To understand the core of Git internals, there are 3 things to we should know: objects, references, the index.

I find this model is elegant. It fits well in a small diagram, as well as in my head.

Objects

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All files that you commited into a Git repository, including the commit info are stored as objects in .git/objects/.

An object is identified by a 40-character-long string – SHA1 hash of the object’s content.

There are 4 types of objects:

1. blob – stores file content.
2. tree – stores direcotry layouts and filenames.
3. commit – stores commit info and forms the Git commit graph.
4. tag – stores annotated tag.

The example will illustrate how these objects relate to each others.

References

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A branch, remote branch or a tag (also called lightweight tag) in Git, is just a pointer to an object, usually a commit object.

They are stored as plain text files in .git/refs/.

Symbolic References

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Git has a special kind of reference, called symbolic reference. It doesn’t point to an object directly. Instead, it points to another reference.

For instance, .git/HEAD is a symbolic reference. It points to the current branch you are working on.

The Index

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The index is a staging area, stored as a binary file in .git/index.

When git add a file, Git adds the file info to the index. When git commit, Git only commits what’s listed in the index.

Examples

Let’s walkthrough a simple example, to create a Git repository, commit some files and see what happened behind the scene in .git directory.

Initialize New Repository

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1	$ git init canai

What happened:

• Empty .git/objects/ and .git/refs/ created.
• No index file yet.
• HEAD symbolic reference created. $ cat .git/HEAD ref: refs/heads/master

Add New File

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1 2	$ echo “A roti canai project.” >> README $ git add README

What happened:

• Index file created.
  It has a SHA1 hash that points to a blob object.
• Blob object created.
  The content of README file is stored in this blob.

First Commit

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$ git commit -m’first commit’ [master (root-commit) d9976cf] first commit 1 files changed, 1 insertions(+), 0 deletions(-) create mode 100644 README

What happened:

• Branch ‘master’ reference created.
  It points to the lastest commit object in ‘master’ branch.
• First commit object created. It points to the root tree object.
• Tree object created.
  This tree represents the ‘canai’ directory.

Add Modified File

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1 2	$ echo “Welcome everyone.” >> README $ git add README

What happened:

• Index file updated.
  Notice it points to a new blob?
• Blob object created.
  The entire README content is stored as a new blob.

Add File into Subdirectory

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$ mkdir doc $ echo “[[TBD]] manual toc” >> doc/manual.txt $ git add doc

What happened:

• Index file updated.
• Blob object created.

Second Commit

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$ git commit -m’second commit’ [master 556eaf3] second commit 2 files changed, 2 insertions(+), 0 deletions(-) create mode 100644 doc/manual.txt

What happened:

• Branch ‘master’ reference updated.
  It points to a lastest commit in this branch.
• Second commit object created. Notice its ‘parent’ points to the first commit object. This forms a commit graph.
• New root tree object created.
• New subdir tree object created.

Add Annotated Tag

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1	$ git tag -a -m’this is annotated tag’ v0.1 d9976