while [ "$os" != "linux32" ]
do
        echo -n "Please enter a valid OS (linux32, linux64, osx32, osx64):"
        read os
        case $os in
            linux32)
                echo "linux 32"
            ;;
            linux64)
                echo "linux 64"
            ;;
            osx32)
                echo "osx 32"
            ;;
            osx64)
                echo "osx 64"
             ;;
            *)
        ;;
        esac
done