each line consist of 6 fields. I shall explain the 6 fields with the help of the example that I have used in the above discussion.