1. Install MPlayer to get mencoder.
2. "mencoder -dvd-device /dev/cdrom dvd://1 -o file.avi -ovc lavc -lavcopts vcodec=mpeg4:vhq:vbitrate=1800 -oac copy"
(You may need to change the value of "/dev/cdrom").
Here's a little script:-

#!/bin/bash
# dvd2avi.sh

indev=$1
outfile=$2

if [ $# != 2 ]; then
	echo
	echo 'dvd2avi.sh:usage:- $0 [dvd device] [output file], e.g "dvdrip.sh /dev/dvd ElephantsDream.avi\" '
	echo
	exit
fi


mencoder -dvd-device $indev dvd://1 -o $outfile \
         -ovc lavc -lavcopts vcodec=mpeg4:vhq:vbitrate=1800 \
         -oac copy

#end dvd2avi.sh