FFMPEG mux video and audio (from another video) - mapping issue

Overview of inputs

input_0.mp4 has the desired video stream and input_1.mp4 has the desired audio stream:

In ffmpeg the streams look like this:

$ ffmpeg -i input_0.mp4 -i input_1.mp4

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input_0.mp4':
  Duration: 00:01:48.50, start: 0.000000, bitrate: 4144 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 4014 kb/s, SAR 115:87 DAR 1840:783, 23.98 fps, 23.98 tbr, 16k tbn, 47.95 tbc (default)
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 124 kb/s (default)

Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'input_1.mp4':
  Duration: 00:00:30.05, start: 0.000000, bitrate: 1754 kb/s
    Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 720x480 [SAR 8:9 DAR 4:3], 1687 kb/s, 59.94 fps, 59.94 tbr, 60k tbn, 119.88 tbc (default)
    Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 55 kb/s (default)

ID numbers

ffmpeg refers to input files and streams with index numbers. The format is input_file_id:input_stream_id. Since ffmpeg starts counting from 0, stream 1:1 refers to the audio from input_1.mp4.

Stream specifiers

This can be enhanced with stream specifiers. For example, you can tell ffmpeg that you want the first video stream from the first input (0:v:0), and the first audio stream from the second input (1:a:0). I prefer this method because it's more efficient. Also, it is less prone to accidental mapping because 1:1 can refer to any type of stream, while 2:v:3 only refers to the fourth video stream of the third input file.

Examples

The -map option instructs ffmpeg what streams you want. To copy the video from input_0.mp4 and audio from input_1.mp4:

$ ffmpeg -i input_0.mp4 -i input_1.mp4 -c copy -map 0:0 -map 1:1 -shortest out.mp4

This next example will do the same thing:

$ ffmpeg -i input_0.mp4 -i input_1.mp4 -c copy -map 0:v:0 -map 1:a:0 -shortest out.mp4

• -map 0:v:0 can be translated as: from the first input (0), select video stream type (v), first video stream (0)

• -map 1:a:0 can be translated as: from the second input (1), select audio stream type (a), first audio stream (0)

Additional Notes

• With -c copy the streams will be stream copied, not re-encoded, so there will be no quality loss. If you want to re-encode, see FFmpeg Wiki: H.264 Encoding Guide.

• The -shortest option will cause the output duration to match the duration of the shortest input stream.

• See the -map option documentation for more info.

The accepted answer is an excellent explanation of ffmpeg's flexible stream selection using the -map option.

However, the documentation linked above also describes a simpler syntax to do what the questioner asks, without -map:

ffmpeg -an -i video1_noAudio.mov -vn -i video2_wAudio.mov -c:a copy -c:v copy video1_audioFromVideo2.mov

Here -an means discard any audio from the first input file, -vn discards any video from the audio recording. ffmpeg then combines them in the obvious way to produce a single output file.

(-c:a and -c:v are just alternatives for -acodec and -vcodec in the question, which can be used to copy for speed, or re-encode a stream if needed.)