arunma/gitplay

Create .git directory

mkdir ./git

Create objects directory

mkdir ./git/objects

Create refs directory

mkdir ./git/refs

Point HEAD to the master branch (or 'whatever' branch)

echo "ref: refs/head/master" > .git/HEAD

argon@argon:oss/gitplay ‹master*›$ git status
On branch master

No commits yet

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

nothing added to commit but untracked files present (use "git add" to track)

Create a new blob

argon@argon:oss/gitplay ‹master*›$ echo "hello world" | git hash-object --stdin
3b18e512dba79e4c8300dd08aeb37f8e728b8dad

Create a new blob and write the blob into the object database

argon@argon:oss/gitplay ‹master*›$ echo "hello world" | git hash-object -w --stdin
3b18e512dba79e4c8300dd08aeb37f8e728b8dad

Verify in .git

argon@argon:oss/gitplay ‹master*›$ tree .git
.git
├── FETCH_HEAD
├── HEAD
├── objects
│   └── 3b
│       └── 18e512dba79e4c8300dd08aeb37f8e728b8dad
└── refs

Cat the object file type

-t - Type

argon@argon:oss/gitplay ‹master*›$ git cat-file -t 3b18e512dba79e4c8300dd08aeb37f8e728b8dad
blob

Cat the object file

-p - Pretty print

argon@argon:oss/gitplay ‹master*›$ git cat-file -p 3b18e512dba79e4c8300dd08aeb37f8e728b8dad
hello world

Git status does not still show any files

argon@argon:oss/gitplay ‹master*›$ git status                                              
On branch master

No commits yet

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

nothing added to commit but untracked files present (use "git add" to track)

Update the index with this new file

argon@argon:oss/gitplay ‹master*›$ git update-index --add --cacheinfo 100644 3b18e512dba79e4c8300dd08aeb37f8e728b8dad hw.txt 

Check out git status and see that the file has been added

argon@argon:oss/gitplay ‹master*›$ git status                                                                               
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
        new file:   hw.txt

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        deleted:    hw.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

We see the file both in "new file" and "deleted" because we created the object and the index but not the physical file

Create a physical file with blob contents

argon@argon:oss/gitplay ‹master*›$ git cat-file -p 3b18e512dba79e4c8300dd08aeb37f8e728b8dad > hw.txt
argon@argon:oss/gitplay ‹master*›$ git status                                                       
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
        new file:   hw.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

Create a tree

A commit has reference to a tree. Let's create a tree

argon@argon:oss/gitplay ‹master*›$ git write-tree 
7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e

Tree and see the structure

argon@argon:oss/gitplay ‹master*›$ tree .git
.git
├── FETCH_HEAD
├── HEAD
├── index
├── objects
│   ├── 3b
│   │   └── 18e512dba79e4c8300dd08aeb37f8e728b8dad
│   └── 7c
│       └── f7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e
└── refs

Cat file to check this new object (tree)

argon@argon:oss/gitplay ‹master*›$ git cat-file -t 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e                                        
tree
argon@argon:oss/gitplay ‹master*›$ git cat-file -p 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e                                                 
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    hw.txt

Commit the tree to the index (staging area)

argon@argon:oss/gitplay ‹master*›$ git commit-tree 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e -m "first commit"
09cc2f691947c0834fcf098381c371c12827a0dd

tree .git

argon@argon:oss/gitplay ‹master*›$ tree .git
.git
├── FETCH_HEAD
├── HEAD
├── index
├── objects
│   ├── 09
│   │   └── cc2f691947c0834fcf098381c371c12827a0dd
│   ├── 3b
│   │   └── 18e512dba79e4c8300dd08aeb37f8e728b8dad
│   └── 7c
│       └── f7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e
└── refs

Cat file to check the new object (commit)

argon@argon:oss/gitplay ‹master*›$ git cat-file -t 09cc2f691947c0834fcf098381c371c12827a0dd                                        
commit
argon@argon:oss/gitplay ‹master*›$ git cat-file -p 09cc2f691947c0834fcf098381c371c12827a0dd
tree 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e
author Arun Manivannan <arun@arunma.com> 1675146888 +0800
committer Arun Manivannan <arun@arunma.com> 1675146888 +0800

first commit

Summary

Commit 09 points to tree 7c Tree 07 points to Blob 3b

Unfortunately the git status does not reflect this

argon@argon:oss/gitplay ‹master*›$ git status                                              
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
        new file:   hw.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

