awkward behaviour in bash find-loop with ffmpeg - bash

Question

There is something weird going on when running following script with Docker.
The dockerfile for this is:
FROM debian:9
WORKDIR /app
RUN apt-get -y update && \
apt-get -y install ffmpeg curl
COPY . /app
The script run.sh:
#!/bin/bash
find /pfs/in -maxdepth 1 -name "*.flac" -print0 | while IFS= read -r -d $'\0' inFile; do
echo "\n##### Process '${inFile}' #####"
ffmpeg -y -i ${inFile} -ar 16000 tmp.wav # use 16kHz - default for EML
done
Starting this, when mounting 3 files into the container:
$ ls pfs/in/
Testaudio16k_2.flac Testaudio16k.flac Testaudio16k.wav TestSprache_Saetze.flac
$ docker run --rm -t -v $(pwd)/pfs/in:/pfs/in test-img:latest /bin/bash run.sh
I get an error on processing the second file: pfs/in/Testaudio16k_2.flac: No such file or directory. The leading / is missing. It is also missing in the preceeding echo. Indeed this happens every second file (if I put more than 3 files in that folder).
Now coming to the counter example:
If I comment out the ffmpeg line in the script, rebuild and run, The echo prints for every file the correct path.
Does anybody have an idea about this?
Is it about the find or is the ffmpegdoing something weird? Something completely different?

Answer 1

Use -nostdin:
ffmpeg -nostdin -i input output
From the ffmpeg documentation:
-stdin
Enable interaction on standard input. On by default unless
standard input is used as an input. To explicitly disable interaction
you need to specify -nostdin.
Disabling interaction on standard input is useful, for example, if
ffmpeg is in the background process group. Roughly the same result
can be achieved with ffmpeg ... < /dev/null but it requires a shell.

Answer 2

I was able to reproduce on a regular Bash console. It's indeed a weird interaction between ffmpeg and the IFS or null delimiter.
This method work as intended
find ~/tmp -maxdepth 1 -name "*.flac" -print0 | xargs -r0 -i bash -c 'echo {}; ffmpeg -hide_banner -y -i {} -ar 16000 tmp.wav'
For your code, add < /dev/null at the end of ffmpeg command as explained here.
ffmpeg -hide_banner -y -i {} -ar 16000 tmp.wav < /dev/null

Answer 3

Can't reproduce that but I strongly recommend to quote the filename when passing it to ffmpeg like this:
ffmpeg -y -i "${inFile}" -ar 16000 tmp.wav.
A space at the beginning or end of the filename would explain your error when you don't use quotes
Btw, you don't need a bash loop. You can just use find:
docker run --rm -t -v $(pwd)/pfs/in:/pfs/in foo \
find /pfs/in -maxdepth 1 -name "*.flac" \
-printf "## Process file %f ##\n" \
-exec ffmpeg -y -i {} -ar 16000 tmp.wav \;