Create a commit

Create a tree object

After creating a blob object for your README.md file, the next step is to create a tree object. A tree object in Git represents a directory and its contents, which could be other directories (sub-trees) or files (blobs). For our simple repository, the tree object will just contain the README.md blob and its name. When we use git commit -m "commit message", it updates the tree objects from index file we just created.

Lets make our tree by running command:

git write-tree

Now our .git folder looks like following:

.git
├── HEAD
├── index
├── objects
│   ├── 35
│   │   └── d7a9a460c597c8a29b210b2bed894a5281088e
│   └── 86
│       └── 046e36f9b11251cf800033cec1c524502a6ce4
└── refs
    └── heads

A new object is now created under .git/objects and the hash was 86046e36f9b11251cf800033cec1c524502a6ce4. We can verify with git cat-file -t 86046e and git cat-file -p 86046e.

Create a Commit Object

Lets have a sync and do a git status, and we should see the result is the same as last time. Same as git add the command git commit -m message performs several operations internally, and update tree structure is part of it. As you may have guessed, lets make a new commit object.

A commit object in Git encapsulates the state of a repository at a given point in time. It includes the following information:

• Tree SHA-1: The SHA-1 hash of the tree object that represents the directory structure of the commit.
• Parent SHA-1: The SHA-1 hash of the commit’s parent(s). For the first commit, this field is absent.
• Author: The name, email of the author of the changes, and the timestamp.
• Committer: The name, email of the person who committed the changes, and the timestamp.
• Commit Message: A descriptive message explaining the commit.

Here’s an example of what a commit object might look like (simplified):

tree 2f3a123456789...
author John Doe <john@example.com> 1625592667 -0400
committer John Doe <john@example.com> 1625592667 -0400

Initial commit

To achive this, we can use plumbing command:

git commit-tree 86046e36f9b11251cf800033cec1c524502a6ce4 -m "initial commit"

This is what .git looks like now:

.git
├── HEAD
├── index
├── objects
│   ├── 35
│   │   └── d7a9a460c597c8a29b210b2bed894a5281088e
│   ├── 79
│   │   └── e1fd1b22c75a812e59672d973cae307c036ef3
│   └── 86
│       └── 046e36f9b11251cf800033cec1c524502a6ce4
└── refs
    └── heads

You should have received a new object with a different hash than what shows above, this is because the content of the commit object is different, like autor and comitter.

Lets verify the new object with git cat-file -t and git cat-file -p