Portable which

which.rb

def which(file)
  exts = [""] + (ENV["PATHEXT"]||"").split(File::PATH_SEPARATOR)
  ENV["PATH"].split(File::PATH_SEPARATOR).map do |path|
    exts.map do |ext|
      filepath = File.join(path, file + ext)
      defined?(File::ALT_SEPARATOR) && File::ALT_SEPARATOR && filepath.tr!(File::SEPARATOR, File::ALT_SEPARATOR)
      filepath if File.exist?(filepath)
    end
  end.flatten.compact.first
end