Linux / UNIX Restrict at / cron Usage To Authorized Users

To restrict the usage of the at and cron commands to authorized users on Linux/UNIX systems, you can use the following steps:

1. Create a group for authorized users:
   sudo groupadd atcronusers

2. Add the authorized users to the group:
   sudo usermod -a -G atcronusers user1
sudo usermod -a -G atcronusers user2
...

3. Modify the permissions of the at and cron commands:
   sudo chown root:atcronusers /usr/bin/at
   sudo chmod 750 /usr/bin/at

   sudo chown root:atcronusers /usr/bin/crontab
sudo chmod 750 /usr/bin/crontab


   These commands set the group ownership of the at and cron commands to atcronusers and set the permissions so that only the root user and members of the atcronusers group can execute them.

4. Edit the /etc/at.allow and /etc/cron.allow files to include the names of the authorized users or group:
   # /etc/at.allow
atcronusers

# /etc/cron.allow
atcronusers


   These files allow only the users or group listed in them to use the at and cron commands.

5. Optionally, you can also edit the /etc/at.deny and /etc/cron.deny files to deny access to specific users or groups:
   # /etc/at.deny
user3

# /etc/cron.deny
user3


   These files deny access to the users or group listed in them.

Note that if the at.deny or cron.deny files exist, only users not listed in these files are allowed to use at or cron. If neither allow nor deny files exist, access to at and cron is granted to all users.

After completing these steps, only the authorized users or group will be able to use the at and cron commands on the system.