When i ssh to server using -X, i always confuse about which display number i should export. It seems to me sometimes the display number has been used by something, so what i can do is only export DISPLAY=localhost:0 && xclock export DISPLAY=localhost:1 && xclock export DISPLAY=localhost:2 && xclock export DISPLAY=localhost:... until the clock appear. Then i will use that display number. Each time log in to the server, the display number which is able to tunnel the gui data correct would be different. I know many of such similar questions has been asked and answer. However I couldn't find answer to my question, anyone know about it? Thanks!