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CRON(8)                      Cronie Users' Manual                      CRON(8)



NAME
       cron - daemon to execute scheduled commands

SYNOPSIS
       cron [-n | -p | -s | -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 /etc/passwd; The founded
       crontabs are loaded into memory.  Cron also searches for /etc/anacrontab  and  the  files  in  the  /etc/cron.d
       directory, which are in a different format (see crontab(5) ).  Cron examines 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).
       Job output can also be sent to syslog by using the -s option.

       There are two ways, how the changes are checked in crontables. The first is checking the modtime  of  file  and
       the  other  is using inotify support.  You can find out which of them are you using, if you check /var/log/cron
       where is (or isn't) inotify mentioned after start of daemon. The inotify support is watching for changes in all
       crontables and touch the disk only in case that something was changed.

       In other case cron checks each minute to see if its crontables modtime have changes and reload those which have
       changes. There is no need to restart cron after some of the crontable is modified. The modtime option  is  used
       also when inotify couldn't be initialized.

       Cron  is  checking  those  files  or  directories: /etc/anacrontab system crontab is usually for running daily,
       weekly, monthly jobs.   /etc/cron.d/ where are system cronjobs stored for different  users.     /var/spool/cron
       that's mean spool directory for user crontables.

       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 frequently 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.

       It's possible to use different time zones for cron tables. More could be found in crontab(5).

   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 appropriate crontab file.

OPTIONS
       As a special case, the string off will disable sending mail.

       -s     This  option  will  direct cron to send job output to the system log using syslog(3).  This is useful if
              your system has no sendmail(8), or if mail is disabled using -m off.

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

       -p     Cron permit any crontab, which user set.

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

       -P     Don't set PATH.  PATH is instead inherited from the environment.

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.  Naturally this is not relevant if cron was built to use syslog(3).

CAVEATS
       The crontab files have to be regular files or symlinks to  regular  files,  they  must  not  be  executable  or
       writable by anyone else than the owner.  This requirement can be overridden by using the -p option on the crond
       command line.  If inotify support is in use, changes in the symlinked crontabs are not automatically noticed by
       the cron daemon. The cron daemon must receive a SIGHUP to reload the crontabs.  This is a limitation of inotify
       API.


SEE ALSO
       crontab(1), crontab(5), inotify(7), pam(8)

AUTHOR
       Paul Vixie <vixieATisc.org>
       Marcela Maslanova <mmaslanoATredhat.com>



Marcela Maslanova                December 2009                         CRON(8)