Something

case $1 in
xfce4)
        exec startxfce4
        ;;
dwm)
        feh --bg-center /home/saboo/Pictures/MegadethBH.jpg
        xfce-mcs-manager &
        while true
           do
             echo `date`
             sleep 1
           done |  exec dwm
        ;;
twm)
        exec twm
        ;;
esac