What does HackerNews think of x2x?
x2x allows the keyboard, mouse on one X display to be used to control another X display.
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People used to use x2x for this: https://github.com/dottedmag/x2x
I've been using x2x https://github.com/dottedmag/x2x
It's really old and has it's perks but it works for my setup.
Hello! I would like to recommend to everyone one of the few utilities I've found in the last few years which blew my socks off, called x2x [0]. It is a small console program you can use to link your X session with a remote session (over SSH), so that you can move your mouse off your current laptop (or whatever) desktop and onto the remote "TV computer" desktop, and type things into that desktop with your keyboard. I'd been dreaming about this for years before I randomly stumbled on it. You can run it and just forget about it since it isn't literally streaming a series of JPGs like VNC. I've been using it with ZERO problems now for a couple of years exactly for this purpose.
xdmx does slightly different thing, it allows one to build X display that uses another X displays as it's screens (everything goes through the xmdx server and last time I tried it it was incredibly slow, as in significantly slower than using normal X11 over the same network).
X11-only equivalent of synergy is for example x2x (https://github.com/dottedmag/x2x).