./crop.sh foo.png 100 100 out.png Crops foo.png by a centered 100 100 rect and stores the result in out.png

crop.sh

#!/bin/bash
IN_FILE="$1"
OUT_FILE="$4"

CROP_W=$2
CROP_H=$3

SIZE=$(identify $IN_FILE | awk '{print $3}')
IMAGE_W=${SIZE%%x*}
IMAGE_H=${SIZE##*x}

let X=($IMAGE_W-$CROP_W)/2
let Y=($IMAGE_H-$CROP_H)/2

CROP="-crop ${CROP_W}x${CROP_H}"

if (( "$X" > 0)); then
    CROP+="+$X"
else
    CROP+="$X"
fi

if (( "$Y" > 0)); then
    CROP+="+$Y"
else
    CROP+="$Y"
fi

convert ${IN_FILE} ${CROP} ${OUT_FILE}

Needs ImageMagick to be installed for convert and identify commands to work.