CRON(8)								      CRON(8)



NAME
       cron - daemon to execute scheduled commands (ISC Cron V4.1)

SYNOPSIS
       cron [-l load_avg] [-n] [-p]

DESCRIPTION
       Cron  should be started from /etc/rc or /etc/rc.local.  It will return
       immediately, so you don't need to start it with '&'.   The  -n  option
       changes	this  default  behavior	 causing it to run in the foreground.
       This can be useful when starting it out of init.

       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 direc-
       tory,  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 exe-
       cuting 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.

   Daylight Saving Time and other time changes
       Local  time  changes of less than three hours, such as those caused by
       the start or end of Daylight Saving Time, are handled specially.	 This
       only applies to jobs that run at a specific time and jobs that are run
       with a granularity greater than one hour.  Jobs	that  run  more	 fre-
       quently are scheduled normally.

       If  time	 has  moved  forward,  those  jobs that would have run in the
       interval that has been skipped will be run  immediately.	  Conversely,
       if time has moved backward, care is taken to avoid running jobs twice.

       Time changes of more than 3 hours are considered to be corrections  to
       the clock or timezone, and the new time is used immediately.

   PAM Access Control
       On  Red	Hat systems, crond now supports access control with PAM - see
       pam(8).	 A  PAM	 configuration	file  for  crond  is   installed   in
       /etc/pam.d/crond	 .   crond loads the PAM environment from the pam_env
       module, but these can be overriden by settings in the crontab file.

SIGNALS
       On receipt of a SIGHUP, the cron daemon will close and reopen its  log
       file.  This is useful in scripts which rotate and age log files.	 Nat-
       urally this is not relevant if cron was built to use syslog(3).

CAVEATS
       In this version of cron , without the -p option, /etc/crontab must not
       be  writable  by	 any  user  other  than root, no crontab files may be
       links, or linked to by any other file, and no  crontab  files  may  be
       executable, or be writable by any user other than their owner.

SEE ALSO
       crontab(1), crontab(5), pam(8)

AUTHOR
       Paul Vixie 



4th Berkeley Distribution      10 January 1996"			      CRON(8)

UNIX/Linux commands referenced on this page:

1. crontab
2. which
3. file
4. time
5. less
6. as
7. at
8. more
9. clock
10. links