Why does a program like avprobe send normal output to &2?

Weapon S · 07-09-2015, 11:45 AM

In this case the program works something like this: you type in `avprobe /foo/bar.wav` and get

Code:

avprobe version 11.4-6:11.4-1~deb8u1, Copyright (c) 2007-2014 the Libav developers
  built on Jun  4 2015 19:39:02 with gcc 4.9.2 (Debian 4.9.2-10)
Input #0, wav, from '/foo/bar.wav':
  Duration: 00:00:04.14, bitrate: 705 kb/s
    Stream #0.0: Audio: pcm_s16le, 44100 Hz, 1 channels, s16, 705 kb/s
# avprobe output

Only the last trivial line comes from standard output; the rest disappears, when using `2> /dev/null`, or when you pipe to `less`.
As far as I know there's a standard output ("&1"), and a standard error output ("&2"). Some programs seem to use the error output for seemingly normal functionality (no errors).
Is there some reason behind it? If so, I'd really like to know, because it's annoying in scripts, and I do some programming as hobby. Or is it just bad practice?

jpollard · 07-09-2015, 12:32 PM

stdout is used for other data - that you weren't asking for, some of the data that can be sent to stdout is intended to be sent to other applications...

Shadow_7 · 07-12-2015, 10:24 AM

In days of old you would pipe your data from ffmpeg/avconv to ffmpeg/avconv. Like if you wanted to change the framerate without skipping or adding frames. So sending messages to stderr lets you keep stdout for your data path. While still being able to see the progress.

Weapon S · 07-12-2015, 10:38 AM

If I understand correctly, stdout used to be reserved for the actual media-data with ffmpeg/avconv, and so the text output goes to stderr. Thanks for explaining.