如何测试使用外部命令行工具的软件

I'm trying to figure out how to test-drive software that launches external processes that take file paths as an input and write output after lengthy processing to stdout or some file? Is there some common patterns on writing tests in this kind of situations? It is hard to create fast executing tests that could verify correct usage of external tools without launching actual tools in tests and inspecting the results.I'm trying to figure out how to test-drive soft