Re: Manually decoding a git object

From: Thomas Rast <trast@inf.ethz.ch>
Date: Feb 20, 2012, 08:29 UTC
Message-ID: <87mx8dj4at.fsf@thomas.inf.ethz.ch>
In-Reply-To: <A716A27D407F401DAAC373027D5CFF2A@PhilipOakley>

"Philip Oakley" <philipoakley@iee.org> writes:

Show 6 quoted lines

> If I have a renamed file which is a git object, such a "Git_Object", was
> 8c-something-or-other, what is the easiest way of examining / decoding /
> recreating the original file (either as its sha1, or a cat-file).
>
> I don't appear to be able to unzip the file in its raw format...  I'm using
> Msysgit on windows XP.

The SHA1 is over the decompressed object contents. The file simply holds a zlib-compressed stream of those contents. (It's pretty much like gzip without the file header.)

You can use any bindings to zlib and something that does sha1, e.g. in python:

  $ cd g/.git/objects/aa/  # my git.git
  $ ls
  592bda986a8380b64acd8cbb3d5bdfcbc0834d  6322a757bee31919f54edcc127608a3d724c99
  $ python
  Python 2.7.2 (default, Aug 19 2011, 20:41:43) [GCC] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import hashlib
  >>> hashlib.sha1(open('592bda986a8380b64acd8cbb3d5bdfcbc0834d').read().decode('zlib')).digest().encode('hex')
  'aa592bda986a8380b64acd8cbb3d5bdfcbc0834d'

Notice that the first byte of the hash goes into the directory name.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch