The at facility submits a command line (or a script) for execution at an arbitrary later time. It has the form:
%atoptions time < scriptfile
This submits the scriptfile for execution at a later time.
The redirection (<) isn't required on BSD and some other UNIX
systems.
If you don't want to write a script, you can omit it and type your commands
on the terminal, terminated by
CTRL-d:
%atoptions time Command 1 Command 2... [CTRL-d]
The time is most commonly a four-digit number representing a
time on a 24-hour clock. For example, 0130 represents 1:30 a.m. and
1400 represents 2 p.m. You can also use abbreviations such as
1am, 130pm, and so on.
- from O'Reilly & Associates' System Performance Tuning, Chapter 3