Working with Crontab

Crontab is the scheduler program used ina unix servers.
Each user can have their own crontab, and though these are files in /var/spool/cron , they are not intended to be edited directly.

If  the  cron.allow  file  exists,  then you must be listed therein in order to be allowed to use this command. 
If the cron.allow file does not exist but the cron.deny file does exist, then you must not be