Crontab on Linux systems allows us to schedule programs at specific times, of our choosing.
The configuration file for crontab is "/etc/crontab".

We can configure crontab tasks using the below command:


We could also list the scheduled tasks using the below command:


To restrict access to the crontab system, we use below files:
  •  "/etc/cron.deny".
  •  "/etc/cron.allow".
Both files contains a list of users, for example:


If only the "/etc/cron.deny" file is present on our system, users in this file can't run the crontab commands.
If only the "/etc/cron.allow" file is present on our system, only the users mentioned in this file can run the crontab commands.

In case none of the files "/etc/cron.deny" and "/etc/cron.allow" are present, usually only the "root" user can use the crontab commands, but if the two files exist on a system, "/etc/cron.allow" take precedence over the 
"/etc/cron.deny" file.

Remark:

On some Linux distribution the crontab security files are located in "/etc/cron.d/cron.allow" and "/etc/cron.d/cron.deny".  


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