How does Git(Hub) handle possible collisions from short SHAs?

git cryptography github sha
Aseem Kishore picture Aseem Kishore · Aug 20, 2011 · Viewed 13k times · Source

Both Git and GitHub display short versions of SHAs -- just the first 7 characters instead of all 40 -- and both Git and GitHub support taking these short SHAs as arguments.

E.g. git show 962a9e8

E.g. https://github.com/joyent/node/commit/962a9e8

Given that the possibility space is now orders of magnitude lower, "just" 268 million, how do Git and GitHub protect against collisions here? And how do they handle them?

Answer

emboss picture emboss · Aug 20, 2011

These short forms are just to simplify visual recognition and to make your life easier. Git doesn't really truncate anything, internally everything will be handled with the complete value. You can use a partial SHA-1 at your convenience, though:

Git is smart enough to figure out what commit you meant to type if you provide the first few characters, as long as your partial SHA-1 is at least four characters long and unambiguous — that is, only one object in the current repository begins with that partial SHA-1.

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