How to schedule tasks in Linux/RHEL6/RHEL5(Crontab configuration)

Crontab is use to define a job which will run at a particular time period.Crontab is use to define jobs or tasks which root wants to run at regular period of time for example daily jobs,weekly jobs,hourly jobs or monthly jobs.For single time schedule tasks we can use "AT" command but the difference between AT command and Crontab is that in AT command we cannot define jobs to run daily,hourly,weekly,monthly .

Now to edit Crontab we need to know about few files before define a job.
/etc/cron.allow
/etc/cron.deny

These two files are use to define Cronjobs Access Control

• If neither /etc/cron.allow nor /etc/cron.deny exists only root is allowed to define a cronjobs.
• If only /etc/cron.deny exists in that case all users except those listed in this file can define cronjobs.
• If only /etc/cron.allow exists in that case root and all the users listed in this file can define cronjobs.
• If both file exists then cron.deny is ignored.

Note:-- By default /etc/cron.deny file exits but its empty that means every user is allow to define a cronjob.Now if you create a new empty /etc/cron.allow file then none of the user will be allow to define a cronjob only root can define.

Steps to define a new cronjob:-

1.  Login with root user and type in gnome-terminal or virtual-terminal the following command
      crontab -e
2. This command open a empty file with vi editor.Now you need to type the time and the comand in this blank file but with a specific syntax.