Undo accidentally running id3tag on an M4A or M4B audio file. Assumes GNU grep and head are available at ggrep and ghead respectively (`brew install coreutils` on macOS).

m4adetag.sh

#!/bin/bash

OFFSET=$(ggrep --only-matching --byte-offset --max-count=1 --binary --text --perl-regexp "\x00\x00\x00\x1c" "$1" | head -1 | cut -d':' -f1)
tail -c +$(( $OFFSET + 1 )) "$1" | ghead -c -128 > untagged.m4b
if ffprobe -loglevel quiet untagged.m4b ; then
  echo "Untagging succeeded for: $1"
  mv untagged.m4b "$1"
else
  echo "Untagging failed for: $1"
fi
rm -f untagged.m4b