CRON(8)			     System Administration		       CRON(8)



NAME
       crond - daemon to execute scheduled commands

SYNOPSIS
       crond [-c | -h | -i | -n | -p | -P | -s | -m]
       crond -x [ext,sch,proc,pars,load,misc,test,bit]

DESCRIPTION
       Cron  is	 started  from	/etc/rc.d/init.d or /etc/init.d when classical
       sysvinit scripts are used. In case systemd is enabled, then  unit  file
       is  installed  into  /lib/systemd/system/crond.service  and  daemon  is
       started by systemctl start crond.service command.  It  returns  immedi-
       ately,  thus, there is no need to need to start it with the '&' parame-
       ter.

       Cron searches /var/spool/cron for crontab files which are  named	 after
       accounts in /etc/passwd; The found crontabs are loaded into the memory.
       Cron also searches for /etc/anacrontab and any files in the /etc/cron.d
       directory,  which have a different format (see crontab(5)).  Cron exam-
       ines all stored crontabs and checks each job to see if it needs	to  be
       run  in	the  current  minute.	When executing commands, any output is
       mailed to the owner of the crontab (or to the  user  specified  in  the
       MAILTO  environment  variable in the crontab, if such exists).  Any job
       output can also be sent to syslog by using the -s option.

       There are two ways how changes in crontables are	 checked.   The	 first
       method  is  checking the modtime of a file.  The second method is using
       the inotify support.  Using of inotify is logged in  the	 /var/log/cron
       log  after  the	daemon	is  started.   The  inotify support checks for
       changes in all crontables and accesses the hard disk only when a change
       is detected.

       When  using  the	 modtime  option, Cron checks its crontables' modtimes
       every minute to check for any changes and reloads the crontables	 which
       have  changed.	There  is  no  need  to restart Cron after some of the
       crontables were modified.  The modtime option is also used when inotify
       can not be initialized.

       Cron checks these files and directories:

       /etc/crontab
	      system  crontab.	Nowadays the file is empty by default.	Origi-
	      nally it was usually used to run daily,  weekly,	monthly	 jobs.
	      By  default  these  jobs are now run through anacron which reads
	      /etc/anacrontab configuration file.  See anacrontab(5) for  more
	      details.

       /etc/cron.d/
	      directory	 that  contains	 system	 cronjobs stored for different
	      users.

       /var/spool/cron
	      directory that contains user crontables created by  the  crontab
	      command.

       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  Daylight  Saving Time changes, are handled in a special way.  This
       only applies to jobs that run at a specific time and jobs that run with
       a granularity greater than one hour.  Jobs that run more frequently are
       scheduled normally.

       If time was adjusted one hour forward, those jobs that would  have  run
       in  the	interval  that has been skipped will be run immediately.  Con-
       versely, if time was adjusted backward, running the same job  twice  is
       avoided.

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

       It is possible  to  use	different  time	 zones	for  crontables.   See
       crontab(5) for more information.

   PAM Access Control
       Cron  supports access control with PAM if the system has PAM installed.
       For more information, see pam(8).  A PAM configuration file  for	 crond
       is installed in /etc/pam.d/crond.  The daemon loads the PAM environment
       from the pam_env module.	 This can be overridden by  defining  specific
       settings in the appropriate crontab file.

OPTIONS
       -h     Prints a help message and exits.

       -i     Disables inotify support.

       -m     This  option  allows  you	 to specify a shell command to use for
	      sending Cron mail output instead of using sendmail(8) This  com-
	      mand  must  accept a fully formatted mail message (with headers)
	      on standard input and send it as a mail message to  the  recipi-
	      ents  specified  in the mail headers.  Specifying the string off
	      (i.e., crond -m off) will disable the sending of mail.

       -n     Tells the daemon to run in the foreground.  This can  be	useful
	      when  starting  it  out  of  init. With this option is needed to
	      change  pam   setting.	/etc/pam.d/crond   must	  not	enable
	      pam_loginuid.so module.

       -p     Allows Cron to accept any user set crontables.

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

       -c     This option enables clustering support, as described below.

       -s     This  option will direct Cron to send the job output to the sys-
	      tem log using syslog(3).	This is useful if your system does not
	      have sendmail(8), installed or if mail is disabled.

       -x     This option allows you to set debug flags.

SIGNALS
       When  the SIGHUP is received, the Cron daemon will close and reopen its
       log file.  This proves to be useful in scripts which rotate and age log
       files.	Naturally,  this is not relevant if Cron was built to use sys-
       log(3).

CLUSTERING SUPPORT
       In this version of Cron it is possible to use a network-mounted	shared
       /var/spool/cron	across a cluster of hosts and specify that only one of
       the hosts should run the crontab jobs in	 this  directory  at  any  one
       time.   This  is done by starting Cron with the -c option, and have the
       /var/spool/cron/.cron.hostname file contain just one line, which repre-
       sents  the  hostname  of	 whichever  host in the cluster should run the
       jobs.  If this file does not exist, or the  hostname  in	 it  does  not
       match  that  returned by gethostname(2), then all crontab files in this
       directory are ignored.  This has no effect on cron  jobs	 specified  in
       the  /etc/crontab file or on files in the /etc/cron.d directory.	 These
       files are always run and considered host-specific.

       Rather than editing /var/spool/cron/.cron.hostname directly, use the -n
       option of crontab(1) to specify the host.

       You should ensure that all hosts in a cluster, and the file server from
       which they mount the shared crontab directory,  have  closely  synchro-
       nised  clocks,  e.g., using ntpd(8), otherwise the results will be very
       unpredictable.

       Using cluster sharing automatically disables inotify  support,  because
       inotify cannot be relied on with network-mounted shared file systems.

CAVEATS
       All  crontab  files  have  to  be  regular files or symlinks to regular
       files, they must not be executable or writable for anyone else but  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 signal to	reload	the  crontabs.
       This is a limitation of the inotify API.

       The  syslog  output  will be used instead of mail, when sendmail is not
       installed.

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

AUTHOR
       Paul Vixie 
       Marcela Malaova 
       Colin Dean 



cronie				  2013-09-26			       CRON(8)