Sometimes “valid” doesn’t mean “illegal to have in a configuration file”, it just means that “in this specific case, the configuration should have a certain value”. For example, on my Mac I should prefix my commands with “g” if I want to access GNU tools, while on most Linux systems I don’t need to do this. Trying to run gsed on Ubuntu would be “invalid” in this case but a parser can’t help me here.
Yes it can: if your config file is more declarative than simply "lists of command lines". The program can determine what is true of its environment, and can construct command lines appropriately given the data about intended outcome that is stored in the configuration file.
