def _get_pid_file_variable(self, db):
        """
        Get the `pid_file` variable
        """
        pid_file = None
        try:
            with closing(db.cursor()) as cursor:
                cursor.execute("SHOW VARIABLES LIKE 'pid_file'")
                pid_file = cursor.fetchone()[1]
        except Exception:
            self.warning("Error while fetching pid_file variable of MySQL.")

        return pid_file
Get the `pid_file` variable