Seems impossible to pass a variable from trigger to cmd

[Sarge2013]

Thanks for you plugin set, first of all.
My environment CentOS 7, MySQL 5.5, plugin lib_mysqludf_sys has installed and works properly.
But I can't pass a variable from trigger to cmd - may be a specific wrapper is needed ?
Please, see the trigger below
AFTER INSERT ON trig_test
FOR EACH ROW
BEGIN
DECLARE cmd CHAR(255);
DECLARE result CHAR(255);
DECLARE var INT;
SET @var = (SELECT MAX(ID) FROM trig_test) ;
-- // this line does not work, tested echo @var, echo "@var" etc - or empty or 0
SET cmd = CONCAT('echo "@var" >> /test/mysqltrigger.txt');
SET result = sys_exec(cmd);
-- // this line work OK
SET cmd = CONCAT('echo "test" >> /test/mysqltrigger.txt');
SET result = sys_exec(cmd);
-- // this line also work OK
INSERT INTO test (column1,column2) VALUES (@var,'test') ;
END;

Please, hint a solution if available,
Best regards,