some gui programs like gparted (very nice harddisk partitioning program) won’t start as root.
hostnamectl; # tested on Operating System: Debian GNU/Linux 10 (buster) Kernel: Linux 4.19.0-13-amd64 Architecture: x86-64 su - root; # become root gvim; # fails with E233: cannot open display gparted; # fails with (gpartedbin:10160): Gtk-WARNING **: 22:38:52.297: cannot open display: # how to fix this export DISPLAY=':0.0' cp -v /home/user-running-x11-session/.Xauthority /root/.Xauthority # then try to start it gparted# or gvim