Reading git objects

By default, git stores objects in .git/objects as their original contents, compressed, and prepended with a few extra characters (see below).

These are easy to read with a small amount of code.

git will read the files for you with git show or git cat-file.

When a repository gets larger, git may use another, less simple format to store the data, called packfiles, in which case these simple code fragments won’t work. The fragments are to show the simplicity of the default way that git stores objects.

git uses a library called zlib for compressing the objects.

To show how this works, first generate a small git repository to make an object file:

$ # Make a new empty git repository
$ mkdir git_example
$ cd git_example
$ git init
Initialized empty Git repository in /Volumes/zorg/mb312/dev_trees/curious-git/working/git_example/.git/

$ # Make and stage a file, to create an object in .git/objects
$ echo "My data fits on one line" > my_file.txt
$ git add my_file.txt

$ # Show the .git/objects directory
$ tree -a .git/objects
.git/objects
├── e2
│   └── 7bb34b0807ebf1b91bb66a4c147430cde4f08f
├── info
└── pack

3 directories, 1 file

We can read this object using some simple Python code. Start up the Python console and:

>>> import zlib  # A compression / decompression library
>>> filename = 'working/git_example/.git/objects/e2/7bb34b0807ebf1b91bb66a4c147430cde4f08f'
>>> compressed_contents = open(filename, 'rb').read()
>>> decompressed_contents = zlib.decompress(compressed_contents)

>>> decompressed_contents
'blob 25\x00My data fits on one line\n'

The contents of the object file is the original contents prepended by blob 25 and a 0 byte, where 25 is the number of characters in the file, not including the housekeeping string:

>>> len(decompressed_contents)  # (length of housekeeping string = 8) + 25
33

The hash value for the file is the hash of the decompressed file, including the housekeeping string:

>>> from hashlib import sha1  # SHA1 hash algorithm
>>> hash_value = sha1(decompressed_contents).hexdigest()
>>> hash_value
'e27bb34b0807ebf1b91bb66a4c147430cde4f08f'