docs: def mkdir(path, create_parent=True, check_if_exists=False):
      """
      Generates a unix command line for creating a directory.
  
      :param path: Directory path.
      :type path: unicode | str
      :param create_parent: Create parent directories, if necessary. Default is ``True``.
      :type create_parent: bool
      :param check_if_exists: Prepend a check if the directory exists; in that case, the command is not run.
        Default is ``False``.
      :type check_if_exists: bool
      :return: Unix shell command line.
      :rtype: unicode | str
      """
      cmd = _format_cmd('mkdir', path, _p=create_parent)
      if check_if_exists:
          return 'if [[ ! -d {0} ]]; then {1}; fi'.format(path, cmd)
      return cmd