“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:
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).”