true, false and nil are like everything else in ruby are objects that have their own classes in which they are the only objects. true and false are both native boolean values. You could set it to variables, use it for methods and what not.

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irb(main):001:0> true.class
=> TrueClass
irb(main):002:0> false.class
=> FalseClass
irb(main):003:0> bool = true
=> true

nil is something different altogether. Nothing is a weird thing in programming and different thing has different programming languages. nil is not zero, zero is a value, it is something but nil is nothing like the absence of anything.

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irb(main):001:0> nil.class
=> NilClass
irb(main):002:0> 0.nil?
=> false
irb(main):003:0> nil.nil?
=> true

Ruby has the following logical operators: && || ! . You could also use the words and or not and they do what you think they'd do.

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irb(main):001:0> true && false
=> false
irb(main):002:0> true and true
=> true
irb(main):003:0> true || false
=> true
irb(main):004:0> true or false
=> true
irb(main):005:0> not true
=> false
irb(main):006:0> !true
=> false

We also have the following comparative operators: \== != < > <= >= Those also do what you'd expect them to do. You could see them in action when we finally get to conditionals in the next section.

Let's say we have a variable called version and wish to print out stuff based on the value of version, then this is how we would do it.

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puts "Enter your Debian version"
version = gets.to_i

if (version < 9)
  puts "Upgrade Debian!"
elsif (version == 9)
  puts "You are using Old-Stable. You should consider upgrading."
elsif (version == 10)
  puts "You are using the latest version of Debian!"
elsif (version == 11)
  puts "You are using Testing"
else
  puts "Enter a version number!"
end

Now if you run that it'll look something like this:

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Enter your Debian version
9
You are using Old-Stable. You should consider upgrading.

Now what if want to check for multiple conditions?

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puts "Enter your Debian version"
version = gets.to_i

if (version < 9)
  puts "Upgrade Debian!"
elsif (version >= 9) && (version < 12)
  puts "You are good for now."
else
  puts "Enter a version number!"
end

And executing that will give us this. By the way you can make ruby scripts by putting .rb at the end of the filename and as you can see you can run scripts by providing the filename as a parameter to the ruby command.

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❯ ruby deb.rb
Enter your Debian version
9
You are good for now.

There are more fun things that we could do with these little words, but it's late here and I'm gonna probaby do that on Day 4.