ho risolto da solo 
comunque se dovesse servire
Codice:
#!/bin/bash
# Author: Andrew Martin
# Credit: http://ubuntuforums.org/showthread.php?t=1309247
echo "Enter the primary display from the following:" # prompt for the display
xrandr --prop | grep "[^dis]connected" | cut --delimiter=" " -f1 # query connected monitors
read choice # read the users's choice of monitor
xrandr --output $choice --primary # set the primary monitor
il tutto da mettere in un file.sh
