Updated: 23 December 2025

From the Ruby Documentation

Interactive Ruby

We can use irb to run an interactive Ruby shell, when open, it will look like this:

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irb(main):001:0>

Once we’ve got that open, we can type a value, and the interactive shell will evaluate it

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irb(main):001:0> "Hello World"
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=> "Hello World"

Though, to formally print something we can use puts like so:

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irb(main):002:0> puts "Hello World"
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Hello World
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=> nil

We can also see that the result of the puts command is nil which is the Ruby version of a nothing value

Math

We can do basic math using IRB as well, like so:

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irb(main):004:0> 5*7
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=> 35
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irb(main):005:0> 5**7
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=> 78125
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irb(main):006:0> Math.sqrt(5)
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=> 2.23606797749979
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irb(main):007:0> Math.sin(0)
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=> 0.0
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irb(main):008:0> Math.acos(1)
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=> 0.0

Methods

Defining a Method

We can define a method using def and end, so a method that prints Hello World would look like this:

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def hi
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  puts "Hello World"
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end

Or, in the shell

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irb(main):017:0> def hi
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irb(main):018:1> puts "Hello World"
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irb(main):019:1> end
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=> :hi

Calling a Method

We can then call a method using either the name of the method, or the name followed by () if the method has no inputs

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irb(main):020:0> hi
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Hello World
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=> nil
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irb(main):021:0> hi()
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Hello World
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=> nil

Method Parameters

We can define hi such that it takes a name parameter like so:

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def hi(name)
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  puts "Hello #{name}"
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end

Also note how string interpolation/templates can be done in the above string

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irb(main):026:0> hi("bob")
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Hello bob
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=> nil

We can also call methods on an object, for example name by using either the method name with or without parenthesis as mentioned before

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name.capitalize

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name.capitalize()

We can then implement that in our hi method

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def hi(name)
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  puts "Hello #{name.capitalize}"
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end

Which can be called like:

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hi("bob")

or

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hi "bob"

Classes

Classes are defined using the class keyword

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class Greeter
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  def initialize(name = "World")
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    @name = name
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  end
5

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  def say_hi()
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    puts "Hi #{@name}"
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  end
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  def say_bye()
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    puts "Hi #{@name}"
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  end
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end

We can create an instance of the Greeter class by using Greeter.new, like so

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greeter = Greeter.new("bob")

And we can then use it with:

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irb(main):029:0> greeter.say_hi
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Hi bob
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=> nil
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irb(main):030:0> greeter.say_bye
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Hi bob
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=> nil