So for anyone who doesn’t know, the Rails console (rails c in Rails3 or script/console in Rails2) runs up an irb process behind the scenes. It also loads Rails into the context, and a few other bits – but mainly it’s an irb session.

I’ve just been getting a couple of “Oh wow, I didn’t know that.” responses when I mention these things in #RubyOnRails, so I thought I’d just post this here.

Thing 1: the underscore method

The return from every command that you enter into irb is available in the underscore method _. So, if you’re like me, you often run a command and then think that you should have assigned it to some variable, or pretty-printed it or something.

The underscore method is perfect for this:

>> City.all
=> [#<City id: 218996838, name: "Chatswood", postcode: "2067", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">, #<City id: 957448139, name: "Roseville", postcode: "2069", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">, #<City id: 1015505586, name: "Willoughby", postcode: "2068", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">]

Bugger me if I didn’t just forget to assign that to something…

>> c = _
=> [#<City id: 218996838, name: "Chatswood", postcode: "2067", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">, #<City id: 957448139, name: "Roseville", postcode: "2069", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">, #<City id: 1015505586, name: "Willoughby", postcode: "2068", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">]

Yay!

The only thing to remember is that if you don’t grab it immediately, then it will be gone next command – it is overwritten with every line.

Thing 2: the y method

This is actually just the Kernel#y method, but it’s particularly useful in irb sessions when you want to print something out in an easy to read (and easy to grab if you want YAML) format.

>> { :foo => 'bar', :woo => { :ooo => 1 } }
=> {:foo=>"bar", :woo=>{:ooo=>1}}
>> y _
---
:foo: bar
:woo:
  :ooo: 1
=> nil

This is especially good for outputting large collections of Rails models.

Thing 3: the pp library

The pp or Pretty-Print lib is also great for printing out collections of data in a neat way. Just require it into your irb session and use it to great effect:

>> require 'pp'
=> true
>> pp City.all
[#<City id: 218996838, name: "Chatswood", postcode: "2067", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">,
 #<City id: 957448139, name: "Roseville", postcode: "2069", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">,
 #<City id: 1015505586, name: "Willoughby", postcode: "2068", state_id: 944944227, created_at: "2009-03-30 01:56:22", updated_at: "2009-03-30 01:56:22">]
=> nil

Voila! To make that even easier, see the next thing.

Thing 4: .irbrc

By default in *nix based operating systems the file .irbrc in your $HOME directory will be loaded by irb when it starts up. (On Windows you need to create your own irbrc anywhere you like and set the IRBRC environment variable).

You can put any Ruby commands you like in here, here’s mine:

require 'rubygems'
require 'irb/completion'
require 'pp'
require 'yaml'

def v(x)
  IO.popen( 'mvim -', 'w') do |io|
    io.puts x.to_yaml
  end
end

def less(x)
  IO.popen( 'less -', 'w') do |io|
    io.puts x.to_yaml
  end
end

You can see that it’s simple to require in your choice of libs in there, and you can even define your own methods, which brings us to the last thing.

Thing 5: total customization

The .irbrc is just Ruby, and as you’ve seen above you can define your own methods. I’ve defined two to make it easy to pipe an object (as yaml) out to an external program (I’ve got mvim and less setup) – but you can have anything you like. This lets you customize your own irb interface with whatever little helpers you want to make your work easier.

Take a look here and here at some powerful examples of what can be done.

Enjoy.