Cksum

Cksum is a special program on a computer that you can use to tell if a file is the same as a copy of that file that you had before. It works by calculating a special number called a checksum. The way it works is if you give it the same file twice, it will calculate the same checksum every time. That number is like a fingerprint that you can use to identify if the file is the same as the copy you had before.