Unexpected shell variable overwrite with `read` and `ffmpeg`

[Jimmy Leung]

Summary of the bug:

As the ticket title suggests, I find that ffmpeg is overwriting my other shell buffers.

Environment

ffmpeg version 7.1.1
macos 15.3.2

How to reproduce:

Below is the script that can reproduce the bug.

#!/bin/sh

set -e

# Or use another other short-enough video to speed up the testing
vid="https://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071"
folder=./temp
_index=0

mkdir -p $folder

    echo "$vid\n$vid\n$vid\n$vid\n$vid" \
|   while read -r vid_item;
do
    echo "vid@$_index: $vid_item"
    ffmpeg -hide_banner -loglevel error -c copy "$folder/${_index}.mp4" -i $vid_item

    _index=$((_index+1));
done

Apparently the $vid_item buffer is overwritten based on command output.

vid@0: https://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071
vid@1: s://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071
[in#0 @ 0x147e337a0] Error opening input: Protocol not found

And if I "patch" the ffmpeg command with echo & xargs, no such error can be found.

echo $vid_item | xargs ffmpeg -hide_banner -loglevel error -c copy "$folder/${_index}.mp4" -i

[MasterQuestionable]

͏    Looks like user instrumented shell error.
͏    Probably something to do with `read`:
͏    ​https://www.gnu.org/software/bash/manual/bash.html#index-read

͏    Maybe it could be irrelevant with FFmpeg at all... (shell bug?)

[Jimmy Leung]

Maybe...(?)

My other script has a loop like this, and the read buffer actually get piped to several utilities (sed, grep, curl, etc), and the missing character issue can only be found at the line I call ffmpeg.

I still suspect it is ffmpeg handing, but anyway I get my workaround. If project maintainer think this error is irrelevant to ffmpeg itself, feel free to close this ticket :)

Btw, this will not happen if I use ffprobe -i $var instead of ffmpeg command, which is also interesting :)

[MasterQuestionable]

͏    Would `ffmpeg -i "$var"` reproduce?
͏    Caveat the shell's field splitting on unquoted variables.

[Cigaes]

ffmpeg reads commands on its standard input. You can disable that with -nostdin.

[Jimmy Leung]

Replying to MasterQuestionable:

͏    Would `ffmpeg -i "$var"` reproduce?
͏    Caveat the shell's field splitting on unquoted variables.

Edit: Ignore the below messages please. Probably I will go with -nostdin for my script

To make the loop actually work, I will need to supply output file argument.

So that's what I could do, changing the original ffmpeg command into the below one.

- ffmpeg -hide_banner -loglevel error -c copy "$folder/${_index}.mp4" -i $vid_item
+ ffmpeg "$folder/${_index}.mp4" -i $vid_item

Without stripping away banner and log, I can reproduce a long piece of error message obtained at the 2nd iteration of the loop (indicated by the label vid@1:).

vid@1: ttps://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071
ffmpeg version 7.1.1 Copyright (c) 2000-2025 the FFmpeg developers
  built with Apple clang version 16.0.0 (clang-1600.0.26.6)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/7.1.1_1 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags='-Wl,-ld_classic' --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libharfbuzz --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-audiotoolbox --enable-neon
  libavutil      59. 39.100 / 59. 39.100
  libavcodec     61. 19.101 / 61. 19.101
  libavformat    61.  7.100 / 61.  7.100
  libavdevice    61.  3.100 / 61.  3.100
  libavfilter    10.  4.100 / 10.  4.100
  libswscale      8.  3.100 /  8.  3.100
  libswresample   5.  3.100 /  5.  3.100
  libpostproc    58.  3.100 / 58.  3.100
[in#0 @ 0x138632ca0] Error opening input: Protocol not found
[in#0 @ 0x138632ca0] Did you mean file:ttps://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071?
Error opening input file ttps://github.com/tsoding/musializer/assets/165283/8b9f9653-9b3d-4c04-9569-338fa19af071.
Error opening input files: Protocol not found

Somehow the first few characters go missing.

Tested it few more times. Seems it only happens to ffmpeg, but not ffprobe nor grep nor other shell utilities.

[Jimmy Leung]

Replying to Cigaes:

ffmpeg reads commands on its standard input. You can disable that with -nostdin.

Ok I just learn this from your reply. Then maybe it is not an issue at all, but my own misuse on ffmpeg :)

[Marton Balint]

Closing as invalid then.