DISPLAY=:0.0 vs DISPLAY=localhost:0.0
 

I'm trying to learn more about X11. Omitting the hostname from the DISPLAY variable should imply the server is running locally, yes?

From a terminal running in Ubuntu:
Run 'xeyes' and the window pops up fine. But neither of the following work:
Can someone elaborate on why this is?

With DISPLAY=localhost:0.0 you are asking to connect to an X11 server via TCP. Your X server is most likely not listening on a TCP socket.

I never did get what the OP was asking.

echo something is you telling the system to report. In the first case it is the name of the terminal but that may be that hostname file is not set.

Set it and your terminal ought to then say the name and the ending such as myscreen:0 or on some systems myscreen:0.0

Then run xeyes -diaplay myscreen:0

It doesn't matter what you say in the host file if X isn't listening on a TCP/IP socket and the normal usage is to not do so.

Run 'xhost +localhost' first. Be careful with xhost..as eeekster eluded to it's generally not a good idea to listen for X connections on TCP.

That only adds permission for localhost; it does not cause the X server to start listening on an IP socket.

Used to have fun with people at work that didn't restrict access on the Windows based X servers. ;)

should be hostfile not host file.

the echo $DISPLAY :0 proves that there is no name on that terminal.

I assumed that the OP has a gui running such as kde or gnome or such. That may be wrong.

I actually did have /etc/hostname set (if that's what you meant), but that doesn't seem to affect whether $DISPLAY contains the hostname or not. This, however:
helped immensely, since I found this and was able to get a TCP connection (working on an X11 proxy for input fuzzing).

Thanks very much!