Create a minimal example and reverse engineer the format
Create a simple repository, and before any packfiles are created (git gc, git config gc.auto, git-prune-packed ...), unpack a commit object with one of the methods from: How to DEFLATE with a command line tool to extract a git object?
export GIT_AUTHOR_DATE="1970-01-01T00:00:00+0000"
export GIT_AUTHOR_EMAIL="[email protected]"
export GIT_AUTHOR_NAME="Author Name" \
export GIT_COMMITTER_DATE="2000-01-01T00:00:00+0000" \
export GIT_COMMITTER_EMAIL="[email protected]" \
export GIT_COMMITTER_NAME="Committer Name" \
git init
# First commit.
echo
touch a
git add a
git commit -m 'First message'
python -c "import zlib,sys;sys.stdout.write(zlib.decompress(sys.stdin.read()))" \
<.git/objects/45/3a2378ba0eb310df8741aa26d1c861ac4c512f | hd
# Second commit.
echo
touch b
git add b
git commit -m 'Second message'
python -c "import zlib,sys;sys.stdout.write(zlib.decompress(sys.stdin.read()))" \
<.git/objects/74/8e6f7e22cac87acec8c26ee690b4ff0388cbf5 | hd
The output is:
Initialized empty Git repository in /home/ciro/test/git/.git/
[master (root-commit) 453a237] First message
Author: Author Name <[email protected]>
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 a
00000000 63 6f 6d 6d 69 74 20 31 37 34 00 74 72 65 65 20 |commit 174.tree |
00000010 34 39 36 64 36 34 32 38 62 39 63 66 39 32 39 38 |496d6428b9cf9298|
00000020 31 64 63 39 34 39 35 32 31 31 65 36 65 31 31 32 |1dc9495211e6e112|
00000030 30 66 62 36 66 32 62 61 0a 61 75 74 68 6f 72 20 |0fb6f2ba.author |
00000040 41 75 74 68 6f 72 20 4e 61 6d 65 20 3c 61 75 74 |Author Name <aut|
00000050 68 6f 72 40 65 78 61 6d 70 6c 65 2e 63 6f 6d 3e |[email protected]>|
00000060 20 30 20 2b 30 30 30 30 0a 63 6f 6d 6d 69 74 74 | 0 +0000.committ|
00000070 65 72 20 43 6f 6d 6d 69 74 74 65 72 20 4e 61 6d |er Committer Nam|
00000080 65 20 3c 63 6f 6d 6d 69 74 74 65 72 40 65 78 61 |e <committer@exa|
00000090 6d 70 6c 65 2e 63 6f 6d 3e 20 39 34 36 36 38 34 |mple.com> 946684|
000000a0 38 30 30 20 2b 30 30 30 30 0a 0a 46 69 72 73 74 |800 +0000..First|
000000b0 20 6d 65 73 73 61 67 65 0a | message.|
000000ba
[master 748e6f7] Second message
Author: Author Name <[email protected]>
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 b
00000000 63 6f 6d 6d 69 74 20 32 32 33 00 74 72 65 65 20 |commit 223.tree |
00000010 32 39 36 65 35 36 30 32 33 63 64 63 30 33 34 64 |296e56023cdc034d|
00000020 32 37 33 35 66 65 65 38 63 30 64 38 35 61 36 35 |2735fee8c0d85a65|
00000030 39 64 31 62 30 37 66 34 0a 70 61 72 65 6e 74 20 |9d1b07f4.parent |
00000040 34 35 33 61 32 33 37 38 62 61 30 65 62 33 31 30 |453a2378ba0eb310|
00000050 64 66 38 37 34 31 61 61 32 36 64 31 63 38 36 31 |df8741aa26d1c861|
00000060 61 63 34 63 35 31 32 66 0a 61 75 74 68 6f 72 20 |ac4c512f.author |
00000070 41 75 74 68 6f 72 20 4e 61 6d 65 20 3c 61 75 74 |Author Name <aut|
00000080 68 6f 72 40 65 78 61 6d 70 6c 65 2e 63 6f 6d 3e |[email protected]>|
00000090 20 30 20 2b 30 30 30 30 0a 63 6f 6d 6d 69 74 74 | 0 +0000.committ|
000000a0 65 72 20 43 6f 6d 6d 69 74 74 65 72 20 4e 61 6d |er Committer Nam|
000000b0 65 20 3c 63 6f 6d 6d 69 74 74 65 72 40 65 78 61 |e <committer@exa|
000000c0 6d 70 6c 65 2e 63 6f 6d 3e 20 39 34 36 36 38 34 |mple.com> 946684|
000000d0 38 30 30 20 2b 30 30 30 30 0a 0a 53 65 63 6f 6e |800 +0000..Secon|
000000e0 64 20 6d 65 73 73 61 67 65 0a |d message.|
000000eb
Then we deduce that the format is as follows:
Top level:
commit {size}\0{content}
where {size} is the number of bytes in {content}.
This follows the same pattern for all object types.
{content}:
tree {tree_sha}
{parents}
author {author_name} <{author_email}> {author_date_seconds} {author_date_timezone}
committer {committer_name} <{committer_email}> {committer_date_seconds} {committer_date_timezone}
{commit message}
where:
{tree_sha}: SHA of the tree object this commit points to.
This represents the top-level Git repo directory.
That SHA comes from the format of the tree object: What is the internal format of a git tree object?
{parents}: optional list of parent commit objects of form:
parent {parent1_sha}
parent {parent2_sha}
...
The list can be empty if there are no parents, e.g. for the first commit in a repo.
Two parents happen in regular merge commits.
More than two parents are possible with git merge -Xoctopus, but this is not a common workflow. Here is an example: https://github.com/cirosantilli/test-octopus-100k
{author_name}: e.g.: Ciro Santilli. Cannot contain <, \n
{author_email}: e.g.: [email protected]. Cannot contain >, \n
{author_date_seconds}: seconds since 1970, e.g. 946684800 is the first second of year 2000
{author_date_timezone}: e.g.: +0000 is UTC
committer fields: analogous to author fields
{commit message}: arbitrary.
I've made a minimal Python script that generates a git repo with a few commits at: https://github.com/cirosantilli/test-git-web-interface/blob/864d809c36b8f3b232d5b0668917060e8bcba3e8/other-test-repos/util.py#L83
I've used that for fun things like:
Here is an analogous analysis of the tag object format: What is the format of a git tag object and how to calculate its SHA?
git grep? – Matthieu Moy