How to poll for Xvfb to be ready?

By default Xvfb will create a Unix Domain socket for clients to connect. On my system this file socket file is created in /tmp/.X11-unix/. You could use inotifywait to listen for events in this directory. For example,

 $ inotifywait -e create /tmp/.X11-unix/

and then run Xvfb :9 (display 9, for example). When it is ready you should see

/tmp/.X11-unix/ CREATE X9

from the inotifywait which will terminate. You should now be able to connect to DISPLAY=:9.