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LinuxLookup.com: Crontab Scheduling Service Guide

Oct 01, 2000, 17:52 (0 Talkback[s])
(Other stories by Vicky)


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"Certain processes must be run at specific times, over and over again. An example of this might be a backup process that is launched each night as the rest of the world sleeps, or a log analyzer that must ran every minute. These processes are ran at certain times or on certain days; the rest of the time, they are not running because they require user interaction (command) or "direction" for lack of a better term. This is where cron comes in handy. It allows you to schedule a specific task to be ran when ever you please, on the minute, hourly, daily, monthly and thus giving you a sense of relief knowing tasks are being executed without you having to baby-sit your system."

"Cron is started from either rc or rc.local and returns immediately, so there is no need to background this command. Cron searches /etc/crontab for system entries and also /var/spool/cron for entries that match users in the /etc/passwd file; found entries are loaded into memory. This occurs every minute as cron "wakes-up" and does several things:

1. It checks the entries that it knows about and runs any commands that are scheduled to run.

2. It determines whether the modtime on the cron directory has changed.

3. If the modtime on the cron directory has changed, cron checks each of the files and reloads any that have changed."

"Since cron checks for changes every minute, it is unnecessary to restart cron when the cron files are changed (edited)."

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