1.\"/* Copyright 1988,1990,1993 by Paul Vixie 2.\" * All rights reserved 3.\" * 4.\" * Distribute freely, except: don't remove my name from the source or 5.\" * documentation (don't take credit for my work), mark your changes (don't 6.\" * get me blamed for your possible bugs), don't alter or remove this 7.\" * notice. May be sold if buildable source is provided to buyer. No 8.\" * warrantee of any kind, express or implied, is included with this 9.\" * software; use at your own risk, responsibility for damages (if any) to 10.\" * anyone resulting from the use of this software rests entirely with the 11.\" * user. 12.\" * 13.\" * Send bug reports, bug fixes, enhancements, requests, flames, etc., and 14.\" * I'll try to keep a version up to date. I can be reached as follows: 15.\" * Paul Vixie <paul@vix.com> uunet!decwrl!vixie!paul 16.\" */ 17.\" 18.\" $Id: cron.8,v 1.8 2000/03/19 17:57:03 aaron Exp $ 19.\" 20.Dd June 6, 1999 21.Dt CRON 8 22.Os 23.Sh NAME 24.Nm cron 25.Nd daemon to execute scheduled commands (Vixie Cron) 26.Sh SYNOPSIS 27.Nm cron 28.Sh DESCRIPTION 29.Nm 30should be started from 31.Pa /etc/rc 32or 33.Pa /etc/rc.local . 34It will return immediately, so you don't need to start it with 35.Ql \&& . 36.Pp 37.Nm 38searches its spool directory 39.Pf ( Pa /var/cron/tabs Ns ) 40for 41.Xr crontab 5 42files which are named after accounts in 43.Pa /etc/passwd ; 44crontabs found are loaded into memory. 45.Nm 46also searches for 47.Pa /etc/crontab 48which is in a different format (see 49.Xr crontab 5 ) . 50.Nm 51then wakes up every minute, examining all loaded crontabs, checking each 52command to see if it should be run in the current minute. 53When executing commands, any output is mailed to the user named in the 54.Ev MAILTO 55environment variable in the crontab, or to the owner of the crontab if 56.Ev MAILTO 57is not present. 58.Pp 59Additionally, 60.Nm 61checks each minute to see if its spool directory's modtime (or the modtime on 62.Pa /etc/crontab ) 63has changed, and if it has, 64.Nm 65examines the modtime on all crontabs and reloads those which have 66changed. 67Thus 68.Nm 69need not be restarted whenever a crontab file is modified. 70Note that the 71.Xr crontab 1 72command updates the modtime of the spool directory whenever it changes a 73crontab. 74.Pp 75Special considerations exist when the clock is changed by less than 3 76hours; for example, at the beginning and end of Daylight Saving 77Time. 78If the time has moved forward, those jobs which would have 79run in the time that was skipped will be run soon after the change. 80Conversely, if the time has moved backward by less than 3 hours, 81those jobs that fall into the repeated time will not be run. 82.Pp 83Only jobs that run at a particular time (not specified as @hourly, nor with 84.Ql * 85in the hour or minute specifier) 86are 87affected. 88Jobs which are specified with wildcards are run based on the 89new time immediately. 90.Pp 91Clock changes of more than 3 hours are considered to be corrections to 92the clock, and the new time is used immediately. 93.Sh SEE ALSO 94.Xr crontab 1 , 95.Xr crontab 5 96.Sh AUTHOR 97Paul Vixie <paul@vix.com> 98