Scheduled Activity: cron and at

Your Linux computer can be made to do the right thing at the right time.
Some Final Thoughts

The user's interface to the crontab and at commands is very similar across different versions of Unix, but implementations of underlying directory structures, daemons, and access controls may differ. Be sure to review your system documentation to take advantage of all aspects of these powerful commands.

There's practically no limit to the use of crontab and at, but let me offer a few words of warning. First, consider security issues when enabling user crontab and at permissions. Obviously, a disgruntled co-worker could leave a “time-bomb” of some sort, limited only by his other permission restrictions. Many versions of crontab and at allow you to specify “allow” and “deny” files to control which users have access to the utilities. You could also use root's crontab to check for and remove user crontabs (or any other files) if you want to.

Also, debug your crontab file entries thoroughly. Check that they are working. These entries are usually scheduled to execute at times of low-usage, so it is unlikely you'll be around to observe them at the time.

John Raithel is a consulting technical writer specializing in documenting the system and network administration of the Unix operating system, and is currently working on World Wide Web and firewall documentation for Silicon Graphics, Inc. He lives in a small town on the central California coast where he plays with his Linux and SunOS “mini-network”. He can be reached via e-mail at