Proof of Concept for storing command-line interaction in YAML files similar to VCR does with HTTP requests.

command_capture.rb

require 'logger'
require 'yaml'

class CommandsFile
  FILE_PATH =  File.join(File.dirname(__FILE__), "commands.yml")

  def self.load
    commands_file = new
    commands_file.load
  end

  def self.write(text)
    commands_file = new
    commands_file.write(text)
  end

  def load
    YAML.load_file(FILE_PATH) || []
  end

  def write(text)
    File.open(FILE_PATH, 'w') { |f|
      f.write(YAML.dump(text))
    }
  end
end

class Captures
  attr_reader :captured_commands, :logger

  def initialize(log_stream = STDOUT)
    @captured_commands = CommandsFile.load
    @logger            = Logger.new(log_stream)
    logger.formatter   = proc do |severity, datetime, progname, msg|
      "#{datetime.strftime("%y%m%e%H%M%S")}: #{msg}\n"
    end
  end

  def create(command, output)
    logger.debug("#create - `#{command}`")
    new_command = { captured_at: Time.now, command: command, output: output }
    captured_commands << new_command
    CommandsFile.write(captured_commands)
    new_command[:output]
  end

  def find(command)
    logger.debug("#find - `#{command}`")
    captured_commands.select { |c| c[:command] == command }.first
  end
end

module CommandCapture
  def `(command)
    captures = Captures.new
    captured = captures.find(command)
    captured ? captured[:output] : captures.create(command, super)
  end
end

Object.send(:include, CommandCapture)

gistfile1.txt

➜  tmp  irb -I . -r 'command_capture'
irb(main):001:0> `date`
130130200203: #find - `date`
130130200203: #create - `date`
=> "Wed Jan 30 20:02:03 CST 2013\n"
irb(main):002:0> `df -h`
130130200217: #find - `df -h`
130130200217: #create - `df -h`
=> "Filesystem      Size   Used  Avail Capacity  Mounted on\n/dev/disk0s2   698Gi   76Gi  622Gi    11%    /\ndevfs          184Ki  184Ki    0Bi   100%    /dev\nmap -hosts       0Bi    0Bi    0Bi   100%    /net\nmap auto_home    0Bi    0Bi    0Bi   100%    /home\nmap -fstab       0Bi    0Bi    0Bi   100%    /Network/Servers\n"
irb(main):003:0> `date`
130130200231: #find - `date`
=> "Wed Jan 30 20:02:03 CST 2013\n"