I've written a script that opens up a file, reads the content and does some operations and calculations and stores them in sets and dictionaries.
How would I write a unit test for such a thing? My questions specifically are:

Would I test that the file opened?
The file is huge (it's the unix dictionary file). How would I unit test the calculations? Do I literally have to manually calculate everything and test that the result is right? I have a feeling that this defeats the whole purpose of unit testing. I'm not taking any input through stdin.
You haven't explained what the calculations are, but I guess your program should be able to work with a subset of the big file, as well. If this is the case, make a unit test which opens up a small file , does the calculations and produces some result, which you can verify is correct.
Unit testing a script that opens a file