git branch

  

Currently, I am curious to find out what Git will do for us behind the scene if we run the 'git branch' command. Below are the logs coming from my experiment. 

Reference: Git Book

Preparation

Create a brand new folder and run the 'git init' command.

Step 1: 

Check the .git tree structure.

$ tree .git/

.git/ ├── HEAD ├── branches ├── config ├── description ├── hooks │   ├── applypatch-msg.sample │   ├── commit-msg.sample │   ├── fsmonitor-watchman.sample │   ├── post-update.sample │   ├── pre-applypatch.sample │   ├── pre-commit.sample │   ├── pre-merge-commit.sample │   ├── pre-push.sample │   ├── pre-rebase.sample │   ├── pre-receive.sample │   ├── prepare-commit-msg.sample │   ├── push-to-checkout.sample │   └── update.sample

├── info │   └── exclude ├── objects

│   ├── info │   └── pack └── refs ├── heads └── tags 9 directories, 17 files

 

HEAD is the special pointer which can point to a branch pointer or a commit (detach HEAD)

  

    $ cat .git/HEAD

ref: refs/heads/master

    $ cat .git/refs/heads/master

cat: .git/refs/heads/master: No such file or directory

Create a new commit in the current branch 'master'.

  

    $ echo 'test' > test.txt

$ git add .

$ git commit -m 'first commit'

[master (root-commit) 7353d8d] first commit 1 file changed, 1 insertion(+) create mode 100644 test.txt

Check the .git tree structure.

$ tree .git/

.git/

├── COMMIT_EDITMSG (a new file) ├── HEAD ├── branches ├── config ├── description ├── hooks │   ├── applypatch-msg.sample │   ├── commit-msg.sample │   ├── fsmonitor-watchman.sample │   ├── post-update.sample │   ├── pre-applypatch.sample │   ├── pre-commit.sample │   ├── pre-merge-commit.sample │   ├── pre-push.sample │   ├── pre-rebase.sample │   ├── pre-receive.sample │   ├── prepare-commit-msg.sample │   ├── push-to-checkout.sample │   └── update.sample

├── index (a new file)

├── info │   └── exclude

├── logs (a new directory)

│   ├── HEAD (a new file)

│   └── refs (a new directory)

│       └── heads (a new directory)

│           └── master (a new file)

├── objects

│   ├── 2b (a new directory) │   │   └── 297e643c551e76cfa1f93810c50811382f9117 (a new file for git tree object) │   ├── 73 (a new directory) │   │   └── 53d8d4755041cbd9600ce8a1c2f5e7e35646ea (a new file for git commit object) │   ├── 9d (a new directory) │   │   └── aeafb9864cf43055ae93beb0afd6c7d144bfa4 (a new file for git blob object)

│   ├── info │   └── pack └── refs ├── heads

    │   └── master (a new file) └── tags 15 directories, 25 files

 

Check the HEAD pointer and branch pointers info.

  

    $ cat .git/HEAD

ref: refs/heads/master

$ cat .git/refs/heads/master

7353d8d4755041cbd9600ce8a1c2f5e7e35646ea

$ git log --oneline --decorate --graph --all

* 7353d8d (HEAD -> master) first commit

Step 2: 

Create a hotfix branch.

(The following command will just create a branch, but the HEAD pointer will still point to the current branch 'master')

  

$ git branch hotfix

Check the .git tree structure.

$ tree .git/

.git/

├── COMMIT_EDITMSG  ├── HEAD ├── branches ├── config ├── description ├── hooks │   ├── applypatch-msg.sample │   ├── commit-msg.sample │   ├── fsmonitor-watchman.sample │   ├── post-update.sample │   ├── pre-applypatch.sample │   ├── pre-commit.sample │   ├── pre-merge-commit.sample │   ├── pre-push.sample │   ├── pre-rebase.sample │   ├── pre-receive.sample │   ├── prepare-commit-msg.sample │   ├── push-to-checkout.sample │   └── update.sample

├── index 

├── info │   └── exclude

├── logs

│   ├── HEAD

│   └── refs

│       └── heads

│   ├── hotfix (a new file)

│           └── master

├── objects

│   ├── 2b │   │   └── 297e643c551e76cfa1f93810c50811382f9117 │   ├── 73 │   │   └── 53d8d4755041cbd9600ce8a1c2f5e7e35646ea │   ├── 9d │   │   └── aeafb9864cf43055ae93beb0afd6c7d144bfa4

│   ├── info │   └── pack └── refs ├── heads

    │   ├── hotfix (a new file)

    │   └── master └── tags 15 directories, 27 files

 

Check the HEAD pointer and branch pointers info.

  

    $ cat .git/HEAD

ref: refs/heads/master

$ cat .git/refs/heads/master

7353d8d4755041cbd9600ce8a1c2f5e7e35646ea

$ cat .git/refs/heads/hotfix

7353d8d4755041cbd9600ce8a1c2f5e7e35646ea

$ git log --oneline --decorate --graph --all

* 7353d8d (HEAD -> master, hotfix) first commit

Add a new commit in the current branch 'master'

  

$ echo 'test2' > test2.txt

$ git add .

