The script launch a web server on port 2000 using WEBrick

webrick1.rb

#!/usr/bin/ruby
#---
# Excerpted from "Programming Ruby 1.9",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material, 
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose. 
# Visit http://www.pragmaticprogrammer.com/titles/ruby3 for more book information.
#---
require 'webrick'
include WEBrick

s = HTTPServer.new(Port: 2000,DocumentRoot: File.join(Dir.pwd, "/html"))

trap("INT") { s.shutdown }
s.start

The first line " #!/usr/bin/ruby " assumes that the script is in the doument root and tell Unix to execute the file as Ruby ( The file will be executed by ruby because Unix can find the compiler(?) in that path )
But how can the ERB file (https://gist.github.com/968621) find the Ruby compiler(?) to be executed? Do I have to set something in the web server (WEBrick)? How does WEBrick find the path to the ruby compiler?