What is the format of a git tag object and how to calculate its SHA?

I am familiar with how Git creates SHA1 hashes for files (blobs), but not how they are created for tag objects. I assume they are, if I create an annotated tag, but what is the recipe? And how might I replicate it outside of Git (e.g., in Perl or Python)?

Answers


The content of a tag object is as follows:

object <commit-sha1>
type commit
tag <tag-name>
tagger <author-with-timestamp>

<tag-message>

Based on that text the SHA1 value is calculated.

how might I replicate it outside of Git (e.g., in Perl or Python)?

Take a look at libgit2 and its various bindings.


Reverse engineer the format from a minimal example

First we determine what the format for tags is:

git tag -as -m abc mytag
cat .git/refs/tags/mytag

outputs the tag sha:

c1d7720e99f9dd1d1c8aee625fd6ce09b3a81fef

so we do as explained at How to DEFLATE with a command line tool to extract a git object? :

python -c "import zlib,sys;sys.stdout.write(zlib.decompress(sys.stdin.read()))" \
    <.git/objects/c1/d7720e99f9dd1d1c8aee625fd6ce09b3a81fef | hd

which gives:

00000000  74 61 67 20 37 39 38 00  6f 62 6a 65 63 74 20 61  |tag 798.object a|
00000010  30 32 63 35 30 32 39 65  30 38 66 37 37 65 61 65  |02c5029e08f77eae|
00000020  35 37 64 62 63 38 31 38  38 61 37 31 31 62 63 39  |57dbc8188a711bc9|
00000030  65 39 62 32 39 30 61 0a  74 79 70 65 20 63 6f 6d  |e9b290a.type com|
00000040  6d 69 74 0a 74 61 67 20  6d 79 74 61 67 0a 74 61  |mit.tag mytag.ta|
00000050  67 67 65 72 20 43 69 72  6f 20 53 61 6e 74 69 6c  |gger Ciro Santil|
00000060  6c 69 20 3c 63 69 72 6f  2e 73 61 6e 74 69 6c 6c  |li <ciro.santill|
00000070  69 40 67 6d 61 69 6c 2e  63 6f 6d 3e 20 31 35 33  |i@blebl.com> 153|
00000080  36 31 37 36 39 30 36 20  2b 30 31 30 30 0a 0a 61  |6176906 +0100..a|
00000090  62 63 0a 2d 2d 2d 2d 2d  42 45 47 49 4e 20 50 47  |bc.-----BEGIN PG|
000000a0  50 20 53 49 47 4e 41 54  55 52 45 2d 2d 2d 2d 2d  |P SIGNATURE-----|
000000b0  0a 0a 69 51 47 7a 42 41  41 42 43 67 41 64 46 69  |..iQGzBAABCgAdFi|
000000c0  45 45 50 6b 64 79 57 72  30 33 4f 6d 39 64 4d 41  |EEPkdyWr03Om9dMA|
000000d0  61 47 51 32 7a 61 2f 4d  2b 77 30 77 30 46 41 6c  |aGQ2za/M+w0w0FAl|
000000e0  75 51 4d 77 6f 41 43 67  6b 51 51 32 7a 61 2f 4d  |uQMwoACgkQQ2za/M|
000000f0  2b 77 0a 30 77 33 77 69  67 76 38 43 6c 31 35 43  |+w.0w3wigv8Cl15C|
00000100  78 4f 6e 71 2f 6c 49 2b  33 4b 47 5a 6e 47 45 66  |xOnq/lI+3KGZnGEf|
00000110  56 47 4b 71 43 46 7a 69  39 4f 66 63 51 48 4d 49  |VGKqCFzi9OfcQHMI|
00000120  77 59 39 6f 78 44 2f 35  68 30 56 37 7a 6b 4a 70  |wY9oxD/5h0V7zkJp|
00000130  78 2f 5a 0a 79 72 52 6d  37 62 41 2f 2f 2b 5a 37  |x/Z.yrRm7bA//+Z7|
00000140  54 59 6c 73 2f 36 54 44  6c 56 53 2f 44 69 72 79  |TYls/6TDlVS/Diry|
00000150  53 39 4d 38 7a 59 45 48  76 44 65 65 71 32 2f 78  |S9M8zYEHvDeeq2/x|
00000160  43 31 71 53 37 47 4e 59  5a 7a 30 36 50 76 2b 79  |C1qS7GNYZz06Pv+y|
00000170  4a 43 54 77 0a 6c 68 32  41 61 6d 7a 33 75 48 55  |JCTw.lh2Aamz3uHU|
00000180  64 6e 4f 37 51 79 70 6f  63 4f 54 6f 5a 4e 53 4a  |dnO7QypocOToZNSJ|
00000190  72 44 2b 2b 48 55 6e 36  77 70 34 54 6f 55 41 35  |rD++HUn6wp4ToUA5|
000001a0  73 37 34 63 46 79 52 58  49 2f 30 49 32 65 59 67  |s74cFyRXI/0I2eYg|
000001b0  44 2b 6b 49 7a 0a 4e 57  64 2b 45 4a 5a 61 52 74  |D+kIz.NWd+EJZaRt|
000001c0  63 55 75 4a 49 55 65 79  75 7a 68 31 64 49 77 6b  |cUuJIUeyuzh1dIwk|
000001d0  73 56 32 52 6e 42 31 37  64 5a 2f 4e 47 57 39 37  |sV2RnB17dZ/NGW97|
000001e0  37 4f 30 69 30 6f 6a 6f  50 6e 74 4e 6e 48 6f 61  |7O0i0ojoPntNnHoa|
000001f0  68 5a 2b 44 6e 4e 0a 71  78 41 37 36 4d 74 6d 43  |hZ+DnN.qxA76MtmC|
00000200  71 41 66 45 6d 68 2b 6b  49 55 74 61 42 44 61 5a  |qAfEmh+kIUtaBDaZ|
00000210  51 36 32 45 66 55 65 76  6c 34 78 6b 72 38 79 4b  |Q62EfUevl4xkr8yK|
00000220  4a 46 6d 53 69 4d 39 76  4c 34 73 7a 4a 75 69 61  |JFmSiM9vL4szJuia|
00000230  4b 56 58 43 71 2b 71 0a  71 33 6d 68 41 41 7a 73  |KVXCq+q.q3mhAAzs|
00000240  70 39 63 6d 32 2b 55 6d  47 7a 70 6b 37 52 51 6f  |p9cm2+UmGzpk7RQo|
00000250  4d 36 50 44 45 73 61 37  43 68 77 52 47 71 53 42  |M6PDEsa7ChwRGqSB|
00000260  78 34 46 48 2f 32 7a 4d  75 49 49 4e 4c 72 72 72  |x4FH/2zMuIINLrrr|
00000270  36 46 64 59 2b 4c 36 71  0a 34 4d 78 67 66 46 38  |6FdY+L6q.4MxgfF8|
00000280  61 6e 47 31 38 42 43 56  65 6d 47 44 65 6d 69 34  |anG18BCVemGDemi4|
00000290  42 71 58 53 4d 73 61 77  35 45 70 77 36 4f 78 43  |BqXSMsaw5Epw6OxC|
000002a0  6c 34 2f 65 33 74 43 48  4b 39 58 78 63 6d 7a 4d  |l4/e3tCHK9XxcmzM|
000002b0  70 62 6a 37 66 77 42 7a  7a 0a 41 76 63 6e 34 48  |pbj7fwBzz.Avcn4H|
000002c0  41 67 52 75 6b 77 7a 54  53 4f 53 59 30 6e 73 74  |AgRukwzTSOSY0nst|
000002d0  67 6b 79 6c 54 4b 69 4d  48 6d 42 4c 4f 2b 49 6b  |gkylTKiMHmBLO+Ik|
000002e0  58 63 4e 47 66 51 6b 46  51 41 67 49 59 67 4d 57  |XcNGfQkFQAgIYgMW|
000002f0  4e 4f 45 2f 68 59 76 73  2b 71 0a 46 51 4d 34 6e  |NOE/hYvs+q.FQM4n|
00000300  6a 45 63 0a 3d 32 50 51  32 0a 2d 2d 2d 2d 2d 45  |jEc.=2PQ2.-----E|
00000310  4e 44 20 50 47 50 20 53  49 47 4e 41 54 55 52 45  |ND PGP SIGNATURE|
00000320  2d 2d 2d 2d 2d 0a                                 |-----.|
00000327

