Sometimes, we need to fire a job at a specific hour without any need to repeat the action, so just a one off. What we can use in this case is a simple utility called at with its companion batch. What does it do? It simply reads from the input or a file on what to execute and when, and it will use /bin/sh to invoke whatever we want. There is a little twist though: batch will do it but not at a specific time. It will be done when the system load drops below 1.5 or any level specified at the atd runtime.
So, we introduced atd; what is this? This is the daemon that executes the one shot jobs defined and put in its queue by the at utility, and so, it is a daemon that usually runs under a dedicated daemon user:
root:# ps -fC atd
UID PID PPID C STIME TTY TIME CMD
daemon 722 1 0 Apr25 ? 00:00:00 /usr/sbin/atd -f
So, this is a daemon that is fired up as a system service, but...