If you want to grep in all output (or want to write all to a file), you always need to 2>&1.

I understand that tools whose purpose is to process some input and write the result to output (e.g. sed) need to handle output and error messages separately. Another example is wget, where you only want to have the downloaded content in stdout. For all these, I think splitting is fine.

But I don't understand this behavior in tools like ffprobe, whose purpose is to analyze a file and then write the result.

STDOUT is intended for "things that you might wish to 'pipe' as input to the next program in the chain." These are "the legitimate outputs" of the program, representing the results that it is tasked to produce.

STDIN is the input-stream of data that a program might expect to have piped-in to it. (If your program needs more-than-one input stream, you have to rig it yourself.)

Meanwhile, STDERR is the chatter-box, where you're talking to the person who is watching the blinking lights. Status messages, error messages, progress messages and so forth. You can instruct the shell to combine the two streams if you wish. You can also divert it to /dev/null, thereby discarding it completely.

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This might be of interest:

https://www.jstorimer.com/blogs/work...tead-of-stdout

Well ... then ffprobe is doing it wrong.

when I use ffprobe -show_streams, both stdout and stderr contain information I'm interested in (and that I pipe to grep or less). In this example, stderr contains info about the streams, not error messages.

I've seen this behavior long time ago in other tools, to, but I don't remember which ones.

Have you tried using the -loglevel quiet parameter to see if this makes things more acceptable?

It just produces less output, in my use case it omits the meta data info for the container.

Ok. If ffprobe is acting in this way then I suggest, in this particular case, that you contact the ffmpeg guys at https://ffmpeg.org/contact.html and ask them if this is normal and, if not, if they would consider changing the behaviour. I would be interested to hear what they say.