cron: Job Scheduler

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Have you ever wandered near your Linux box in the middle of the night, only to discover the hard disk working furiously? If you have, or just want a way for some task to occur at regular intervals, cron is the answer.
Pre-Configured cron Jobs

The Debian and Red Hat distributions come with several pre-configured cron jobs to help control disk usage. Other distributions may provide similar help. The Debian dh_installcron command will install these jobs. Normally, you should not need to run this command—installing the cron package will take care of it for you.

These jobs, located in the file /etc/crontab, use the run-parts command to call all the scripts in directories /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly. For the most part, these scripts control disk usage, compressing and pruning log files in /var/log and cleaning up after indices from the man(1) command. The package maintainers who created these jobs configured them to run during the night, normally a slow time for other system activity. Some of the scripts generate a lot of disk activity, which can slow other I/O-intensive jobs. If you want them to run at other times, edit /etc/crontab or move the scripts among the directories that contain them.

Additional Reading

See the list of references in the sidebar for additional reading on cron. Additionally, the man page for the at facility may prove useful. at provides a one-time job-scheduling facility. If you do not keep your Linux system running 24 hours per day, you may want to review Anacron, which does not depend on specific time events to get its work done.

Conclusion

I have provided a brief introduction to the cron facility, a typical part of Linux and other UNIX operating systems. It will provide a starting point for time-related work you want your Linux system to perform. In brief, if you want to schedule repetitive tasks so as not to type the same commands again and again, use cron.

Resources

Michael S. Keller works as a technical analyst with Sprint Paranet, a wholly owned subsidiary of Sprint, a nationwide network services provider based in Houston. He has used UNIX variants for nearly nine years and enjoys communing with cats, motorcycles and the universe. You may reach him at mskeller@sprintparanet.com.

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