Automatically grayscale images which fall under a set saturation threshold

autogray.sh

#!/bin/bash
# https://www.imagemagick.org/discourse-server/viewtopic.php?t=19580#p81226
# https://unix.stackexchange.com/a/9499
# http://www.fmwconcepts.com/imagemagick/tidbits/image_info.php#gray

find . -type f \( -name '*.jpg' -o -name '*.png' \) -exec sh -c '
    for file do
        graytest=$(identify -format "%[colorspace]" "$file")
        if [[ $graytest != Gray ]]; then
            sattest=$(convert "$file" -colorspace HSL -channel g -separate +channel -format "%[fx:mean<.12?1:0]" info:)
            if [[ $sattest -eq 1 ]]; then
                [[ $file == *.jpg ]] && echo "$file: converting to grayscale" && jpegtran -grayscale -outfile "$file" "$file"
                [[ $file == *.png ]] && echo "$file: converting to grayscale" && convert "$file" -colorspace Gray "$file"
            else
                echo "$file: skipping, exceeds saturation threshold"
            fi
        else
            echo "$file: skipping, colorspace is already Gray"
        fi
    done
' exec-sh {} +