cron

SYNOPSIS

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

DESCRIPTION

       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 crontabs found are loaded into memory.  Cron also  searches
       for /etc/crontab and the files in the directory, which are in a differ-
       ent format (see crontab(5) ).  Cron then wakes up every minute, examin-
       ing  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.

   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.

OPTIONS

       -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
       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 <vixie@isc.org>

4th Berkeley Distribution 10 January 2007 CRON(8)






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