what about adding the following feature to s6-echo? command line option -D enables double-output mode, so instead of using stdout for regular output, and stderr for warnings and errors, everything is sent both to stdout and stderr. This is intended to redirect stderr to a log file, so full output can be both shown on console and logged.
The command you're looking for is named "tee". :) I'd rather keep s6-echo as simple and intuitive as possible - it's meant to write stuff to stdout and nothing else. I'm already pretty angry that this command exists - the only reason why s6-echo needs to be a thing is that no two implementations of the "echo" command do the exact same thing, so "echo" is unusable when you have a string that starts with a dash (or a string that you *don't know*, because it could start with a dash and yield unpredictable outcomes.) If it weren't for that, I would gladly scrap s6-echo. So I'm not going to add stuff to it and make *more* software depend on it. -- Laurent