It works but the documentation is wrong.
The default socket name depends on the user name.
For non-root this will typically be $XDG_RUNTIME_DIR/mir_socket
For root (where XDG_RUNTIME_DIR isn't set) this will typically be /tmp/mir_socket
So you need "bin/mir_demo_client_egltriangle -m /tmp/mir_socket"
It works but the documentation is wrong.
The default socket name depends on the user name.
For non-root this will typically be $XDG_RUNTIME_ DIR/mir_ socket
For root (where XDG_RUNTIME_DIR isn't set) this will typically be /tmp/mir_socket
So you need "bin/mir_ demo_client_ egltriangle -m /tmp/mir_socket"