Reflection

HEAD is just a pointer to the branch (one of the heads). heads must point to a commit

Create a new branch

A branch is nothing but a named file inside the .git/refs/heads directory.

argon@argon:oss/gitplay ‹master*›$ mkdir -p .git/refs/heads                                                
argon@argon:oss/gitplay ‹master*›$ echo "09cc2f691947c0834fcf098381c371c12827a0dd" > .git/refs/heads/master

Git status

The hw.txt is not staged anymore. It is committed.

argon@argon:oss/gitplay ‹master*›$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

nothing added to commit but untracked files present (use "git add" to track)

Create a new branch

Again, a named file in .git/refs/heads/test.

Let's point the same commit from this new branch.

argon@argon:oss/gitplay ‹master*›$ echo "09cc2f691947c0834fcf098381c371c12827a0dd" > .git/refs/heads/test  

Git log

Both master and test point to the same commit hash.

commit 09cc2f691947c0834fcf098381c371c12827a0dd (HEAD -> master, test)
Author: Arun Manivannan <arun@arunma.com>
Date:   Tue Jan 31 14:34:48 2023 +0800

    first commit

Switch branch from master to test

argon@argon:oss/gitplay ‹master*›$ echo "ref: refs/heads/test" > .git/HEAD                         
argon@argon:oss/gitplay ‹test*›$ git status
On branch test
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

nothing added to commit but untracked files present (use "git add" to track)

Create a new file

argon@argon:oss/gitplay ‹test*›$ echo "new file" | git hash-object -w --stdin
fa49b077972391ad58037050f2a75f74e3671e92
argon@argon:oss/gitplay ‹test*›$ git update-index --add --cacheinfo 100644 fa49b077972391ad58037050f2a75f74e3671e92 new.txt  
argon@argon:oss/gitplay ‹test*›$ git cat-file -p fa49b077972391ad58037050f2a75f74e3671e92                                        
new file
argon@argon:oss/gitplay ‹test*›$ git write-tree                                          
b57ada7fde872f65a3b9f28995d945923f6ae1a1
argon@argon:oss/gitplay ‹test*›$ git commit-tree b57ada7fde872f65a3b9f28995d945923f6ae1a1 -m "test commit"                                         
627b929dd8453f13d68e511a3477e4aa65c33041
argon@argon:oss/gitplay ‹test*›$ echo "627b929dd8453f13d68e511a3477e4aa65c33041" > .git/refs/heads/test 
argon@argon:oss/gitplay ‹test*›$ git status                                                               
On branch test
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        deleted:    new.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

no changes added to commit (use "git add" and/or "git commit -a")
argon@argon:oss/gitplay ‹test*›$ git cat-file -p fa49b077972391ad58037050f2a75f74e3671e92 > new.txt                              
argon@argon:oss/gitplay ‹test*›$ git status                                                        
On branch test
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

nothing added to commit but untracked files present (use "git add" to track)

Git log

commit 627b929dd8453f13d68e511a3477e4aa65c33041 (HEAD -> test)
Author: Arun Manivannan <arun@arunma.com>
Date:   Tue Jan 31 14:54:41 2023 +0800

    test commit

Compare branches

argon@argon:oss/gitplay ‹test*›$ cat .git/HEAD                                           
ref: refs/heads/test
argon@argon:oss/gitplay ‹test*›$ cat .git/refs/heads/test
627b929dd8453f13d68e511a3477e4aa65c33041
argon@argon:oss/gitplay ‹test*›$ git cat-file -p 627b929dd8453f13d68e511a3477e4aa65c33041
tree b57ada7fde872f65a3b9f28995d945923f6ae1a1
author Arun Manivannan <arun@arunma.com> 1675148081 +0800
committer Arun Manivannan <arun@arunma.com> 1675148081 +0800

test commit
argon@argon:oss/gitplay ‹test*›$ git cat-file -p b57ada7fde872f65a3b9f28995d945923f6ae1a1                                        
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    hw.txt
100644 blob fa49b077972391ad58037050f2a75f74e3671e92    new.txt
 
