xclock

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

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

How to run xclock

In a terminal, type:

xclock

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: