Scrub metadata from m4a files by copying the AAC stream to a new file without reencoding, using ffmpeg. This script replaces the file you're scrubbing, so duplicate your data beforehand. I'm no shell expert and wanted something quick, so it's not very polished, e.g. it won't work with wildcards.

scrubm4a

#!/bin/sh

INFILE="$@"
TEMPFILE=/tmp/$RANDOM
echo "processing file: " $INFILE
echo "extracting aac"
ffmpeg -i "$INFILE" -codec: copy $TEMPFILE.aac
echo "stuffing aac into m4a container"
ffmpeg -i $TEMPFILE.aac -bsf:a aac_adtstoasc -codec: copy $TEMPFILE.m4a
echo "replacing old with new"
rm "$INFILE"
mv $TEMPFILE.m4a "$INFILE"

Very useful! Thank you for making this.