Running Like Clockwork
When you start working with scripts, you may want to run a script at a preset time, usually at a time when you're not there. The Linux system provides a couple of ways to run a script at a preselected time: the at command and the cron table. Each method uses a different technique for scheduling when and how often to run scripts. The following sections describe each of these methods.
Scheduling a job using the at command
The at command allows you to specify a time when the Linux system will run a script. The at command submits a job to a queue with directions on when the shell should run the job. The at daemon, atd, runs in the background and checks the job queue for jobs to run. Most Linux distributions start this daemon automatically at boot time.
The atd daemon checks a special directory on the system (usually /var/spool/at) for jobs submitted using the at command. By default, the atd daemon checks this directory every 60 seconds. When a job is present,...