How hard would this be?

[Red Squirrel]

I've never coded at the hardware level so I'm really not sure if this is easy or not.

Say I want to write a custom UPS software, how hard would it be to talk to a UPS via serial or USB? (guessing both are completely different ways of talking) Is it similar to handling a tcp session, with send/recv? Guessing this requires some assembly level coding as well.

This would be for Linux.

[henky@nok.co.id]

Not so hard. Almost similar to using TCP/IP socket.
You just open the serial/USB then do writing/reading
using write() or read() instead of send() or recv().