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CRON(8)			  BSD System Manager's Manual		       CRON(8)

NAME
     cron — daemon to execute scheduled commands (Vixie Cron)

SYNOPSIS
     cron [-j jitter] [-J rootjitter] [-m mailto] [-s] [-o]
	  [-x debugflag[,...]]

DESCRIPTION
     The cron utility should be started from /etc/rc or /etc/rc.local.	It
     will return immediately, so you do not need to start it with '&'.

     The cron utility searches /var/cron/tabs for crontab files which are
     named after accounts in /etc/passwd; crontabs found are loaded into mem‐
     ory.  The cron utility also searches for /etc/crontab which is in a dif‐
     ferent format (see crontab(5)).

     The cron utility then wakes up every minute, examining all stored
     crontabs, checking each command to see if it should be run in the current
     minute.  Before running a command from a per-account crontab file, cron
     checks the status of the account with pam(3) and skips the command if the
     account is unavailable, e.g., locked out or expired.  Commands from
     /etc/crontab bypass this check.  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).

     Additionally, cron checks each minute to see if its spool directory's
     modification time (or the modification time on /etc/crontab) has changed,
     and if it has, cron will then examine the modification time on all
     crontabs and reload those which have changed.  Thus cron need not be
     restarted whenever a crontab file is modified.  Note that the crontab(1)
     command updates the modification time of the spool directory whenever it
     changes a crontab.

     Available options:

     -j jitter
	     Enable time jitter.  Prior to executing commands, cron will sleep
	     a random number of seconds in the range from 0 to jitter.	This
	     will not affect superuser jobs (see -J).  A value for jitter must
	     be between 0 and 60 inclusive.  Default is 0, which effectively
	     disables time jitter.

	     This option can help to smooth down system load spikes during
	     moments when a lot of jobs are likely to start at once, e.g., at
	     the beginning of the first minute of each hour.

     -J rootjitter
	     Enable time jitter for superuser jobs.  The same as -j except
	     that it will affect jobs run by the superuser only.

     -m mailto
	     Overrides the default recipient for cron mail.  Each crontab(5)
	     without MAILTO explicitly set will send mail to the mailto mail‐
	     box.  Sending mail will be disabled by default if mailto set to a
	     null string, usually specified in a shell as '' or "".

     -s	     Enable special handling of situations when the GMT offset of the
	     local timezone changes, such as the switches between the standard
	     time and daylight saving time.

	     The jobs run during the GMT offset changes time as intuitively
	     expected.	If a job falls into a time interval that disappears
	     (for example, during the switch from standard time) to daylight
	     saving time or is duplicated (for example, during the reverse
	     switch), then it is handled in one of two ways:

	     The first case is for the jobs that run every at hour of a time
	     interval overlapping with the disappearing or duplicated inter‐
	     val.  In other words, if the job had run within one hour before
	     the GMT offset change (and cron was not restarted nor the
	     crontab(5) changed after that) or would run after the change at
	     the next hour.  They work as always, skip the skipped time or run
	     in the added time as usual.

	     The second case is for the jobs that run less frequently.	They
	     are executed exactly once, they are not skipped nor executed
	     twice (unless cron is restarted or the user's crontab(5) is
	     changed during such a time interval).  If an interval disappears
	     due to the GMT offset change, such jobs are executed at the same
	     absolute point of time as they would be in the old time zone.
	     For example, if exactly one hour disappears, this point would be
	     during the next hour at the first minute that is specified for
	     them in crontab(5).

     -o	     Disable the special handling of situations when the GMT offset of
	     the local timezone changes, to be compatible with the old
	     (default) behavior.  If both options -o and -s are specified, the
	     option specified last wins.

     -x debugflag[,...]
	     Enable writing of debugging information to standard output.  One
	     or more of the following comma separated debugflag identifiers
	     must be specified:

	     bit   currently not used
	     ext   make the other debug flags more verbose
	     load  be verbose when loading crontab files
	     misc  be verbose about miscellaneous one-off events
	     pars  be verbose about parsing individual crontab lines
	     proc  be verbose about the state of the process, including all of
		   its offspring
	     sch   be verbose when iterating through the scheduling algorithms
	     test  trace through the execution, but do not perform any actions

FILES
     /etc/crontab     System crontab file
     /etc/pam.d/cron  pam.conf(5) configuration file for cron
     /var/cron/tabs   Directory for personal crontab files

SEE ALSO
     crontab(1), pam(3), crontab(5), pam.conf(5)

AUTHORS
     Paul Vixie ⟨paul@vix.com⟩

BSD				 June 29, 2008				   BSD
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