A bash script to batch resize images on macOS, possibly through an Automator Service (uses sips, ImageOptim)

resize_images.sh

for f in "$@"
do

	dir=$(dirname "${f}")
	filename=$(basename -- "$f")
	extension="${filename##*.}"
	filename="${filename%.*}"

	# array of pixel sizes (widths) to generate
	sizes=(600 1200)
	for s in "${sizes[@]}"
	do
		newfilename="$dir"/"$filename"_"${s}"."$extension"

		cp "$f" "$newfilename" # make a copy of the file first
		sips -Z "${s}" -s formatOptions 60 "$newfilename" # resize the copy
		/Applications/ImageOptim.app/Contents/MacOS/ImageOptim "$newfilename" # run imageoptim on the resized image
	done
done

osascript -e 'display notification "Image resizing complete!"'