I'm making a script that handles a predefined set of data, outputting to a file. I want to pop up a warning when one datum (which is always "Regular" in every set that I've had access to) is different stating that this value is unhandled (since I don't know how it affects the data). Should I output this warning to stderr or stdout?
If I save the output of this script (i.e. stdout only) so that I could process it later, would that warning interfere with how the output is parsed? Moreover, if output is piped to another process, the warning should show up on the terminal, so the user sees it immediately.
For those reasons, in general, you output warnings to stderr.