mkvsub filename.mkv

function mkvsub(){
  t=$(f=$(mkvinfo $1); n=$(echo "$f" | grep -n subtitle | grep -oP "\d+"); 
  echo "$f" | head -n $(($n - 2)) | tail -n 1 | grep -oP "\d+" | tail -n 1)
  mkvextract tracks $1 $t:sub.srt
  # strip sdh subtitle lines
  cat sub.srt | perl -pe "s/,,\[.+/,,/" >> sub2.srt
}