serial/parallel loopback testing
 

Is there any software I can run under Linux to allow testing serial and parallel ports with loopback plugs attached?

You don't really need special software for that. You could just use minicom or a similar terminal emulator. You could even forgo that if you ran (in two separate terminals):

and

The concept of the loopback tester is that anything sent over TX is wired directly into RX. So sending anything out should cause a duplicate of it to be received if the port is working correctly.

another way
 

you can also use the read and write function to test the ports by opening them using the file open command...