criquet's Journal: Generating files when referenced

I want a means to generate a file's contents whenever it's referenced (and have it cached unless something has changed). I think this can be accompilshed with devices (at least on Linux) but I'm not sure that that's the right approach. Ideally I'd like to be able to write a script in, say, Bash, python or perl, that produced output that, for all intents and purposes, was the contents of a readonly file.