CROND linux command manual

CRON(8)                                                                     CRON(8)

       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.

       crontab(1), crontab(5)

       Paul Vixie 

4th Berkeley Distribution      20 December 1993                       CRON(8)