check_executable_overwrite

# File lib/rubygems/installer.rb, line 199
  def check_executable_overwrite filename # :nodoc:
    return if @force

    generated_bin = File.join @bin_dir, formatted_program_filename(filename)

    return unless File.exist? generated_bin

    ruby_executable = false
    existing = nil

    File.open generated_bin, 'rb' do |io|
      next unless io.gets =~ /^#!/ # shebang
      io.gets # blankline

      # TODO detect a specially formatted comment instead of trying
      # to run a regexp against Ruby code.
      next unless io.gets =~ /This file was generated by RubyGems/

      ruby_executable = true
      existing = io.read.slice(%{
          ^\s*(
            gem \s |
            load \s Gem\.bin_path\( |
            load \s Gem\.activate_bin_path\(
          )
          (['"])(.*?)(\22)),
        }, 3)
    end

    return if spec.name == existing

    # somebody has written to RubyGems' directory, overwrite, too bad
    return if Gem.default_bindir != @bin_dir and not ruby_executable

    question = "#{spec.name}'s executable \"#{filename}\" conflicts with ".dup

    if ruby_executable then
      question << (existing || 'an unknown executable')

      return if ask_yes_no "#{question}\nOverwrite the executable?", false

      conflict = "installed executable from #{existing}"
    else
      question << generated_bin

      return if ask_yes_no "#{question}\nOverwrite the executable?", false

      conflict = generated_bin
    end

    raise Gem::InstallError,
      "\"#{filename}\" from #{spec.name} conflicts with #{conflict}"
  end