Calculate the aspect ratio of an image

image_ratio.sh

#!/bin/bash
# Called with ./image_ratio.sh IMAGEFILE

# convert is from ImageMagick
image_width () {
    convert "$1" -format "%w" info:
}
image_height () {
    convert "$1" -format "%h" info:
}

# Stolen (and edited) from http://www.tldp.org/LDP/abs/html/ops.html
gcd () {
    local dividend=$1             #  Arbitrary assignment.
    local divisor=$2              #! It doesn't matter which of the two is larger.
    #  Why not?

    local remainder=1             #  If an uninitialized variable is used inside
    #+ test brackets, an error message results.

    until [ "$remainder" -eq 0 ]
    do    #  ^^^^^^^^^^  Must be previously initialized!
        let "remainder = $dividend % $divisor"
        dividend=$divisor     # Now repeat with 2 smallest numbers.
        divisor=$remainder
    done                    # Euclid's algorithm

    echo $dividend
}

IMAGE=$1

WIDTH=$(image_width $IMAGE)
HEIGHT=$(image_height $IMAGE)

GCD=$(gcd $WIDTH $HEIGHT)
echo "GCD: $GCD"

let "A = $WIDTH / $GCD"
let "B = $HEIGHT / $GCD"

echo $A:$B