shebang

# File lib/rubygems/installer.rb, line 500
  def shebang(bin_file_name)
    ruby_name = RbConfig::CONFIG['ruby_install_name'] if @env_shebang
    path = File.join gem_dir, spec.bindir, 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 which = Gem.configuration[:custom_shebang]
      # replace bin_file_name with "ruby" to avoid endless loops
      which = which.gsub(/ #{bin_file_name}$/," #{RbConfig::CONFIG['ruby_install_name']}")

      which = which.gsub(/\$(\w+)/) do
        case $1
        when "env"
          @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
        when "ruby"
          "#{Gem.ruby}#{opts}"
        when "exec"
          bin_file_name
        when "name"
          spec.name
        end
      end

      "#!#{which}"
    elsif 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