cron



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 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.


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.  Natu-
       rally 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 <vixie@isc.org>



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

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