cron.8   [plain text]

.\"/* 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          <>          uunet!decwrl!vixie!paul
.\" */
.\" $FreeBSD: src/usr.sbin/cron/cron/cron.8,v 1.16 2001/03/09 11:02:01 ru Exp $
.Dd December 20, 1993
.Dt CRON 8
.Nm cron
.Nd daemon to execute scheduled commands (Vixie Cron)
.Op Fl s
.Op Fl o
.Fl x
.Ar debugflag Ns Op , Ns Ar ...
.Nm Cron
should be started from
.Pa /etc/rc
.Pa /etc/rc.local .
It will return immediately,
so you don't need to start it with '&'.
.Nm Cron
.Pa /var/cron/tabs
for crontab files which are named after accounts in
.Pa /etc/passwd ;
crontabs found are loaded into memory.
.Nm Cron
also searches for
.Pa /etc/crontab
which is in a different format (see
.Xr crontab 5 ) .
.Nm 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
environment variable in the crontab, if such exists).
checks each minute to see if its spool directory's modtime (or the modtime
.Pa /etc/crontab )
has changed, and if it has,
will then examine the modtime on all crontabs and reload those which have
changed.  Thus
need not be restarted whenever a crontab file is modified.  Note that the
.Xr crontab 1
command updates the modtime of the spool directory whenever it changes a
Available options:
.Bl -tag -width indent
.It Fl 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's 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 interval.
In other words, if the job had run within one hour before the GMT offset change
(and cron was not restarted nor the 
.Xr 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
.Xr 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
.Xr crontab 5 .
.It Fl 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
.Fl o
.Fl s
are specified, the option specified last wins.
.Xr crontab 1 ,
.Xr crontab 5 
.An Paul Vixie Aq