[matheusmoreira]

That's the right approach. Output goes to standard output, and user facing messages go to standard error. That way output can still be piped or redirected while the program talks to the user, and messages can also be suppressed by redirecting to the null device.

I've always been annoyed by the fact file descriptor 2 is called the "error" stream. Should have been called the "user" stream.

[kazinator]

If a tool unconditionally produces diagnostics on standard error, whether there is an exceptional situation or not, I consider that poor behavior, contrary to the quiet tool philosophy from Unix.

Standard error is special output that the user should be able to somehow see (e.g. on a terminal) even if the regular output is redirected (as you note above).

That doesn't mean standard error is above the quiet tool guideline.

[roundabout-host]

Or the "status" stream (you can still redirect it and it is useful sometimes).