decrypt.rb

#!/usr/bin/env ruby

def main
    if ARGV.length != 2
        puts 'Invalid number of arguments'
        exit 1
    end

    crypt = ARGV[0]
    caesar = ARGV[1].to_i

    crypt.chars do |c|
        diff = (c.bytes[0] - 65 + caesar) % 26
        print (65 + diff).chr
    end
    puts
end

main