What is the unless in Ruby command?

In a nutshell it is an if on the contrary, where it performs the content within the conditional if the condition is not true .

Example:

#!/usr/bin/ruby

x = 1 
unless x == 2
   puts "x é diferente de 2"
 else
   puts "x é igual a 2"
end

Source:

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unless is equivalent to using if not, unless and if not (if! condition) is a personal preference. I personally use unless when you do not need an else .