Apply watermark.png at 25% size in lower left corner, and rename output to EXIF date

watermark.sh

#!/bin/bash
for FILE in "$@"; do
  ww=`convert $FILE -format "%[fx:w]" info:`
  hh=`convert $FILE -format "%[fx:h]" info:`
  datepath="$(identify -verbose $FILE | grep DateTimeOri | awk '{print $2"_"$3 }' | sed s%:%_%g)"
  convert $FILE -auto-orient \( watermark.png -resize $((ww / 4))x$((hh / 4)) \) -gravity SouthWest -composite ${datepath}_overlay.jpg
done