I'm trying to understand how Git calculates the hash of refs.
$ git ls-remote https://github.com/git/git
....
29932f3915935d773dc8d52c292cadd81c81071d refs/tags/v2.4.2
9eabf5b536662000f79978c4d1b6e4eff5c8d785 refs/tags/v2.4.2^{}
....
Clone the repo locally. Check the refs/tags/v2.4.2^{} ref by sha
$ git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785
tree 655a20f99af32926cbf6d8fab092506ddd70e49c
parent df08eb357dd7f432c3dcbe0ef4b3212a38b4aeff
author Junio C Hamano <gitster@pobox.com> 1432673399 -0700
committer Junio C Hamano <gitster@pobox.com> 1432673399 -0700
Git 2.4.2
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Copy the decompressed content so that we can hash it. (AFAIK Git uses the uncompressed version when it's hashing)
git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785 > fi
Let's SHA-1 the content using Git's own hash command
git hash-object fi
3cf741bbdbcdeed65e5371912742e854a035e665
Why is the output not [9e]abf5b536662000f79978c4d1b6e4eff5c8d785? I understand the first two characters (9e) are the length in hex. How should I hash the content of fi so that I can get the Git ref abf5b536662000f79978c4d1b6e4eff5c8d785?
As described in "How is git commit sha1 formed ", the formula is:
(printf "<type> %s\0" $(git cat-file <type> <ref> | wc -c); git cat-file <type> <ref>)|sha1sum
In the case of the commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 (which is v2.4.2^{}, and which referenced a tree) :
(printf "commit %s\0" $(git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 | wc -c); git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 )|sha1sum
That will give 9eabf5b536662000f79978c4d1b6e4eff5c8d785.
As would:
(printf "commit %s\0" $(git cat-file commit v2.4.2{} | wc -c); git cat-file commit v2.4.2{})|sha1sum
(still 9eabf5b536662000f79978c4d1b6e4eff5c8d785)
Similarly, computing the SHA1 of the tag v2.4.2 would be:
(printf "tag %s\0" $(git cat-file tag v2.4.2 | wc -c); git cat-file tag v2.4.2)|sha1sum
That would give 29932f3915935d773dc8d52c292cadd81c81071d.