Some Linux systems do not have the at command installed.

1. Install at by typing in:
   yum install at

2. Start the atd daemon by typing in:
/etc/init.d/atd start

3. Make sure atd is set to start each time the server is rebooted

https://unix.stackexchange.com/questions/86016/how-to-install-and-use-at-job-scheduling