Gradient map with ImageMagick

#!/bin/sh

# Imagemagick analogue of Adobe Photoshop Gradient map with 75% opacity on it
#  you will need the input.jpg file with original color image
#  and gradient_line.png file with 256 x 1 px image of gradient

# make image grayscale
convert input.jpg -colorspace gray draft.jpg

# apply the gradient lut
convert draft.jpg gradient_line.png -clut draft.jpg

# set 75% opacity
convert draft.jpg -alpha on -channel a -evaluate set 75% draft.png

# impose result image on original image
convert the_in.jpg temp.png -gravity center -composite -format jpg -quality 90 output.jpg

# remove temporary files
rm draft.jpg && rm draft.png