The Patient List Viewer harness the at commands to place jobs in the background. This frees up the UI while long queries are running.

Some Linux systems do not have the at command installed.

The ATD service is installed along with the at command. This service is required to be running for the at command to work

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