Mine

#!/bin/sh
LEFT=$1
RIGHT=$2
TARGET=$3
cat /dev/null > $TARGET
for FIRST in `cat $LEFT`
do
FOUND=no
	for SECOND in `cat $RIGHT`
	do
	if [ "$FIRST" = "$SECOND" ]
	then
		FOUND=yes
	fi
	done
	if [ "$FOUND" = "no" ]
	then
		echo "$FIRST" >> $TARGET
	fi
done