Take a full page screenshot of a website.

screenshot-web.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'trollop'
require 'vapir'

def filename_for_url(url)
  url.
    downcase.
    gsub(/https?:\/\//,'').
    gsub(/\//, '_').
    gsub(/[^0-9A-za-z.]/,'') + ".png"
end

def print_file_name(file)
  puts file
end

opts = Trollop::options do
  opt :url, "URL to screenshot", :default => ''
  opt :force, "force saving", :default => false
end

Trollop::die :url, "url required" if opts[:url].nil? || opts[:url] == ''
file = filename_for_url(opts[:url])
Trollop::die :url, "file exists, use --force to override" if (File.exists?(file) && !opts[:force])

begin
  browser = Vapir::Firefox.new
  browser.goto opts[:url]
  browser.screen_capture file
  print_file_name(file)
ensure
  browser.close
end

For this you can use the gem gastly.