← Back to context

Comment by ryandrake

15 hours ago

Another thing that gets screwed up a lot is: Comand line usage help/information should be printed to stderr if it was invoked because the user passed an invalid option to the command line (in other words, the usage text counts as diagnostic[1] info), but it should be printed to stdout if the user invoked the application with -h, --help or similar. Reasons:

1. If you mess up the command line to the program in a script or pipe, and get a bunch of usage output in stdout, a downstream consumer of that stdout might think its legit program output and try to parse it.

2. If your user actually calls the program with -h or --help, they might want to |less through it to read it on a small terminal screen. Output that to stdout.

3. Generally, you can always tell if something is going wrong by grepping for errors or warnings a single stream (stderr), or by looking for a nonzero exit code.

But your general principle applies: Output expected by the user -> stdout. Diagnostic output or output incidental to the program's operation or errors -> stderr.

1: https://pubs.opengroup.org/onlinepubs/9799919799/functions/s...

12 comments

ryandrake

Reply
dylan604  13 hours ago

> 3. Generally, you can always tell if something is going wrong by grepping for errors or warnings a single stream (stderr), or by looking for a nonzero exit code.

I'll use ffmpeg as an example of being an edge case. It's hard to get ffmpeg to give a nonzero exit code. What might be a problem for the user wasn't necessarily a problem for the app, so the app thinks it is completed and does its thing exiting with zero. For example, if a file is being read as input that is corrupted causing ffmpeg to no longer be able to read from the source, it will happily close your file cleanly so it is usable (just shorter than expected) and report it completed successfully. If all you do is check the exit code, you'll think your file is completed. Much more due diligence is necessary to be sure.

anyfoo  15 hours ago

Disagree. stdout is only reserved for actual processed command output. It may be empty, it may be invalid because the input was invalid (shit in, shit out), but it may never be things intended for a human to read.

If one wants to use a pager (like I sometimes do, though most of the time I just scroll up), they'll just use `foo 2>&1 | less`.

  • archargelod  2 hours ago

    > stdout is only reserved for actual processed command output

    If user asks a program to print help message, the help text is the processed command output!

  • ramses0  12 hours ago

    In the case of `git log -100 | grep FOO`, the log output should go to stdout.

    In the case of `git diff | grep FOO`, the diff output should go to stdout.

    In the case of `git --help | grep FOO` the help output should go to stdout.

    In the case of `git --omg-wtf | grep FOO`, it's fine if there is only output on stderr.