Configuring the at Command Service
crontab is great for processes that must be run on a regular schedule, but is a poor tool for something you want to run only once. The at tool enables you to specify a command to run at a certain time; the time can be the current time or it can be a specified time in the future.
To go along with the at command is the batch command, which executes commands when system load levels permit—that is, when the load average (measured by /proc/loadavg) drops below 1.5 or some other value specified at the invocation of atrun.
Enabling at Command Service
The at command service (as well as the batch command service) reads commands from standard input or a specified file that is to be executed at a later time via /bin/sh