Re: just curious: what influences a commit hash?

From: Matthieu Moy <matthieu.moy@imag.fr>
Date: Mar 5, 2009, 10:38 UTC
Message-ID: <vpq7i3445m7.fsf@bauges.imag.fr>
In-Reply-To: <20090305063632.42880@gmx.net>

stoecher@gmx.at writes:

Show 7 quoted lines

> Hi,
>
> being new to git I did some experiments with commits looking at the hashes. What I observed:
> * The same commit (same file, same committer, same message) into
> different empty repositories (git init) gives different hashes. So I
> assume that also the time of the commit influences the hash. Is this
> intended? For what reason?

You should distinguish "commit objects" from "tree objects". You can see the "commit" object with, for example:

$ git cat-file -p HEAD tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904 author Matthieu Moy <Matthieu.Moy@imag.fr> 1236249249 +0100 committer Matthieu Moy <Matthieu.Moy@imag.fr> 1236249249 +0100

foo

See: it contains a reference to the tree, possibly references to the
parents, and a timestamp. So, hashing it takes the timestamp into
account. A consequence of this is that you can not change the
timestamp for a commit without changing the "revision identifier"
(i.e. the hash of the commit object).

But the empty tree object is deterministically 4b825dc642cb6eb9a060e54bf8d69288fbee4904.

-- 
Matthieu