ffmpeg audio and video out of sync fix

This guide for how to mix audio and video with ffmpeg and fix out of sync problems

First get audio and video total time

ffprobe -i video.mp4 -show_format | grep duration
ffprobe -i audio.mp4 -show_format | grep duration

Devide audio length by video length and set the atempo audio filter variable, in this example 0.999

ffmpeg -i video.mp4 -i audio.mp4 -c:v copy -c:a aac -af atempo=0.999 output.mp4

I made a script to do that automatically example usage: script_name.sh video.mp4 audio.mp3 output.mp4

VIDEO_LEN=$(ffprobe -loglevel 0 -i $1 -show_format | grep duration)
AUDIO_LEN=$(ffprobe -loglevel 0 -i $2 -show_format | grep duration)

VIDEO_LEN=${VIDEO_LEN:9}
AUDIO_LEN=${AUDIO_LEN:9}

echo $VIDEO_LEN
echo $AUDIO_LEN

RATIO=$(echo "$AUDIO_LEN / $VIDEO_LEN" | bc -l)

echo $RATIO

ffmpeg -i $1 -i $2 -c:v copy -c:a aac -af atempo=$RATIO $3