Periodic command execution: cron  and at

Both users and system administrators often need to run commands periodically. For example, the system administrator might want to run a command to clean the directories with temporary files (/tmp  and /var/tmp ) from old files, to keep the disks from filling up, since not all programs clean up after themselves correctly.      

The cron  service is set up to do this. Each user has a crontab , where he lists the commands he wants to execute and the times they should be executed. The cron  daemon takes care of starting the commands when specified.  

The at  service is similar to cron , but it is once only: the command is executed at the given time, but it is not repeated.