Find a free X11 display number

Question

I have some unit tests that need an X11 display so I plan to start Xvfb before running them, but to start Xvfb I will need a free display number to connect it to. My best guess

Answer 1

Why not exploit the fact that every X11 server puts a "Lock" file into /tmp?

This is called /tmp/.Xn-lock where "n" is the Display id. (Also note the leading . in the filename).

This is the mechanism that Xserver itself uses to check for duplication, and it appears to be consistent on all *nix platforms I have tried (HP-UX, Linux, ...)

So you could adapt your script thus (forgive me for syntax errors, I'm nore accustomed to C shell than Bourne/Korn shell scripting)

DISPLAY_NUM=0

do

  if ( -e /tmp/.X$DISPLAY_NUM-lock ) then

     let DISPLAY_NUM=$DISPLAY_NUM+1

  else

     Xvfb :$DISPLAY_NUM -screen 0 1280x1024x24 -ac   (or whatever args take your fancy)

  fi

done