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Git Objects

Git is a file system and it stores data as objects in .git/objects. To dig deeper, let's create an empty repository

Initialise a git repo

git init
Initialized empty Git repository in ~/code-geass/git-objects/.git/

ls .git/objects/
total 12K
info  pack

Commit something

echo "Hello World" > sample.txt
git add .
git commit -m "Initial commit"

[master (root-commit) 70f928c] Initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 sample.txt

ls -la .git/objects/

4e  4f  70  info  pack

As you can see three new directories were created after the Initial Commit. You can check the files by

find .git/objects -type f
.git/objects/70/f928c3b426a4949d8968334e4d30e92bc31687
.git/objects/4e/bf5763311653c68f44db6b66c300192a7e11c4
.git/objects/4f/52b57b2a3a96457d18049ea34c6085de0e09a4

Each new directories has a file inside it.

Check object type, size and contents

You can check git objects type, size and contents.

  • type git cat-file -t <object>
  • size git cat-file -s <object>
  • contents git cat-file -p <object>

where <object> is dir-name followed by first two chars of file-name. If referrign to the example above

git cat-file -p 70f9

tree 4ebf5763311653c68f44db6b66c300192a7e11c4
author husky-parul <[email protected]> 1570471501 -0400
committer husky-parul <[email protected]>1570471501 -0400

Initial commit

To know about all the objects types run,

for i in 70f9 4ebf 4f52; do git cat-file -t $i; done
commit
tree
blob

for i in 70f9 4ebf 4f52; do git cat-file -s $i; done
181
38
13

Blob objects

Blob is an abbreviation of “Binary Large Object”. It contains contents of the file only.

Now let's try to update sample.txt, create a new file and make another commit and see what happens to these objects.

echo "Update a sent" > sample.txt
echo "Hello World2" > sample2.txt
git add .
git commit -m "Second commit"
[master a0cf9b0] Second commit
 2 files changed, 2 insertions(+), 1 deletion(-)
 create mode 100644 sample2.txt

find .git/objects -type f
.git/objects/70/f928c3b426a4949d8968334e4d30e92bc31687
.git/objects/b2/b6f00d3432b3a12bc47e2ae31ee679f2baae92
.git/objects/a0/cf9b04d176d7a6dd1ccc16cbd035bb9592d6de
.git/objects/f8/6effb19a7ee6cea51166c3a1438ba313794fc8
.git/objects/4e/bf5763311653c68f44db6b66c300192a7e11c4
.git/objects/4f/52b57b2a3a96457d18049ea34c6085de0e09a4
.git/objects/d2/fc8330756fc2dd131ff428a48e5a402d515cfe

for i in 70f9 b2b6 a0cf f86e 4ebf 4f52 d2fc; do git cat-file -t $i; done
commit
blob
commit
blob
tree
blob
tree

As you can see two blob objects have been added. Note that the existing blob object 4f52 has not been updated or deleted. That means a blob object is a snapshot of the file.

Commit objects

Similarly you can see commit objects. The commit objects contain the reference to a tree object, author, committer and commit message. Except for the initial commit, commits also have the reference to the tree object of the parent. There may be multiple parents.

git cat-file 70f9 -p
tree 4ebf5763311653c68f44db6b66c300192a7e11c4
author husky-parul <[email protected]> 1570471501 -0400
committer husky-parul <[email protected]> 1570471501 -0400

Initial commit

git cat-file a0cf -p
tree d2fc8330756fc2dd131ff428a48e5a402d515cfe
parent 70f928c3b426a4949d8968334e4d30e92bc31687
author husky-parul <[email protected]> 1570472704 -0400
committer husky-parul <[email protected]> 1570472704 -0400

Second commit

Unlike blob, commit object is metadata and has no content.

Tree objects

Tree objects contain one line per file or subdirectory. Each line has file permission, object type, and object hash and filename.

git cat-file d2fc -p
100644 blob f86effb19a7ee6cea51166c3a1438ba313794fc8	sample.txt
100644 blob b2b6f00d3432b3a12bc47e2ae31ee679f2baae92	sample2.txt

Refrences

https://git-scm.com/book/en/v2/Git-Internals-Git-Objects