If it’s empty or incorrect, and you are physically at the machine, you can try setting it manually:
Bash
export DISPLAY=:0
./ProjectLibre
3. SSH with X11 Forwarding
If you are connecting to this machine remotely via SSH, you can’t just run a GUI app without “tunneling” the graphics back to your local machine.
Reconnect using the -X or -Y flag:
Bash
ssh -X rob@192.168.x.x
Once connected, the DISPLAY variable is usually set automatically (often to something like localhost:10.0).
Why this happens (The “X11 Architecture”)
In the X Window System, the Server is actually the software managing your monitor and keyboard, while the Client is the application (ProjectLibre). Even if they are on the same physical box, they communicate via a protocol. If the “security cookies” (MIT-MAGIC-COOKIE) don’t match or the path is blocked, the Client (Java) panics because it has nowhere to “draw” its buttons and charts.
A Quick Note on “Headless”
If you are trying to run ProjectLibre on a server without a monitor just to process files, it won’t work this way. ProjectLibre requires a graphical environment to initialize. You would need a virtual framebuffer like xvfb to trick the app into thinking a screen exists.