Linux / UNIX Restrict at / cron Usage To Authorized Users

Posted on in Categories , , , , , , , , last updated July 18, 2009

The cron and at services are used to allow commands to be executed at a later time. How do I restrict them to selected users such as root, opt1, opt2 and so on? How do I make sure user php can run php script but cannot modify or install a new job?

The cron service is required by almost all UNIX / Linux / BSD oses to perform necessary maintenance tasks. Both cron and anacron make use of a number of configuration files and directories. Regular users can modify and install their own cron configuration or jobs.

  1. /etc/cron.allow – Put users who are allowed to use cron.
  2. /etc/at.allow – Put users who are allowed to use at.

If these files exist and if the corresponding files /etc/cron.deny and /etc/at.deny do not exist, then only users listed in the relevant allow files can run the crontab and at commands to submit their jobs.

How Do I Restrict at And cron To Authorized Users Only?

First, delete both /etc/cron.deny and /etc/at.deny files.
# rm /etc/cron.deny /etc/at.deny
Now, open /etc/cron.allow, enter:
# vi /etc/cron.allow
Add one line for each user allowed to use the crontab command to create cron jobs. Finally, edit /etc/at.allow using a text editor such as vi, enter:
# vi /etc/at.allow
Add one line for each user allowed to use the at command to create at jobs:


Note that even if a user called jerry is not listed in cron.allow, cron jobs, he can still be run as that user. The cron.allow file controls only administrative access to the crontab command for scheduling and modifying cron jobs. In other words you can set / modify cron jobs on behalf of jerry. Jerry can only run it but cannot set or modify jobs.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

Leave a Comment