How does Git compute file hashes?

Question

The SHA1 hashes stored in the tree objects (as returned by git ls-tree) do not match the SHA1 hashes of the file content (as returned by sha1sum):

$ git cat-file blob 4716ca912495c805b94a88ef6dc3fb4aff46bf3c | sha1sum
de20247992af0f949ae8df4fa9a37e4a03d7063e  -

How does Git compute file hashes? Does it compress the content before computing the hash?

Answer 1

Git prefixes the object with "blob ", followed by the length (as a human-readable integer), followed by a NUL character

$ echo -e 'blob 14\0Hello, World!' | shasum 8ab686eafeb1f44702738c8b0f24f2567c36da6d

Source: http://alblue.bandlem.com/2011/08/git-tip-of-week-objects.html

Answer 2

I needed this for some unit tests in Python 3 so thought I'd leave it here.

def git_blob_hash(data):
    if isinstance(data, str):
        data = data.encode()
    data = b'blob ' + str(len(data)).encode() + b'\0' + data
    h = hashlib.sha1()
    h.update(data)
    return h.hexdigest()

I stick to \n line endings everywhere but in some circumstances Git might also be changing your line endings before calculating this hash so you may need a .replace('\r\n', '\n') in there too.

Answer 3

This is a python3 version for binary hash calculation (the above example is for text)

For purpose of readability put this code in your own def. Also note, the code is a snippet, not a complete script. For your inspiration.

    targetSize: int
exists: bool
if os.path.exists(targetFile):
    exists = True
    targetSize = os.path.getsize(targetFile)
else:
    exists = False
    targetSize = 0
openMode: str
if exists:
    openMode = 'br+'
else:
    openMode = 'bw+'
with open(targetFile, openMode) as newfile:
    if targetSize > 0:
        header: str = f"blob {targetSize}\0"
        headerBytes = header.encode('utf-8')
        headBytesLen = len(headerBytes)
        buffer = bytearray(headBytesLen + targetSize)
        buffer[0:0+headBytesLen] = headerBytes
        buffer[headBytesLen:headBytesLen+targetSize] = newfile.read()
        sha1Hash = hashlib.sha1(buffer).hexdigest()
        if not sha == sha1Hash:
            newfile.truncate()
        else:
            continue
    with requests.get(fullFile) as response2:            
        newfile.write(response2.content)

Answer 4

Based on Leif Gruenwoldt answer, here is a shell function substitute to git hash-object :

git-hash-object () { # substitute when the `git` command is not available
    local type=blob
    [ "$1" = "-t" ] && shift && type=$1 && shift
    # depending on eol/autocrlf settings, you may want to substitute CRLFs by LFs
    # by using `perl -pe 's/\r$//g'` instead of `cat` in the next 2 commands
    local size=$(cat $1 | wc -c | sed 's/ .*$//')
    ( echo -en "$type $size\0"; cat "$1" ) | sha1sum | sed 's/ .*$//'
}

Test:

$ echo 'Hello, World!' > test.txt
$ git hash-object test.txt
8ab686eafeb1f44702738c8b0f24f2567c36da6d
$ git-hash-object test.txt
8ab686eafeb1f44702738c8b0f24f2567c36da6d

Answer 5

I am only expanding on the answer by @Leif Gruenwoldt and detailing what is in the reference provided by @Leif Gruenwoldt

Do It Yourself..

• Step 1. Create an empty text document (name does not matter) in your repository
• Step 2. Stage and Commit the document
• Step 3. Identify the hash of the blob by executing git ls-tree HEAD
• Step 4. Find the blob's hash to be e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
• Step 5. Snap out of your surprise and read below

How does GIT compute its commit hashes

    Commit Hash (SHA1) = SHA1("blob " + <size_of_file> + "\0" + <contents_of_file>)

The text blob⎵ is a constant prefix and \0 is also constant and is the NULL character. The <size_of_file> and <contents_of_file> vary depending on the file.

See: What is the file format of a git commit object?

And thats all folks!

But wait!, did you notice that the <filename> is not a parameter used for the hash computation? Two files could potentially have the same hash if their contents are same indifferent of the date and time they were created and their name. This is one of the reasons Git handles moves and renames better than other version control systems.

Do It Yourself (Ext)

• Step 6. Create another empty file with a different filename in the same directory
• Step 7. Compare the hashes of both your files.

Note:

The link does not mention how the tree object is hashed. I am not certain of the algorithm and parameters however from my observation it probably computes a hash based on all the blobs and trees (their hashes probably) it contains

Answer 6

git hash-object

This is a quick way to verify your test method:

s='abc'
printf "$s" | git hash-object --stdin
printf "blob $(printf "$s" | wc -c)\0$s" | sha1sum

Output:

f2ba8f84ab5c1bce84a7b441cb1959cfc7093b7f
f2ba8f84ab5c1bce84a7b441cb1959cfc7093b7f  -

where sha1sum is in GNU Coreutils.

Then it comes down to understanding the format of each object type. We have already covered the trivial blob, here are the others:

• commit: What is the file format of a git commit object?
• tree: What is the internal format of a git tree object?
• tag: How is a Git Tag Object SHA1 Created?