A `which` command for Ruby

which.rb

module FileUtils
  # Returns the full path to an executable command if it exists in the PATH.
  #
  #   FileUtils.which('ruby') # => "/usr/bin/ruby"
  #   FileUtils.which('matz') # => nil
  #
  # Keyword arguments:
  #
  # - <tt>env: 'PATH'</tt> - the PATH environment variable.
  # - <tt>separator: File::PATH_SEPARATOR</tt> - the PATH separator.
  #
  def which(command, env: 'PATH', separator: File::PATH_SEPARATOR)
    ENV.fetch(env) { return nil }
       .split(separator)
       .filter_map { |dir| Dir.exist?(dir) && File.join(dir, command) }
       .find { |path| File.file?(path) && File.executable?(path) }
  end
  module_function :which
end