xeyes

xeyes is a program that shows two eyes. The x in its name refers to the X11 display server, which is one of many ways to display graphics on screen.

xeyes is used mostly diagnostically, i.e. to find out if one used SSH with X-forwarding. When xeyes is run, but does not show the eyes, it means that SSH with X-forwarding does not work.

How to run xeyes

In a terminal, type:

xeyes

If you've logged in via SSH with X-forwarding and it works correctly, you will see this:

If you've logged in without SSH with X-forwarding or the SSH client is not setup correctly, you will see:

The line that indicates the error is:

Error: Can't open display: