Add a checksum file check.
user
```ruby
#!/usr/bin/env ruby
#
# Author: AJ Bourg <aj@ajbourg.com>
#
# Checks the checksums of files.

require 'rubygems' if RUBY_VERSION < '1.9.0'
require 'sensu-plugin/check/cli'
require 'digest'

class Checksum < Sensu::Plugin::Check::CLI

  option :files,
    :description => "Comma separated list of files to check.",
    :short => '-f FILES',
    :long => '--files FILES',
    :required => true
  option :hash,
    :description => "The hash these files must hash as. If unspecified the files will be compared to the first file.",
    :short => '-h SHA2HASH',
    :long => '--hash SHA2HASH'
  option :warn_only,
    :description => "Warn instead of critical if they don't match",
    :short => '-w',
    :long => '--warn-only',
    :boolean => true

  def run
    files = config[:files].split(',')

    if files.length == 1 and !config[:hash]
      unknown "We have nothing to compare this file with."
    end

    hash = config[:hash] || Digest::SHA2.file(files.first).hexdigest

    errors = []

    files.each do |file|
      if File.exists?(file)
        file_hash = Digest::SHA2.file(file).hexdigest

        errors << "#{file} does not match" if file_hash != hash
      else
        errors << "#{file} does not exist"
      end
    end

    if errors.length > 0 and config[:warn_only]
      warning errors.join("\n")
    elsif errors.length > 0
      critical errors.join("\n")
    else
      ok "Files match."
    end
  end

end```
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