cron

CRON(8)                      System Administration                     CRON(8)



NAME
       crond - daemon to execute scheduled commands

SYNOPSIS
       crond [-c | -h | -i | -n | -p | -P | -s | -m<mailcommand>]
       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.

       -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.

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

AUTHOR
       Paul Vixie ⟨vixie@isc.org⟩
       Marcela MaÅ¡láÅová ⟨mmaslano@redhat.com⟩
       Colin Dean ⟨colin@colin-dean.org⟩
       Tomáš Mráz ⟨tmraz@fedoraproject.org⟩



cronie                            2013-09-26                           CRON(8)