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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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