Overview
By the end of this article you should be able to answer the following questions:
Announcement
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$ cat /etc/cron.deny
david
$ touch /etc/cron.allow
# Append their username to the following file:
/etc/at.deny
/etc/at.allow
Controlling who can create cron jobs
By default all users are allowed to use the crontab command to manage their respective cron jobs. To restrict a user from using the crontab command, we can add their names to the /etc/cron.deny file:
$ cat /etc/passwd | egrep 'tom|jerry' tom:x:1003:1003::/home/tom:/bin/bash jerry:x:1004:1004::/home/jerry:/bin/bash $ cat /etc/cron.deny jerry tom
Notice, each user name is on it’s own line.
After that, the listed users can no longer use the crontab command, which in turn means that they can’t manage their cron jobs:
$ su - jerry Last login: Sat May 9 21:48:35 BST 2015 on pts/1 $ crontab -e You (jerry) are not allowed to use this program (crontab) See crontab(1) for more information $ crontab -l You (jerry) are not allowed to use this program (crontab) See crontab(1) for more information
Notice, you can’t even using crontab for viewing a list of cron jobs.
By default everyone has crontab access, and you remove access by adding usernames to the /etc/cron.deny. If you have a lot of users and the majority of them are not supposed to have crontab access, then a better option is to change the default, so that by default user’s don’t have crontab access. This is done by simply creating the /etc/cron.allow file:
$ su - homer Last login: Sat Apr 11 18:23:26 BST 2015 from powershellpc.codingbee.dyndns.org on pts/7 $ crontab -l no crontab for homer $ exit logout $ touch /etc/cron.allow $ su - homer Last login: Sat May 9 22:09:19 BST 2015 on pts/1 $ crontab -l You (homer) are not allowed to use this program (crontab) See crontab(1) for more information
Now you can give crontab access to users by explicitly adding their name to the /etc/cron.allow file.
The equivalent privelege system exists for the “at” system. By default, anyone can create an “at” job unless their name is added to:
/etc/at.deny
If you want to change the default so that no one can set “at” jobs, then you need to delete the above file and replace it with:
/etc/at.allow
Note: You can either have the .allow file or the .deny file, but you can’t have them both exist at the same time. I think the same applies to cron.allow and cron.deny.