How to Write End-to-End Tests for Legacy Code

The approach is not something I invented. There's a really good book about this called, working effectively with Legacy Code by Michael Feathers. So if I approach an existing system that was written in TTD, I would say that no one needs me there. If someone called me in, it's because they are unable to change their software and they need help to make their software changeable. The test is going to have to be disgusting. A lot of compromises might have to be made. We might have to keep some parts of the system untested, or you might have to test them manually to begin with.