How to assign a Git SHA1's to a file without G

Question

As I understand it when Git assigns a SHA1 hash to a file this SHA1 is unique to the file based on its contents.

As a result if a file moves from one repository to another the SHA1 for the file remains the same as its contents have not changed.

How does Git calculate the SHA1 digest? Does it do it on the full uncompressed file contents?

I would like to emulate assigning SHA1's outside of Git.

Answer 1

This is how Git calculates the SHA1 for a file (or, in Git terms, a "blob"):

sha1("blob " + filesize + "\0" + data)

So you can easily compute it yourself without having Git installed. Note that "\0" is the NULL-byte, not a two-character string.

For example, the hash of an empty file:

sha1("blob 0\0") = "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"

$ touch empty
$ git hash-object empty
e69de29bb2d1d6434b8b29ae775ad8c2e48c5391

Another example:

sha1("blob 7\0foobar\n") = "323fae03f4606ea9991df8befbb2fca795e648fa"

$ echo "foobar" > foo.txt
$ git hash-object foo.txt 
323fae03f4606ea9991df8befbb2fca795e648fa

Here is a Python implementation:

from hashlib import sha1
def githash(data):
    s = sha1()
    s.update("blob %u\0" % len(data))
    s.update(data)
    return s.hexdigest()

Answer 2

You can make a bash shell function to calculate it quite easily if you don't have git installed.

git_id () { printf 'blob %s\0' "$(ls -l "$1" | awk '{print $5;}')" | cat - "$1" | sha1sum | awk '{print $1}'; }

Answer 3

In JavaScript

const crypto = require('crypto')
const bytes = require('utf8-bytes')

function sha1(data) {
    const shasum = crypto.createHash('sha1')
    shasum.update(data)
    return shasum.digest('hex')
}

function shaGit(data) {
    const total_bytes = bytes(data).length
    return sha1(`blob ${total_bytes}\0${data}`)
}

Answer 4

In Perl:

#!/usr/bin/env perl
use Digest::SHA1;

my $content = do { local $/ = undef; <> };
print Digest::SHA1->new->add('blob '.length($content)."\0".$content)->hexdigest(), "\n";

As a shell command:

perl -MDigest::SHA1 -E '$/=undef;$_=<>;say Digest::SHA1->new->add("blob ".length()."\0".$_)->hexdigest' < file

Answer 5

Take a look at the man page for git-hash-object. You can use it to compute the git hash of any particular file. I think that git feeds more than just the contents of the file into the hash algorithm, but I don't know for sure, and if it does feed in extra data, I don't know what it is.

Answer 6

Using Ruby, you could do something like this:

require 'digest/sha1'

def git_hash(file)
  data = File.read(file)
  size = data.bytesize.to_s
  Digest::SHA1.hexdigest('blob ' + size + "\0" + data)
end