This is a serious post about a serious issue -- restarted due to
distracting responses.
The best way to see and replicate the problem is to run this script
(which creates three AC3s and a FOO.TXT on c:\):
ffmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t
240.0 -c ac3 "c:\file1.ac3"
ffmpeg -y -f lavfi -i anullsrc=channel_layout=2:sample_rate=48000 -t
240.0 -c ac3 "c:\file2.ac3"
echo ffconcat version 1.0>c:\concat.txt
echo file 'file1.ac3'>>c:\concat.txt
echo file 'file2.ac3'>>c:\concat.txt
ffmpeg -y -safe 0 -i "c:\concat.txt" "c:\concat.ac3"
echo this is foo>c:\FOO.TXT
On my computer, '-t 240.0' created silent AC3s that were long-large
enough to provoke the issue -- on your system, '-t' may need to be bigger.
In the terminal output shown below, note these lines:
<quote>
Enter command: <target>|all <time>|-1 <command>[ <argument>]
Parse error, at least 3 arguments were expected, only 1 given in string
'ho this is foo>c:\foo.txt'
</quote>
Egad! That should not be happening. To me, it indicates that after a
certain length of execution time, FFmpeg goes into some sort of command
mode during which it fetches the 'echo' string and responds to it as
though it was a command to FFmpeg. WTF? If FFmpeg does have some sort of
automatic command mode that it monitors, isn't that asking for trouble?
Certainly I consider it a bug (or design flaw).
C:\Windows\System32>ffmpeg -y -f lavfi -i
anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file1.ac3"
ffmpeg version 2025-07-23-git-829680f96a-full_build-www.gyan.dev
Copyright (c) 2000-2025 the FFmpeg developers
built with gcc 15.1.0 (Rev6, Built by MSYS2 project)
configuration: --enable-gpl --enable-version3 --enable-static
--disable-w32threads --disable-autodetect --enable-fontconfig
--enable-iconv --enable-gnutls --enable-lc
ms2 --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma
--enable-libsnappy --enable-zlib --enable-librist --enable-libsrt
--enable-libssh --enable-libzmq --enabl
e-avisynth --enable-libbluray --enable-libcaca --enable-libdvdnav
--enable-libdvdread --enable-sdl2 --enable-libaribb24
--enable-libaribcaption --enable-libdav1d --enab
le-libdavs2 --enable-libopenjpeg --enable-libquirc --enable-libuavs3d
--enable-libxevd --enable-libzvbi --enable-liboapv --enable-libqrencode
--enable-librav1e --enable
-libsvtav1 --enable-libvvenc --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid
--enable-libaom --enable-libjxl --en
able-libvpx --enable-mediafoundation --enable-libass --enable-frei0r
--enable-libfreetype --enable-libfribidi --enable-libharfbuzz
--enable-liblensfun --enable-libvidst
ab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm
--enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va
--enable-ffnvcodec --enable-libvpl
--enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc
--enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio
--enable-openal --enable-libgme --
enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb
--enable-libmp3lame --enable-libshine --enable-libtheora
--enable-libtwolame --enable-libvo-amrwbenc --
enable-libcodec2 --enable-libilbc --enable-libgsm --enable-liblc3
--enable-libopencore-amrnb --enable-libopus --enable-libspeex
--enable-libvorbis --enable-ladspa --ena
ble-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband
--enable-libsoxr --enable-chromaprint
libavutil 60. 6.100 / 60. 6.100
libavcodec 62. 8.100 / 62. 8.100
libavformat 62. 1.103 / 62. 1.103
libavdevice 62. 0.100 / 62. 0.100
libavfilter 11. 3.100 / 11. 3.100
libswscale 9. 0.100 / 9. 0.100
libswresample 6. 0.100 / 6. 0.100
[out @ 0000000001cabf40] The "sample_fmts" option is deprecated: set the
supported sample formats
[out @ 0000000001cabf40] The "all_channel_counts" option is deprecated:
accept all channel counts
Input #0, lavfi, from 'anullsrc=channel_layout=2:sample_rate=48000':
Duration: N/A, start: 0.000000, bitrate: 384 kb/s
Stream #0:0: Audio: pcm_u8, 48000 Hz, 1 channels (FR), u8, 384 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (pcm_u8 (native) -> ac3 (native))
Press [q] to stop, [?] for help
Output #0, ac3, to 'c:\file1.ac3':
Metadata:
encoder : Lavf62.1.103
Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
Metadata:
encoder : Lavc62.8.100 ac3
[out#0/ac3 @ 0000000001cac040] video:0KiB audio:5625KiB subtitle:0KiB
other streams:0KiB global headers:0KiB muxing overhead: 0.000000%
size= 5625KiB time=00:03:59.99 bitrate= 192.0kbits/s speed= 329x
elapsed=0:00:00.72
C:\Windows\System32>fmpeg -y -f lavfi -i
anullsrc=channel_layout=2:sample_rate=48000 -t 240.0 -c ac3 "c:\file2.ac3"
'fmpeg' is not recognized as an internal or external command,
operable program or batch file.
