Script for resizing a directory of images using Ruby. Note requires the RMagick gem to be installed and assumes Rbenv is used for managing the Ruby Version. Blog post: http://www.calebwoods.com/2015/02/01/batch-resizing-images-ruby/

resize.rb

#!/usr/bin/ruby
require 'RMagick'
require 'pathname'

@directory = Pathname(File.expand_path(ARGV[0]))
@size      = ARGV.fetch(1) { 1025 }

def resize_image(file)
  img = Magick::Image.read(file).first

  resized = img.resize_to_fit(@size)

  resized_path = @directory.join('resized', File.basename(file))
  resized.write(resized_path) do
    self.quality = 100
  end

  # free up RAM
  img.destroy!
  resized.destroy!
end

resize_dir = "#{@directory}/resized"
unless File.directory? resize_dir
  puts "Creating #{resize_dir}/"
  Dir.mkdir resize_dir
end

files = Dir.glob "#{@directory}/*.{jpg,png,gif}"

puts "Resizing #{files.size} images..."

files.each do |file|
  puts "Resizing #{file}"
  resize_image(file)
end

puts "Finished resizing #{files.size} images"