Resize an image and constrain the image proportionately to a specified width, using ImageMagick (convert) from the command line.

convert.sh

#!/usr/bin/env bash
#
# The gist example here does not perform any sanity or error checking. It's just a gist.
#
# example: resize an image file to 150px width...
# ./convert.sh example.jpg 150
#
$FILENAME=$1
$WIDTH=$2

convert -geometry $WIDTH $FILENAME $FILENAME;
echo -e "\nDone!\n";