[d3VwsX]

Some applications print usage information (e.g. with --help flag) to stderr. That is a bit annoying when there is a lot of output and I just want to grep for some flag, since the first attempt will fail and then I have to try again after adding 2>&1.

[matheusmoreira]

If I pass the --help flag, I'm explicitly asking the program to print the help text. In this case it's appropriate for the help text to be written to standard output since the help text is the output.

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.

[quectophoton]

Then there's `ssh-keyscan`, printing the "real" output to stdout, but also printing "comments" to stderr.

Nothing wrong with that, but it was a bit confusing the first time I saw that behavior.