Oracle Database Discussions

Hi experts,
i am using oracle 11g(releasae 2)
when i run the oracle opatch utility by the following command...i get the error...
SQL> $opatch lsinventory -detail
'opatch' is not recognized as an internal or external command,
operable program or batch file.

please tell me what settings i need to do before running this command so that i could check installed patches and apply new patches.......


Thanks ad regards,
MDBA