IRBEdit

IRB is short for Interactive Ruby.

You can start an Interactive Ruby session by issuing irb at the command line.

Example session

$ irb
irb(main):001:0> puts "Hello, world!"
Hello, world!
=> nil
irb(main):002:0> exit

Tips

Access the result of the last expression with _

$ 4 + 9
=> 13
$ _
=> 13

Source: http://rubyquicktips.tumblr.com/post/342527837/console-tip-retrieve-the-last-return-value-with