def shebang(bin_file_name)
    ruby_name = Gem::ConfigMap[:ruby_install_name] if @env_shebang
    path = spec.bin_file bin_file_name
    first_line = File.open(path, "rb") {|file| file.gets}

    if /\A#!/ =~ first_line then
      # Preserve extra words on shebang line, like "-w".  Thanks RPA.
      shebang = first_line.sub(/\A\#!.*?ruby\S*((\s+\S+)+)/, "#!#{Gem.ruby}")
      opts = $1
      shebang.strip! # Avoid nasty ^M issues.
    end

    if not ruby_name then
      "#!#{Gem.ruby}#{opts}"
    elsif opts then
      "#!/bin/sh\n'exec' #{ruby_name.dump} '-x' \"$0\" \"$@\"\n#{shebang}"
    else
      # Create a plain shebang line.
      @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
      "#!#{@env_path} #{ruby_name}"
    end
  end
#
Generates a #! line for +bin_file_name+'s wrapper copying arguments if
necessary.