4. A Little Theory

The magic word is DISPLAY. In the X window system, a display consists (simplified) of a keyboard, a mouse and a screen. A display is managed by a server program, known as an X server. The server serves displaying capabilities to other programs that connect to it.

A display is indicated with a name, for instance:

• DISPLAY=light.uni.verse:0
• DISPLAY=localhost:4
• DISPLAY=:0

The display consists of a hostname (such as light.uni.verse and localhost), a colon (:), and a sequence number (such as 0 and 4). The hostname of the display is the name of the computer where the X server runs. An omitted hostname means the local host. The sequence number is usually 0 -- it can be varied if there are multiple displays connected to one computer.

If you ever come across a display indication with an extra .n attached to it, that's the screen number. A display can actually have multiple screens. Usually there's only one screen though, with number n=0, so that's the default.

Other forms of DISPLAY exist, but the above will do for our purposes.

For the technically curious:

• hostname:D.S means screen S on display D of host hostname; the X server for this display is listening at TCP port 6000+D.
• host/unix:D.S means screen S on display D of host host; the X server for this display is listening at UNIX domain socket /tmp/.X11-unix/XD (so it's only reachable from host).
• :D.S is equivalent to host/unix:D.S, where host is the local hostname.