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CRON(8)                                         System Administration                                         CRON(8)

       crond - daemon to execute scheduled commands

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

       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  sys‐
       temctl  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:

              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.

              directory that contains system cronjobs stored for different users.

              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.

              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 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.

       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).

       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

       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  unpre‐

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

       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.

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