from which we deduce the format is:

tag ${size}\0object ${sha_of_commit_it-points-to}
type ${type_of_object_it_points_to}
tag ${name_of_tag}
tagger ${username_and_email} ${seconds_since_utc} ${timezone}

${commit_message_including_gpg}

From the hd we see that the full output is 0x326 == 806 bytes long which minus eight bytes from the prefix: tag 798. makes the size 798.

Analogous analysis for Git commit objects: What is the file format of a git commit object?

Minimal Python script that creates a tag object and gets its SHA

To ensure that we understood it correctly, here is a Python script that generates a working git repo from scratch, including a tag:

You can then cd into it and verify that all works normally with git commands.

Tested on git 2.16.1, Ubuntu 18.04.


It's pretty much the same, although the smallish header prepended to the commit object is different. You can use git cat-file to see the actual format.


The pattern is basically:

sha1("tag " + datasize + "\0" + data)

Where data is the output of git cat-file. One can produce this by piping that output to git-hash-object like so:

git cat-file tag v0.30 | git hash-object -t tag --stdin

And the equivalent a perl one-liner is:

git cat-file tag v0.30 | perl -MDigest::SHA1 -E '$/=undef;$_=<>;say Digest::SHA1->new->add("tag ".length()."\0".$_)->hex digest'

It seems that one can do this same thing with any of the types objects simply by replacing "tag " with the proper object name: "blob ", "tree ", or "commit ".


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