mysql> show variables like 'tmpdir';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| tmpdir    | /tmp |
+---------------+-------+
1 row in set (0.00 sec)

mysql> set global tmpdir = '/Data/app/mysql5.6.25/mysqltmp';
ERROR 1238 (HY000): Variable 'tmpdir' is a read only variable

mysql> show variables like 'tmpdir';
+---------------+--------------------------------+
| Variable_name | Value             |
+---------------+--------------------------------+
| tmpdir    | /mysql5.6/var/tmp |
+---------------+--------------------------------+
1 row in set (0.00 sec)