code: public static UnixUserGroupInformation login(Configuration conf, boolean save
        ) throws LoginException {
      UnixUserGroupInformation ugi = readFromConf(conf, UGI_PROPERTY_NAME);
      if (ugi == null) {
        ugi = login();
        LOG.debug("Unix Login: " + ugi);
        if (save) {
          saveToConf(conf, UGI_PROPERTY_NAME, ugi);
        }
      }
      return ugi;
    }
docs: Get a user's name & its group names from the given configuration;
  If it is not defined in the configuration, get the current user's
  information from Unix.
  If the user has a UGI in the ugi map, return the one in
  the UGI map.
  
  @param conf either a job configuration or client's configuration
  @param save saving it to conf?
  @return UnixUserGroupInformation a user/group information
  @exception LoginException if not able to get the user/group information