I can connect to the db via sqlplus, however, whenever I issue commands like 'imp', 'cd', 'dir'...etc, I get the error in the subject i.e., 'unknown command'....
Kindly help me to resolve?
SQL*Plus command line <> OS command line
All commands you stated are OS ones. You do not need to run SQL*Plus to run them - they may be called from OS command line directly.
Anyway, if you need to invoke them from SQL*Plus, use HOST command. For more details, have a look into documentation: http://download.oracle.com/docs/cd/B...13.htm#1010003