sync/scripts/reencode_m4a.sh

#!/usr/bin/env bash
set -euo pipefail

# evaluate options through given arguments
map=
vbr=
while [[ $# > 0 ]]; do
   case $1 in
      -map) map='-map 0:0';                              shift;;
      -vbr) vbr='libfdk_aac -profile:a aac_he -b:a 64k'; shift;;
      *   )                                              break ;; # Anything else stops command line processing.
   esac
done

# if no variable bitrate, just copy audio
if [[ -z $vbr ]]; then
   map="$map copy"
fi

# ensure existence of destination folder
dir="$(pwd)"
dest=${dir%/*}/${dir##*/}_reencode
if [[ !(-d $dest) ]]; then
   mkdir $dest
fi

for file in ./*.m4a; do
   echo "ffmpeg -i '$file' -c:a $vbr $map -movflags +faststart '$dest/$file'";
   ffmpeg -i "$file" -c:a $vbr $map -movflags +faststart "$dest/$file";
done