I`m relatively new to the concept of unit-testing and have very little experience in the same. I have been looking at lots of articles on how to write unit-tests; however, I still have difficulty in writing tests where conditions like the following arise:-

1. Test user Input.
2. Test input read from a file.
3. Test input read from an environment variable.

Itd be great if someone could show me how to approach the above mentioned scenarios; itd still be awesome if you could point me to a few docs/articles/blog posts which I could read.