Scheduling a Single Job: at

The at command allows you to schedule a job for execution at a later time. The job may consist of a single command or a script.

Like crontab, at allows you to schedule the automatic completion of routine commands. However, unlike crontab files, at files execute their commands once, and then are removed from their directory. Therefore, at is most useful for running simple commands or scripts that direct output into separate files for later examination.

Submitting an at job involves entering a command, following the at command syntax to specify options to schedule the time your job will be executed. For more information about submitting at jobs, see "at Command Description".

The at command stores the command or script you entered, along with a copy of your current environment variables in either /usr/spool/cron/atjobs or /var/spool/cron/atjobs. As a file name, your at job file is given a long number specifying its location in the at queue, followed by the .a extension, such as 793962000.a.

The cron daemon periodically executes the atrun program, usually at 15-minute intervals. atrun then executes at jobs at their scheduled times. After your at job has been executed, its file is removed from the atjobs directory.