mkvmerge -o output.mkv input.mkv -S # remove all subtitle tracks
mkvmerge -o output.mkv input.mkv -s 3,4 # remove tracks 3 and 4
mkvmerge -o output.mkv input.mkv -s '!3' # remove all subtitle tracks except 3
mkvmerge -i input.mkv # show track numbers

mkvmerge -o output.mkv input.mkv subs.srt
mkvmerge -o output.mkv input.mkv --language 0:ger --track-name 0:German subs.srt

mkvextract tracks input.mkv 3:subs.srt

for f in *.mkv; do
  sub=$(mkvmerge -i "$f" | awk '$4=="subtitles"{print;exit}')
  [[ $sub ]] || continue
  [[ $sub =~ S_TEXT/ASS ]] && ext=ass || ext=srt
  track=$(awk -F '[ :]' '{print $3}' <<< "$sub")
  mkvextract tracks "$f" "$track:${f%mkv}$ext"
done

find -E . -maxdepth 1 -type f -iregex '.*.(ASS)$' |
  while read FILEPATH
  do
    DIRNAME=$(dirname "${FILEPATH}");
    BASENAME=$(basename "${FILEPATH}");
    FILENAME="${BASENAME%.*}";
    EXTENSION="${BASENAME##*.}"
    mkvmerge -o "/Users/jack/${FILENAME}"-NEW.mkv "${FILENAME}".mkv --language 0:eng --track-name 0:English "${FILENAME}".ass
  done