Create several sizes of an image for use with the HTML srcset attribute.

srcset-generator.sh

#!/bin/bash

function usage {
  cat << EOM

  Usage: $0 filename size [quality, default: 75] [count, default: 3]

  For example:

    $0 rad.jpg 1024 75 3

  Will create rad-1024.jpg, rad-2048.jpg, and rad-3072.jpg with 75% quality.
EOM
}

if [ "$#" -eq 0 ]; then
  usage
  exit 0
elif [ "$#" -lt 2 ]; then
  usage
  exit 1
fi

FILENAME=$1
EXTENSION="${FILENAME##*.}"
SIZE=$2
QUALITY=${3:-75}
COUNT=${4:-3}

for i in $(seq 1 $COUNT); do 
  sz=$(($SIZE * $i))
  CMD="convert \"$FILENAME\" -resize ${sz}x${sz} -quality $QUALITY \"${FILENAME%.$EXTENSION}-$sz.$EXTENSION\""
  echo Running $CMD...
  eval $CMD
done