Simple ruby script that generates all necessary iOS icons.

iosicons.rb

#!/usr/bin/env ruby

require 'fileutils'
require 'json'

# MARK: Classes
class Size
  attr_reader :base_size
  attr_reader :scale
  def initialize(base_size, scale)
    @base_size = base_size
    @scale = scale
  end

  def filename
    "Icon-App-#{@base_size}x#{@base_size}@#{@scale}x.png"
  end
end

class IconType
  attr_reader :idiom
  attr_reader :sizes
  def initialize(idiom, sizes)
    @idiom = idiom
    @sizes = sizes
  end
end

# MARK: Program
unless ARGV.length >= 1
  puts "Usage: #{__FILE__} source [destination]"
  puts "\tsource - raster (largest available) or vector representation of the source icon"
  puts "\tdestination - directory relative to current directory (#{Dir.pwd}) where to store the resultant icons, default is current directory"
  exit 1
end

source = ARGV[0]
destination = ARGV[1] || '.'

FileUtils.mkpath(destination) unless File.exists?(destination)

icon_types = [
  IconType.new('iphone', [
      Size.new(20, 2),
      Size.new(20, 3),
      Size.new(29, 1),
      Size.new(29, 2),
      Size.new(29, 3),
      Size.new(40, 2),
      Size.new(40, 3),
      Size.new(60, 2),
      Size.new(60, 3),
  ]),
  IconType.new('ipad', [
      Size.new(20, 1),
      Size.new(20, 2),
      Size.new(29, 1),
      Size.new(29, 2),
      Size.new(40, 1),
      Size.new(40, 2),
      Size.new(76, 1),
      Size.new(76, 2),
      Size.new(83.5, 2),
  ]),
  IconType.new('ios-marketing', [Size.new(1024, 1)])
]

sizes = icon_types.flat_map { |icon_type| icon_type.sizes }.uniq { |size| [size.base_size, size.scale] }

sizes.each do |size|
  real_size = size.base_size * size.scale
  exit 1 unless system("convert \"#{source}\" -resize #{real_size}x#{real_size} -unsharp 1x4 #{destination}/#{size.filename}")
end

contents = {
  :images => icon_types.flat_map do |icon_type|
    icon_type.sizes.map do |icon_size|
      {
        :size => "#{icon_size.base_size}x#{icon_size.base_size}",
        :idiom => icon_type.idiom,
        :filename => icon_size.filename,
        :scale => "#{icon_size.scale}x",
      }
    end
  end,
  :info => {
    :version => 1,
    :author => 'xcode'
  }
}

IO.write("#{destination}/Contents.json", JSON.pretty_generate(contents))