ffmpeg - overwrite audio in video until audio ends

Question

I have a video file, say it's called video.mp4. The video has its own audio. I also have an audio file, say audio.aac, shorter in length than the video.

I want to use ffmpeg to overwrite the audio of video.mp4 with the audio from audio.aac. But since the audio.aac is shorter than video.mp4, after it ends I want the original audio the video had to continue (not from the beginning of course, I don't want to shift it).

Preferably without reencoding.

EDIT: video.mp4 and audio.aac are not files I have. I want to make a script that can work with any input files. Here are some scripts I currently have:

I can extract the audio from the video like this:

ffmpeg -i $1 -vn -acodec copy $2

I can replace the audio on a video with one from another file like this (keeping shortest length):

ffmpeg -i "$1" -i "$2" -vcodec copy -acodec copy -shortest -map 0:v -map 1:a "$3"

I can replace the audio on a video with one from another file like this (without keeping shortest length):

ffmpeg -i "$1" -i "$2" -vcodec copy -acodec copy -map 0:v -map 1:a "$3"

I can trim a video like this (I know it may not be very accurate without reencoding but works for me):

ffmpeg -ss "$1" -i "$2" -ss 0 -c copy -to "$3" -avoid_negative_ts make_zero "$4"

I want to do something like the third script but instead of silence after the audio ends, I want the original audio of the video.

Answer 1

1. Get info about audio.aac including duration.

   ffprobe -v error -show_streams -show_entries stream=codec_name,sample_rate,channels:format=duration audio.aac
   

   Example output:

   [STREAM]
   codec_name=aac
   sample_rate=44100
   channels=2
   [/STREAM]
   [FORMAT]
   duration=29.463936
   [/FORMAT]
   

2. Conform audio from other input to match attributes of audio.aac:

   ffmpeg -i input.mp4 -ss 29.463936 -map 0:a -c:a aac -ar 44100 -ac 2 conformed.m4a
   

3. Make input.txt:

   file 'audio.aac'
   file 'conformed.m4a'
   

4. Concatenate:

   ffmpeg -i input.mp4 -f concat -i input.txt -map 0:v -map 1:a -c copy -movflags +faststart output.mp4