bash

#!/bin/bash
#variable1="pike horse trout blackbird cow hallibut crow turkey mackerel chaffinch"
#variable2="salmon mackerel tuna hallibut trout pike sardine"

variable1=$1
variable2=$2

for word in $variable1;do
        if [ "${variable2//$word/}" != "$variable2" ];then
                variable1=${variable1//$word/}
        fi
done

echo "$variable1"