C:\Windows\System32>echo ffconcat version 1.0>c:\concat.txt
C:\Windows\System32>echo file 'file1.ac3'>>c:\concat.txt
C:\Windows\System32>echo file 'file2.ac3'>>c:\concat.txt
C:\Windows\System32>ffmpeg -y -safe 0 -i "c:\concat.txt" "c:\concat.ac3"
ffmpeg version 2025-07-23-git-829680f96a-full_build-www.gyan.dev
Copyright (c) 2000-2025 the FFmpeg developers
built with gcc 15.1.0 (Rev6, Built by MSYS2 project)
configuration: --enable-gpl --enable-version3 --enable-static
--disable-w32threads --disable-autodetect --enable-fontconfig
--enable-iconv --enable-gnutls --enable-lc
ms2 --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma
--enable-libsnappy --enable-zlib --enable-librist --enable-libsrt
--enable-libssh --enable-libzmq --enabl
e-avisynth --enable-libbluray --enable-libcaca --enable-libdvdnav
--enable-libdvdread --enable-sdl2 --enable-libaribb24
--enable-libaribcaption --enable-libdav1d --enab
le-libdavs2 --enable-libopenjpeg --enable-libquirc --enable-libuavs3d
--enable-libxevd --enable-libzvbi --enable-liboapv --enable-libqrencode
--enable-librav1e --enable
-libsvtav1 --enable-libvvenc --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid
--enable-libaom --enable-libjxl --en
able-libvpx --enable-mediafoundation --enable-libass --enable-frei0r
--enable-libfreetype --enable-libfribidi --enable-libharfbuzz
--enable-liblensfun --enable-libvidst
ab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm
--enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va
--enable-ffnvcodec --enable-libvpl
--enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc
--enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio
--enable-openal --enable-libgme --
enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb
--enable-libmp3lame --enable-libshine --enable-libtheora
--enable-libtwolame --enable-libvo-amrwbenc --
enable-libcodec2 --enable-libilbc --enable-libgsm --enable-liblc3
--enable-libopencore-amrnb --enable-libopus --enable-libspeex
--enable-libvorbis --enable-ladspa --ena
ble-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband
--enable-libsoxr --enable-chromaprint
libavutil 60. 6.100 / 60. 6.100
libavcodec 62. 8.100 / 62. 8.100
libavformat 62. 1.103 / 62. 1.103
libavdevice 62. 0.100 / 62. 0.100
libavfilter 11. 3.100 / 11. 3.100
libswscale 9. 0.100 / 9. 0.100
libswresample 6. 0.100 / 6. 0.100
[ac3 @ 00000000026f2ac0] Estimating duration from bitrate, this may be
inaccurate
Input #0, concat, from 'c:\concat.txt':
Duration: N/A, start: 0.000000, bitrate: 192 kb/s
Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (ac3 (native) -> ac3 (native))
Press [q] to stop, [?] for help
Output #0, ac3, to 'c:\concat.ac3':
Metadata:
encoder : Lavf62.1.103
Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
Metadata:
encoder : Lavc62.8.100 ac3
[ac3 @ 00000000026f8240] Estimating duration from bitrate, this may be
inaccurate00.50
Enter command: <target>|all <time>|-1 <command>[ <argument>]
Parse error, at least 3 arguments were expected, only 1 given in string
'ho this is foo>c:\foo.txt'
[out#0/ac3 @ 000000000280bd00] video:0KiB audio:8438KiB subtitle:0KiB
other streams:0KiB global headers:0KiB muxing overhead: 0.000000%
size= 8438KiB time=00:05:59.99 bitrate= 192.0kbits/s speed=300x
elapsed=0:00:01.20
C:\Windows\System32>
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