Git tree objects, how are they stored?

A git tree is git’s internal representation for directories. Trees have entries which are either trees themselves, or blobs.

The general format is: First 4 bytes declaring the object type. In our case, those four bytes are “tree”, ASCII-encoded. Then comes a space, and then the entries, separated by nothing.
The exact format is the following. All capital letters are “non-terminals” that I’ll explain shortly.

    tree (ZNA FNS)*

where

• N is the NUL character


• Z is the size of the object in bytes


• A is the unix access code, ASCII encoded, for example> 100644 for a vanilla file.


• F is the filename, (I’m not sure about the encoding. It’s definitely ASCII-compatible), NUL-terminated.


• S is the 20 byte SHA hash of the entry pointed to, 20 bytes long.

Here’s an example. Say we have a directory with two files, called test and test2. The SHA of the directory is f0e12ff4a9a6ba281d57c7467df585b1249f0fa5. You can see the SHA-hashes of the entries in the output of

    $ git cat-file -p f0e12ff4a9a6ba281d57c7467df585b1249f0fa5
    100644 blob 9033296159b99df844df0d5740fc8ea1d2572a84    test
    100644 blob a7f8d9e5dcf3a68fdd2bfb727cde12029875260b    test2

(To reproduce this, the file test should contain the ASCII-encoded characters “hallo” (not \n-terminated), the file test2 should contain the ASCII-encoded characters “bla\n” (yes, \n-terminated)).

That is converted into the following object (uncompressed):



Finally, the file is deflated and stored on disk. For deflation, zlib is used. For my experiments I used the ruby zlip package. The following tiny script reads the tree from disk and inflates it, so you can look at it:

    require 'zlib'
    require 'fileutils'
    File.open("objects/f0/e12ff4a9a6ba281d57c7467df585b1249f0fa5") { |f|
        puts Zlib::Inflate.inflate(f.read)
    }