IRB stands for “interactive Ruby” and is a tool to interactively execute Ruby expressions read from the standard input.
irb command from your shell will start the interpreter.
When executing irb, prompts are displayed as follows. Then, enter the Ruby expression. An input is executed when it is syntactically complete.
$ irb irb(main):001:0> 1+2 #=> 3 irb(main):002:0> class Foo irb(main):003:1> def foo irb(main):004:2> print 1 irb(main):005:2> end irb(main):006:1> end #=> nil
The Readline extension module can be used with irb. Use of Readline is default if it’s installed.
There are a few variables in every Irb session that can come in handy:
_: The value command executed, as a local variable
__: The history of evaluated commands
__[line_no]: Returns the evaluation value at the given line number,
line_nois a negative, the return value
line_nomany lines before the most recent return value.
Another popular Ruby interactive console is pry.