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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 get_user_id(username):
 """ Returns the user id to a given user name. Returns ``None`` if the user does not exist. :param username: User name. :type username: unicode :return: User id. :rtype: int """

    uid = single_line_stdout('id -u {0}'.format(username), expected_errors=(1,), shell=False)
    return check_int(uid)