Checking if a variable is defined?

Use the defined? keyword (documentation). It will return a String with the kind of the item, or nil if it doesn’t exist.

>> a = 1
 => 1
>> defined? a
 => "local-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"

As skalee commented: "It is worth noting that variable which is set to nil is initialized."

>> n = nil  
>> defined? n
 => "local-variable"

This is useful if you want to do nothing if it does exist but create it if it doesn't exist.

def get_var
  @var ||= SomeClass.new()
end

This only creates the new instance once. After that it just keeps returning the var.

defined?(your_var) will work. Depending on what you're doing you can also do something like your_var.nil?

Try "unless" instead of "if"

a = "apple"
# Note that b is not declared
c = nil

unless defined? a
    puts "a is not defined"
end

unless defined? b
    puts "b is not defined"
end

unless defined? c
    puts "c is not defined"
end

You can try:

unless defined?(var)
  #ruby code goes here
end
=> true

Because it returns a boolean.

defined? is great, but if you are in a Rails environment you can also use try, especially in cases where you want to check a dynamic variable name:

foo = 1
my_foo = "foo"
my_bar = "bar"
try(:foo)        # => 1
try(:bar)        # => nil
try(my_foo)      # => 1
try(my_bar)      # => nil

Leaving an incredibly simple example in case it helps.

When variable doesn't exist:

if defined? a then "hi" end
# => nil

When variable does exist:

a = 2
if defined? a then "hi" end
# => "hi"