CROND linux command manual
cron - daemon to execute scheduled commands (Vixie Cron)
Cron should be started from /etc/rc or /etc/rc.local. It will return
immediately, so you don't need to start it with '&'.
Cron searches /var/spool/cron for crontab files which are named after
accounts in /etc/passwd; crontabs found are loaded into memory. Cron
also searches for /etc/crontab and the files in the /etc/cron.d/
directory, which are in a different format (see crontab(5)). Cron
then wakes up every minute, examining all stored crontabs, checking
each command to see if it should be run in the current minute. When
executing commands, any output is mailed to the owner of the crontab
(or to the user named in the MAILTO environment variable in the
crontab, if such exists).
Additionally, cron checks each minute to see if its spool directory's
modtime (or the modtime on /etc/crontab) has changed, and if it has,
cron will then examine the modtime on all crontabs and reload those
which have changed. Thus cron need not be restarted whenever a
crontab file is modified. Note that the Crontab(1) command updates
the modtime of the spool directory whenever it changes a crontab.
4th Berkeley Distribution 20 December 1993 CRON(8)