# cron

## NAME



           crond - daemon to execute scheduled commands
    


## SYNOPSIS



           **crond**[**-c** | **-h** | **-i** | **-n** | **-p** | **-P** | **-s** | **-m** _< mail_**command >**]
           **crond -x**[ext,sch,proc,pars,load,misc,test,bit]
           **crond -V**
    


## 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 immediately, thus, there is no need to need to start it
           with the '&' parameter.
    
           _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_ examines 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.
                  Originally 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.
           Conversely, 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 command must accept a fully formatted mail message
                  (with headers) on standard input and send it as a mail
                  message to the recipients 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.
    
           **-f** the same as -n, consistent with other crond
                  implementations.
    
           **-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
                  system 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.
    
           **-V** Print version and exit.
    


## 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 syslog(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 represents 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
           synchronised 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.
    


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