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.\"/* Copyright 1988,1990,1993 by Paul Vixie
.\" * All rights reserved
.\" *
.\" * Distribute freely, except: don't remove my name from the source or
.\" * documentation (don't take credit for my work), mark your changes (don't
.\" * get me blamed for your possible bugs), don't alter or remove this
.\" * notice. May be sold if buildable source is provided to buyer. No
.\" * warrantee of any kind, express or implied, is included with this
.\" * software; use at your own risk, responsibility for damages (if any) to
.\" * anyone resulting from the use of this software rests entirely with the
.\" * user.
.\" *
.\" * Send bug reports, bug fixes, enhancements, requests, flames, etc., and
.\" * I'll try to keep a version up to date. I can be reached as follows:
.\" * Paul Vixie <paul@vix.com> uunet!decwrl!vixie!paul
.\" */
.\"
.\" $Id: cron.8,v 1.4 1999/06/05 22:16:34 aaron Exp $
.\"
.TH CRON 8 "20 December 1993"
.UC 4
.SH NAME
cron \- daemon to execute scheduled commands (Vixie Cron)
.SH SYNOPSIS
cron
.SH DESCRIPTION
.I Cron
should be started from /etc/rc or /etc/rc.local. It will return immediately,
so you don't need to start it with '&'.
.PP
.I Cron
searches /var/cron/tabs for crontab files which are named after accounts in
/etc/passwd; crontabs found are loaded into memory.
.I Cron
also searches for /etc/crontab which is in a different format (see
.IR crontab(5)).
.I Cron
then wakes up every minute, examining all stored crontabs, checking each
command to see if it should be run in the current minute. 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).
.PP
Additionally,
.I cron
checks each minute to see if its spool directory's modtime (or the modtime
on
.IR /etc/crontab)
has changed, and if it has,
.I cron
will then examine the modtime on all crontabs and reload those which have
changed. Thus
.I cron
need not be restarted whenever a crontab file is modified. Note that the
.IR Crontab (1)
command updates the modtime of the spool directory whenever it changes a
crontab.
.PP
Special considerations exist when the clock is changed by less than 3
hours, for example at the beginning and end of Daylight Saving
Time. If the time has moved forwards, those jobs which would have
run in the time that was skipped will be run soon after the change.
Conversely, if the time has moved backwards by less than 3 hours,
those jobs that fall into the repeated time will not be run.
.PP
Only jobs that run at a particular time (not specified as
@hourly, nor with '*' in the hour or minute specifier) are
affected. Jobs which are specified with wildcards are run based on the
new time immediately.
.PP
Clock changes of more than 3 hours are considered to be corrections to
the clock, and the new time is used immediately.
.SH "SEE ALSO"
crontab(1), crontab(5)
.SH AUTHOR
.nf
Paul Vixie <paul@vix.com>
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