At

The at scheduler is used to schedule a job for one-time-only running at a later date. For comparison, the cron scheduler is used to schedule jobs for repeated execution at regular intervals. One very handy use of the at scheduler is to schedule lengthy jobs to run in the background immediately - that way even if you need to (or are forced to) close your shell session, your job will continue running. You could force a job to the background without using at simply by adding an ampersand to the end of your command line - for example cvsup /usr/share/examples/cvsup/ports-supfile & - but if forced to the background that way, it is still a child process of your shell, and thus will die a sudden death if you choose to log out before it is done.

Access to either cron or the at scheduler may be open to all users or restricted only to certain users at the system administrator's discretion.