Ruby and IRB

IRB is a Ruby tool that I’m not a fan. When a Ruby newbie uses it, there are some kind of behavior that most confuses then clarify.

But there are a lot of users, so I need to understand and explain how it works.

IRB, created by Keiju Ishitsuka, stands for Interactive Ruby Shell, and acts like expected, you send it a Ruby statement, that statement is executed and IRB return the last evaluated expression and some other tips.

But How ?

By nature, if used without arguments the Ruby interpreter wait for additional information.

D:\>ruby  
3.times do
puts 'step'
end
^D
step
step
step
D:\>

The Ruby interpreter, has two useful switches to achieve that:

• -e let you specify a script on the command line to be executed.
• -n wraps the specified script in a "while gets()" loop.

So, Keiju has used these to implement this tool. Try it yourself with the following command:

ruby -n -e 'puts eval($_)'

And IRB gives some useful tips on the command prompt. Just after started, IRB will show a command prompt like this:

irb(main):001:0>

This prompt has three parts delimited by “:” “irb(main)” are the actual program and session names, “001” is the actual line number and the final “0” is the indent level or in other words, the number of open blocks you currently have.

Indent level sample:

D:\>irb
irb(main):001:0> level = 0
=> 0
irb(main):002:0> if level == 0
irb(main):003:1> level = 1
irb(main):004:1> if level == 1
irb(main):005:2> level = 2
irb(main):006:2> end
irb(main):007:1> end
=> 2
irb(main):008:0> exit
D:\>

Program level and session name sample:

D:\>irb
irb(main):001:0> class Number
irb(main):002:1> def choice
irb(main):003:2> 'even'
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> mynumber = Number.new
=> #
irb(main):007:0> puts mynumber.choice
even
=> nil
irb(main):008:0> irb Number
irb#1(Number):001:0> def choice
irb#1(Number):002:1> 'odd'
irb#1(Number):003:1> end
=> nil
irb#1(Number):004:0> irb_exit
=> # irb::irb: context="# IRB::Context:0x2e46b58" , @signal_status=:IN_EVAL, @scanner=# rubylex:0x2e467fc
irb(main):009:0> puts mynumber.choice
odd
=> nil
irb(main):010:0> exit
D:\>