cron automatically executes jobs at a given time (see: man cron)
as specified in the crontab file.

crontab file is edited by the special editor `crontab'
This editor has the same syntax as the `vi' editor (see: man vi, man crontab)
This file can ususually be edited only as root.

Command `crontab -l' lists the crontab file.
Commend `crontab -e' edits the crontab file.


Example of the crontab file (obtained by the command `crontab -l'):


# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.7768 installed on Tue Mar 21 23:59:22 2000)
# (Cron version -- $Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $)
0 0  * * *      /root/bin/analyze
23  */4  * * *  /root/bin/clear_tmp


First line executes the command `/root/bin/analyze' each midnight.
Second line executes the command `/root/bin/clear_tmp' every for hours at 
the hour and 23 minutes.

The file `/root/bin/clear_tmp' containd the following two lines:

rm -rf /usr/local/httpd/htdocs/netplot/tmp/*
rm -rf /usr/local/httpd/htdocs/FAni/tmp/*