Convert the audio of a single video into AAC, retaining original audio as well

tvshow.sh

#!/bin/sh
# Example: ~/tvshow.sh file-without-extention mkv mp4
input="$1.$2"
output="$1.$3"
echo "$input --> $output"
if [ -f "$input" ];
then
    ffmpeg -hide_banner -i "$input" -map 0:v -c:v copy -map 0:a -c:a copy -map 0:a:0 -c:a:0 aac -map 0:s -c:s mov_text -map_metadata 0 -movflags +faststart "$output" && \
    rm "$input"
else
    echo "$input does not exist"
fi