I was taught many moons ago that configuration, like ogres and onions, is best considered in layers:

1. default values: What will most users in most places find most useful/least infuriating?

2. configuration files (system-wide, then user): What will most users on this system want most of the time? What will this particular user want most of the time?

3. environment variables: How should this session (i.e., a potentially large series of related executions) be tailored?

4. command line options: What is most useful for this particular run?

I was also taught that:

- figuring out how to go from an option to the name of a corresponding environment variable to a line in a config file should be both straightforward and well documented; and

- sometimes you need a more complex configuration than is cleanly supportable through any other method than a file. In such a case, the location of that file can itself be passed through options and the environment.