----------------------

argon@argon:oss/gitplay ‹test*›$ cat .git/refs/heads/master 
09cc2f691947c0834fcf098381c371c12827a0dd
argon@argon:oss/gitplay ‹test*›$ git cat-file -p 09cc2f691947c0834fcf098381c371c12827a0dd
tree 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e
author Arun Manivannan <arun@arunma.com> 1675146888 +0800
committer Arun Manivannan <arun@arunma.com> 1675146888 +0800

first commit
argon@argon:oss/gitplay ‹test*›$ git cat-file -p 7cf7564d5a372dd8dcc4a0ba9300ec22ed7f7e5e
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    hw.txt

Notice that both hw.txt and new.txt are blobs

Brand new branch with commit tree inheritance

argon@argon:oss/gitplay ‹foobar*›$ echo "foo" | git hash-object -w --stdin                 
257cc5642cb1a054f08cc83f2d943e56fd3ebe99
argon@argon:oss/gitplay ‹foobar*›$ git cat-file -p 257cc5642cb1a054f08cc83f2d943e56fd3ebe99                                        
foo
argon@argon:oss/gitplay ‹foobar*›$ git cat-file -p 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 > foo.txt
argon@argon:oss/gitplay ‹foobar*›$ gst                                                               
On branch foobar
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md
        foo.txt

nothing added to commit but untracked files present (use "git add" to track)
argon@argon:oss/gitplay ‹foobar*›$ git update-index --add --cacheinfo 100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 foo.txt
argon@argon:oss/gitplay ‹foobar*›$ git write-tree                                                                            
6bc243aeb80f08ab2b42d7bd067059cce86283b6
argon@argon:oss/gitplay ‹foobar*›$ git commit-tree 6bc243aeb80f08ab2b42d7bd067059cce86283b6 -p 09cc2f691947c0834fcf098381c371c12827a0dd -m "foo branch first commit" 
9a8e2079c3386d3ab7b73d948a93e6244eebbcd9
argon@argon:oss/gitplay ‹foobar*›$ gst
On branch foobar
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        new file:   foo.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

argon@argon:oss/gitplay ‹foobar*›$ git cat-file -p 9a8e2079c3386d3ab7b73d948a93e6244eebbcd9                                                  
tree 6bc243aeb80f08ab2b42d7bd067059cce86283b6
parent 09cc2f691947c0834fcf098381c371c12827a0dd
author Arun Manivannan <arun@arunma.com> 1675154747 +0800
committer Arun Manivannan <arun@arunma.com> 1675154747 +0800

foo branch first commit
argon@argon:oss/gitplay ‹foobar*›$ echo "9a8e2079c3386d3ab7b73d948a93e6244eebbcd9" > .git/refs/heads/foo          
argon@argon:oss/gitplay ‹foobar*›$ gst                                                     
On branch foobar
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        new file:   foo.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        README.md

argon@argon:oss/gitplay ‹foobar*›$ echo "ref: refs/heads/foo" > .git/HEAD                            
argon@argon:oss/gitplay ‹foo*›$ git log

commit 9a8e2079c3386d3ab7b73d948a93e6244eebbcd9 (HEAD -> foo)
Author: Arun Manivannan <arun@arunma.com>
Date:   Tue Jan 31 16:45:47 2023 +0800

    foo branch first commit

commit 09cc2f691947c0834fcf098381c371c12827a0dd (master)
Author: Arun Manivannan <arun@arunma.com>
Date:   Tue Jan 31 14:34:48 2023 +0800

    first commit

Summary

1. git log goes to HEAD
2. HEAD points to one of the ./git/refs/heads aka "branch"
3. heads are just named files that has pointers to a commit tree
4. commit tree points to a tree
5. tree points to sub tree or blob objects
6. blob stores the hashes of the files