git-noob git-noob - 1 month ago 4
Git Question

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

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.


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

Another example:

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

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

Here is a Python implementation:

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