How the command "at" works?

[Karl Auer]

On Fri, 2016-08-05 at 20:27 -0500, Peng Yu wrote:
> I am asking how "at" works internally not how to use it. It seems man
> does not help much for this purpose.

If you read the "at" man page it says you should "see also" atd, and
that answers pretty much all your questions.

"at" is actually two parts - the user interface "at" and a daemon
"atd".

They communicate through job files: "at" puts jobs in a directory
(/var/spool/cron/atjobs), "atd" reads them out of there and runs them.

Each job file is an ordinary shell script (sh not bash!). It's mostly
commands to set the environment and change to the right directory; the
last lines are whatever you typed into "at".

You can find out more about atd through "man atd" and "systemctl status
atd". You might also want to check out at.allow and at.deny.

Interestingly, atd is started with the -f option, which is supposed to
run it in the foreground. Odd. It's definitely not in the foreground.

Regards, K.

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