Puzzled with file descriptor in Bash (ffmpeg video capture) - linux

Question

I am trying to use file descriptors in Bash and found a problem I cannot solve.
I have to read a video stream coming from the standard output of a command executed in a coproc. This piece of code works as expected :
ffmpeg \
-i <(exec cat <&${COPROC[0]}) \
-c:v $ENCODE_VIDEO_FORMAT_LOSSLESS $ENCODE_VIDEO_OPTIONS_LOSSLESS \
-c:a copy \
-progress /dev/fd/1 \
"${capfile}"
But the cat process is not really useful since ffmpeg -i pipe:<file descriptor> seems to do the same. So I tried the following code which fails with pipe:63: Bad file descriptor
error.
ffmpeg \
-i pipe:"${COPROC[0]}" \
-c:v $ENCODE_VIDEO_FORMAT_LOSSLESS $ENCODE_VIDEO_OPTIONS_LOSSLESS \
-c:a copy \
-progress /dev/fd/1 \
"${capfile}"
The actual script is something a bit complicated but here is a minimal testing code for this issue :
#!/bin/bash
#
ENCODE_VIDEO_FORMAT_LOSSLESS=ffv1
ENCODE_VIDEO_OPTIONS_LOSSLESS="-level 3 -threads 7 -coder 1 -context 1 -g 1 -slices 30 -slicecrc 1"
capfile=capure.mkv
coproc ffmpeg -i file:'Camomille.mkv' -c:v copy -c:a copy -f matroska pipe:1
capture_fd=${COPROC[0]}
echo "hk_capture_pid=${COPROC_PID}"
ffmpeg \
-i pipe:${COPROC[0]} \
-c:v $ENCODE_VIDEO_FORMAT_LOSSLESS $ENCODE_VIDEO_OPTIONS_LOSSLESS \
-c:a copy \
-progress /dev/fd/1 \
"${capfile}"
This is the output of the second ffmpeg command :
ffmpeg version 4.1.4-1build2 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9 (Ubuntu 9.2.1-4ubuntu1)
configuration: --prefix=/usr --extra-version=1build2 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
libavutil 56. 22.100 / 56. 22.100
libavcodec 58. 35.100 / 58. 35.100
libavformat 58. 20.100 / 58. 20.100
libavdevice 58. 5.100 / 58. 5.100
libavfilter 7. 40.101 / 7. 40.101
libavresample 4. 0. 0 / 4. 0. 0
libswscale 5. 3.100 / 5. 3.100
libswresample 3. 3.100 / 3. 3.100
libpostproc 55. 3.100 / 55. 3.100
pipe:63: Bad file descriptor
av_interleaved_write_frame(): Broken pipe
Error writing trailer of pipe:1: Broken pipe
frame= 4 fps=0.0 q=-1.0 Lsize= 48kB time=00:00:00.03 bitrate=10051.1kbits/s speed=3.44x
video:86kB audio:1kB subtitle:0kB other streams:0kB global headers:2kB muxing overhead: unknown
Conversion failed!
This one fails and if you replace -i pipe:${COPROC[0]} by -i <(exec cat <&${COPROC[0]}) a capture.mkv file is created.
I run ubuntu eoan and bash version is : GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu). I upgraded several times since I started with this issue, so it wouldn't be related too much to bash and ffmpeg versions.
If someone can point me to what I do wrong with bash file descriptors I would be grateful.

Answer 1

The file descriptor ${COPROC[0]} is valid only for the shell, not for ffmpeg.
So what you need is :
ffmpeg \
-i pipe:0 \
-c:v $ENCODE_VIDEO_FORMAT_LOSSLESS $ENCODE_VIDEO_OPTIONS_LOSSLESS \
-c:a copy \
-progress /dev/fd/1 \
"${capfile}" <&${COPROC[0]}