How do you define Class Variable, Instance Variable and Global Variable in Ruby?

Question

Ruby Class Variables
Class variables begin with @@ and must be initialized before they can be used in method definitions.
Referencing an uninitialized class variable produces an error. Class variables are shared among descendants of the class or module in which the class variables are defined.

Overriding class variables produce warnings with the -w option.

Here is an example showing usage of class variable:

#!/usr/bin/ruby

class Customer
   @@no_of_customers=0
   def initialize(id, name, addr)
      @cust_id=id
      @cust_name=name
      @cust_addr=addr
   end
   def display_details()
      puts "Customer id #@cust_id"
      puts "Customer name #@cust_name"
      puts "Customer address #@cust_addr"
    end
    def total_no_of_customers()
       @@no_of_customers += 1
       puts "Total number of customers: #@@no_of_customers"
    end
end

# Create Objects
cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

# Call Methods
cust1.total_no_of_customers()
cust2.total_no_of_customers()

Here @@no_of_customers is a class variable. This will produce following result:

Total number of customers: 1
Total number of customers: 2

Ruby Instance Variables:
Instance variables begin with @. Uninitialized instance variables have the value nil and produce warnings with the -w option.
Here is an example showing usage of Instance Variables.

#!/usr/bin/ruby

class Customer
   def initialize(id, name, addr)
      @cust_id=id
      @cust_name=name
      @cust_addr=addr
   end
   def display_details()
      puts "Customer id #@cust_id"
      puts "Customer name #@cust_name"
      puts "Customer address #@cust_addr"
    end
end

# Create Objects
cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

# Call Methods
cust1.display_details()
cust2.display_details()
Here @cust_id, @cust_name and @cust_addr are instance variables. This will produce following result:
Customer id 1
Customer name John
Customer address Wisdom Apartments, Ludhiya
Customer id 2
Customer name Poul
Customer address New Empire road, Khandala

Ruby Global Variables
Global variables begin with $. Uninitialized global variables have the value nil and produce warnings with the -w option.
Assignment to global variables alters global status. It is not recommended to use global variables. They make programs cryptic.
Here is an example showing usage of global variable.

#!/usr/bin/ruby

$global_variable = 10
class Class1
  def print_global
     puts "Global variable in Class1 is #$global_variable"
  end
end
class Class2
  def print_global
     puts "Global variable in Class2 is #$global_variable"
  end
end

class1obj = Class1.new
class1obj.print_global
class2obj = Class2.new
class2obj.print_global

Here $global_variable is a global variable. This will produce following result:

NOTE: In Ruby you CAN access value of any variable or constant by putting a hash (#) character just before that variable or constant.
Global variable in Class1 is 10
Global variable in Class2 is 10