Can you provide a concrete example of such an application?
> Unless your app doesn't generate [stdout] output,then grepping the [visible] output will not work if it's stderr and it will be confusing
With the first "output" meaning "specifically stdout", and the second meaning "what you see on the terminal".
As in, you run a command, see it printed a lot of stuff, so then you do it again but piping it to `grep` or `less`, and realize none of that output was actually going to stdout.
The output of a package manager is the packages. The text it writes to the terminal is not part of the output, it's just the program's log. I haven't thought about what's appropriate in this situation. By unix standards package managers shouldn't even be printing anything to begin with.
If the user passes invalid options and inputs, then the program should produce no output and any error or help messages should be written to the error stream. It's important that these messages end up on the terminal when one gets the command invocation wrong while trying to pipe data into another program. If they're written to standard output, the program on the other end of the pipe will slurp all the error messages up and try to parse them as though they were valid inputs.
Nothing wrong with that, but it was a bit confusing the first time I saw that behavior.
