Resize all image files to half of the sizes

resize.bash

#!/usr/bin/env bash
set -euxo pipefail

# Usage:
# cd /path/to/images/directory
# bash resize.bash

shopt -s extglob
for filename in [^resize]*.?(jpg|jpeg|png); do
    newFile="resized-$filename"
    if [ ! -f "$newFile" ]; then
      convert $filename -resize 50% $newFile
    fi
done