add ops in to expand your truth table

truth_table.rb

#!/usr/bin/env ruby
ops = [
  "l && r",
  "l || r",
  "l ^ r",
  "!l && !r",
  "!(l ^ r)"
]

ops.each do |key|
  puts key.center(12)
  puts "=" * 20
  [true, false].product([true, false]).each do |l, r|
    printf "|%5s|%5s|%5s|\n", l, r, eval(key)
  end
  puts "=" * 20
end