Why GIT calculates checksum (SHA-1) of a file?

Question

I would like to know why GIT calculates checksum of a file.Typically, checksum is used for the purpose of integrity. An example would really help.

Answer 1

YOU HAVE IT IN ONE.
YES THAT IS THE REASON THAT GIT COMPUTES THE SHA1 OF THE FILE'S CONTENTS - IT PROVIDES INTEGRITY, VERACITY AND NON-REPUDIATION (THE LAST ONE IS STILL TRUE THOUGH CRYO-ANALYSIS IS GETTING BETTER, SO SHA1 IS NO LONGER RECOMMENDED, AND GIT IS LOOKING AT HOW TO PROGRESS TO NEWER CRYPTO-HASHES). ONCE GIT HAS THE SHA1'S OF THE FILES IN A DIRECTORY, IT DOES THE SAME AGAIN FOR THE 'FILE' THAT LISTS THE FILE NAMES, MODE BITS AND THEIR CONTENT'S SHA1S, AND EVER ONWARDS UP THE TREES TO THE COMMIT, WHICH LISTS THE SHA1S OF ITS PARENTS. SO IT YOU HAVE THE SHA1 OF THE TIP OF A BRANCH, SUCH AS MASTER, AND YOU HAVE A REPO THAT HOLDS THAT SHA1, THEN YOU HAVE THE FULL CRYPTO INTEGRITY THAT YOUR COPY (WITH ALL ITS HISTORY) IS IDENTICAL TO THAT OF THE ORIGINATORS - YOUR OWN DALI, REMBRANT, GOGIN, HANGING IN YOUR HALL... AND IT ISN'T EVEN A REPLICA, IT'S THE REAL THING!

Answer 2

An example? Ok. Back when something else was using a simple CRC, someone tried to replace a file with another, bypassing the normal history system. The CRC was good enough to detect it; so, something was needed that was good enough to detect/stop this.

But more importantly: The hash is the filename of the file. It is critical that the hash be good enough that you won't get duplicate filenames. CRC doesn't do that. Sha-1 does.

The checksum has to be good enough to make a unique filename in normal use.
It does not have to be good enough to guarantee non-alteration, but that's a really good secondary; it does have to be good enough to detect accidental damage (such as memory/disk/network/driver/etc corruption).

Now, a secondary benefit of the whole "layer upon layer" approach: The hash of the last commit is only valid if every file and commit to date is accurate. If you know the hash of your last commit (20 bytes, I think), and you can validate all the hashes in the past, then you know that nothing has altered any file outside of the git mechanism.