Script used in ifeelmaps.com to optimise images as suggested by PageSpeed Insights

convert-imgs.sh

#!/bin/bash

# Convert images received as list of filenames over stdin
#
#   Uses imagemagick's convert script https://www.imagemagick.org/script/convert.php
#   Follows suggestions on https://developers.google.com/speed/docs/insights/OptimizeImages
#
# Usage:
#   find . -path './*original*' -type f | sed 's/\.\///' | ./convert-imgs.sh 615 > converted_images
#
# Arguments:
#   stdin - list of filenames
#   width - number to resize image's width to. Optional
#
#   NOTE: the filenames should not start with `./`, in general filenames should not have dots, only the one for the file extension
#         since the script relies on finding the dot in filename to get the image extension and apply the correct conversion
#         (thus the piping to `sed 's/\.\///'` above in Usage)
#
#    e.g. a good list of images is:
#
#           image1.png
#           image2.png
#
#    AND NOT (as it will break the script):
#           ./image1.png
#           ./image2.png

suffix=imagemagick_converted # the suffix used to generate converted images

# generate convert's arguments for images extensions
png_args="-strip"
jpg_args="-strip -sampling-factor 4:2:0 -quality 85 -interlace JPEG"
if [ -z $1 ] # was width passed?
then
  # https://www.imagemagick.org/script/command-line-processing.php#geometry "Width given, height automagically selected to preserve aspect ratio."
  png_args="$png_args -resize $1"
  jpg_args="$jpg_args -resize $1"
fi

while read -r filename; # loop over filenames passed over stdin
do
  echo $filename # print file processed

  dot_index=`expr index $filename .` # find the index of dot in filename string
  name=${filename:0:dot_index-1}     # name = characters before the dot
  extension=${filename:dot_index}    # extension = characters after the dot

  converted_img="$name"_"$suffix".$extension

  case $extension in
  *jpg|JPG|jpeg|JPEG*)
    # TODO use Progressive JPG ? read http://itstillworks.com/progressive-jpeg-12211723.html pros and cons
    `convert $filename $jpg_args $converted_img`
    ;;
  *png|PNG|gif|GIF*)
    `convert $filename $png_args $converted_img`
    ;;
  *) ;;
  esac
done