You scale, crop or overlay a video with -filter_complex, label the result [v], and -map "[v]". The audio disappears, so you add -map 0 to bring it back. The exit code is 0 and nothing is printed, but the file has two video streams, the original and yours, and the (default) flag sits on the original.

Below I reproduce it with a generated test clip, explain why -map 0 does this, and show the fixes and two related traps I hit while testing.

The ffmpeg I used:

$ ffmpeg -version | head -1
ffmpeg version 7.1 Copyright (c) 2000-2024 the FFmpeg developers

Everything below runs in one empty folder, in this order.

A test clip and a way to list streams

Two seconds of test pattern (320x240) with a 440 Hz tone, plus the same video without audio for later:

$ ffmpeg -hide_banner -loglevel error -f lavfi -i testsrc=s=320x240:d=2:r=25 -f lavfi -i sine=f=440:d=2 -c:v libx264 -c:a aac -shortest in.mp4
$ ffmpeg -hide_banner -loglevel error -f lavfi -i testsrc=s=320x240:d=2:r=25 -c:v libx264 noaudio.mp4

This lists the streams with ffmpeg -i, so it works without ffprobe; the grep keeps only the stream lines:

# streams.sh — list the streams in a file (no ffprobe needed)
ffmpeg -hide_banner -i "$1" 2>&1 | grep 'Stream #'

$ sh streams.sh in.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 320x240 [SAR 1:1 DAR 4:3], 47 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)

One video, one audio.

Reproduce: -map "[v]" -map 0

The filter halves the size to 160x120, so it’s easy to tell which stream is which:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0 1.mp4; echo "exit $?"
exit 0
$ sh streams.sh 1.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn
  Stream #0:1[0x2](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 320x240 [SAR 1:1 DAR 4:3], 46 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  Stream #0:2[0x3](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)

Three streams: the filtered 160x120 video, the original 320x240 video, and the audio. The (default) flag is on the 320x240 one, not on the filtered one.

With the maps the other way round, the original comes first:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map 0 -map "[v]" 2.mp4; echo "exit $?"
exit 0
$ sh streams.sh 2.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 320x240 [SAR 1:1 DAR 4:3], 46 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)
  Stream #0:2[0x3](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn

Now the unfiltered video is stream 0 and carries the (default) flag.

Why: -map 0 means every stream of input 0

With a -map, ffmpeg takes only what you mapped, which is why -map "[v]" alone drops the audio:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" 3.mp4; echo "exit $?"
exit 0
$ sh streams.sh 3.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn (default)

-map 0 then adds all of input 0: its audio, and also its video. The filter reading [0:v] doesn’t take that stream out of the list. So the original video goes into the output a second time, next to the filtered one.

Why the (default) flag ends up on the original rather than the filtered stream in 1.mp4 I didn’t check; I only see that it does.

Fix: map only the audio

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0:a 4.mp4; echo "exit $?"
exit 0
$ sh streams.sh 4.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)

One filtered video, one audio, and the default flag on the filtered video.

If some inputs have no audio, -map 0:a is an error:

$ ffmpeg -hide_banner -loglevel error -i noaudio.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0:a 5.mp4; echo "exit $?"
Stream map '' matches no streams.
To ignore this, add a trailing '?' to the map.
Failed to set value '0:a' for option 'map': Invalid argument
Error parsing options for output file 5.mp4.
Error opening output files: Invalid argument
exit 234

(The empty '' in the first line is what this ffmpeg build actually printed.)

Add the ? the message suggests. In zsh, quote it, or zsh treats ? as a wildcard and never runs ffmpeg:

$ ffmpeg -hide_banner -loglevel error -i noaudio.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0:a? 9.mp4; echo "exit $?"
zsh:1: no matches found: 0:a?
exit 1

Quoted:

$ ffmpeg -hide_banner -loglevel error -i noaudio.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map "0:a?" 6.mp4; echo "exit $?"
exit 0
$ sh streams.sh 6.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn (default)

A negative map works only before the label

You can also keep -map 0 and remove its video with a negative map, -map -0:v. Placed after -map "[v]", it broke the command:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0 -map -0:v 7.mp4; echo "exit $?"
[fc#0 @ 0x60000336c0a0] Filter scale:default has an unconnected output
Error binding filtergraph inputs/outputs: Invalid argument
exit 234

The filter’s output ended up unconnected, as if the negative map had also removed the [v] mapping; I didn’t dig further. Placed before -map "[v]", it works, but the audio becomes stream 0:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map 0 -map -0:v -map "[v]" 8.mp4; echo "exit $?"
exit 0
$ sh streams.sh 8.mp4
  Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)
  Stream #0:1[0x2](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn (default)

-map "[v]" -map 0:a is shorter and keeps the video first.

Trap while testing: a failed run leaves the old file

If you check your fix by overwriting the same out.mp4, a failed run makes it look like the fix didn’t work. First the broken command, then a “fixed” one that fails because the input has no audio:

$ ffmpeg -hide_banner -loglevel error -i in.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0 -y out.mp4; echo "exit $?"
exit 0
$ ffmpeg -hide_banner -loglevel error -i noaudio.mp4 -filter_complex "[0:v]scale=160:120[v]" -map "[v]" -map 0:a -y out.mp4; echo "exit $?"
Stream map '' matches no streams.
To ignore this, add a trailing '?' to the map.
Failed to set value '0:a' for option 'map': Invalid argument
Error parsing options for output file out.mp4.
Error opening output files: Invalid argument
exit 234
$ sh streams.sh out.mp4
  Stream #0:0[0x1](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 160x120 [SAR 1:1 DAR 4:3], 32 kb/s, 25 fps, 25 tbr, 12800 tbn
  Stream #0:1[0x2](und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p(progressive), 320x240 [SAR 1:1 DAR 4:3], 46 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  Stream #0:2[0x3](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)

-y didn’t help: the second run failed while parsing its options, before it opened out.mp4, so the three-stream file from the first run is still there.

The failures don’t all behave the same. Here is what the folder holds now:

$ ls *.mp4
1.mp4
2.mp4
3.mp4
4.mp4
6.mp4
7.mp4
8.mp4
in.mp4
noaudio.mp4
out.mp4
$ stat -f '%N %z bytes' 7.mp4
7.mp4 0 bytes

The option error (5.mp4) created no file. The filtergraph error (7.mp4) left an empty one. Check the exit code before you look at the file, or delete the output first.

Checklist

  • Extra video stream after -filter_complex: look for -map 0 (or -map 1 …). It maps every stream of that input, including the video your filter already used.
  • Keep the audio with -map "[v]" -map 0:a; use -map "0:a?" (quoted in zsh) if an input may have no audio.
  • -map 0 -map "[v]" puts the unfiltered video first, with the (default) flag.
  • -map -0:v after -map "[v]" gave Filter ... has an unconnected output. Put it before, or don’t use it.
  • List the streams of the result (ffmpeg -i out.mp4 2>&1 | grep 'Stream #'); the exit code is 0 either way.
  • A failed run may leave the previous out.mp4 untouched, or a 0-byte file. Check $? first.
  • Filter addresses like 0x60000336c0a0 differ between runs.

Tested on macOS 26.5.2 (Apple silicon) with ffmpeg 7.1, the static arm64 build shipped in the imageio-ffmpeg Python package (version line above), from zsh. Test clips are generated with lavfi; no other ffmpeg builds tested.