指定したディスプレイ番号のXの共有が出来る．自分自身でもOK

$ x11vnc -display $DISPLAY

既定値では接続終了後に終了してしまう

$ x11vnc --help | grep "^-once" -A1
-once                  Exit after the first successfully connected viewer
                       disconnects, opposite of -forever. This is the Default.

-forever オプションを利用する

$ x11vnc --help | grep "^-forever" -A1
-forever               Keep listening for more connections rather than exiting
                       as soon as the first client(s) disconnect. Same as -many

-shared オプションを利用する

$ x11vnc --help | grep "^-shared" -A1
-shared                VNC display is shared, i.e. more than one viewer can
                       connect at the same time (default off).