$ env -i update-manager
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused
(update-manager:13433): Gtk-WARNING **: 08:47:49.719: cannot open display:
Works for me:
$ env -i update-manager
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused
(update- manager: 13433): Gtk-WARNING **: 08:47:49.719: cannot open display: