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CRON(8)								       CRON(8)

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

       cron [-n | -p | -m<mailcommand>]
       cron -x [ext,sch,proc,pars,load,misc,test,bit]

       Cron should be started from /etc/rc.d/init.d or /etc/init.d

       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 execut-
       ing  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 inter-
       val 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.

       -m     This option allows you to specify a shell command string to  use
	      for  sending cron mail output instead of sendmail(8).  This com-
	      mand must accept a fully formatted mail message  (with  headers)
	      on  stdin and send it as a mail message to the recipients speci-
	      fied in the mail headers.

       -n     This option changes default behavior causing it to run crond  in
	      the  foreground.	 This  can  be	useful when starting it out of

       -p     Cron permit any crontab, which user set.

       -x     With this option is possible to set debug flags.

       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.	 Natu-
       rally this is not relevant if cron was built to use syslog(3).

       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.

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

       Paul Vixie <vixie@isc.org>

4th Berkeley Distribution      15 September 2011		       CRON(8)
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