$ git commit -m 'second commit from master branch'

[master eef89eb] second commit from master branch 1 file changed, 1 insertion(+) create mode 100644 test2.txt

Check the .git tree structure.

$ tree .git/

.git/

├── COMMIT_EDITMSG  ├── HEAD ├── branches ├── config ├── description ├── hooks │   ├── applypatch-msg.sample │   ├── commit-msg.sample │   ├── fsmonitor-watchman.sample │   ├── post-update.sample │   ├── pre-applypatch.sample │   ├── pre-commit.sample │   ├── pre-merge-commit.sample │   ├── pre-push.sample │   ├── pre-rebase.sample │   ├── pre-receive.sample │   ├── prepare-commit-msg.sample │   ├── push-to-checkout.sample │   └── update.sample

├── index 

├── info │   └── exclude

├── logs

│   ├── HEAD

│   └── refs

│       └── heads

│   ├── hotfix

│           └── master

├── objects

│   ├── 18 (a new directory)

│   │   └── 0cf8328022becee9aaa2577a8f84ea2b9f3827 (a new file for git blob object)

│   ├── 2b │   │   └── 297e643c551e76cfa1f93810c50811382f9117 │   ├── 59 (a new directory) │   │   └── 50f5c8ec9efcb96612f89264fd72994badb828 (a new file for git tree object)

│   ├── 73 │   │   └── 53d8d4755041cbd9600ce8a1c2f5e7e35646ea │   ├── 9d │   │   └── aeafb9864cf43055ae93beb0afd6c7d144bfa4

│   ├── ee (a new directory) │   │   └── f89ebbefa9af58ca3f5c7a86a12416bf9c74aa (a new file for git commit object)

│   ├── info │   └── pack └── refs ├── heads

    │   ├── hotfix

    │   └── master └── tags 18 directories, 30 files

 

Check the HEAD pointer and branch pointers info.

  

    $ cat .git/HEAD

ref: refs/heads/master

$ cat .git/refs/heads/master

eef89ebbefa9af58ca3f5c7a86a12416bf9c74aa

$ cat .git/refs/heads/hotfix

7353d8d4755041cbd9600ce8a1c2f5e7e35646ea

$ git log --oneline --decorate --graph --all

* eef89eb (HEAD -> master) second commit from master branch

* 7353d8d (hotfix) first commit

From the result, it is a linear graph.

The current branch will keep moving to point to the latest commit.

Step 3: 

Run the 'git checkout' command to switch branch.

  

$ git checkout hotfix

Switched to branch 'hotfix'

Check the HEAD info.

  

    $ cat .git/HEAD

ref: refs/heads/hotfix

Create a new commit.

  

    $ echo 'test3' > test3.txt

    $ git add .

    $ git commit -m 'fix bug in hotfix branch'

[hotfix b135500] fix bug in hotfix branch 1 file changed, 1 insertion(+) create mode 100644 test3.txt

Check the .git tree structure.

$ tree .git/

.git/

├── COMMIT_EDITMSG  ├── HEAD ├── branches ├── config ├── description ├── hooks │   ├── applypatch-msg.sample │   ├── commit-msg.sample │   ├── fsmonitor-watchman.sample │   ├── post-update.sample │   ├── pre-applypatch.sample │   ├── pre-commit.sample │   ├── pre-merge-commit.sample │   ├── pre-push.sample │   ├── pre-rebase.sample │   ├── pre-receive.sample │   ├── prepare-commit-msg.sample │   ├── push-to-checkout.sample │   └── update.sample

├── index 

├── info │   └── exclude

├── logs

│   ├── HEAD

│   └── refs

│       └── heads

│   ├── hotfix

│           └── master

├── objects

│   ├── 18

│   │   └── 0cf8328022becee9aaa2577a8f84ea2b9f3827

│   ├── 2b │   │   └── 297e643c551e76cfa1f93810c50811382f9117 │   ├── 59 │   │   └── 50f5c8ec9efcb96612f89264fd72994badb828

│   ├── 73 │   │   └── 53d8d4755041cbd9600ce8a1c2f5e7e35646ea │   ├── 8c (a new directory) │   │   └── 281897622f3e3750998f1509b7c441cbb1f3e2 (a new file for git tree object)

│   ├── 9d │   │   └── aeafb9864cf43055ae93beb0afd6c7d144bfa4

│   ├── b1 (a new directory) │   │   └── 35500a8511bfa6f8a75d219a0d7e3951bedf78 (a new file for git commit object) │   ├── df (a new directory) │   │   └── 6b0d2bcc76e6ec0fca20c227104a4f28bac41b (a new file for git blob object)

│   ├── ee │   │   └── f89ebbefa9af58ca3f5c7a86a12416bf9c74aa

│   ├── info │   └── pack └── refs ├── heads

    │   ├── hotfix

    │   └── master └── tags 21 directories, 33 files

 

Check the HEAD pointer and branch pointers info.

  

    $ cat .git/HEAD

ref: refs/heads/hotfix

$ cat .git/refs/heads/master

eef89ebbefa9af58ca3f5c7a86a12416bf9c74aa

$ cat .git/refs/heads/hotfix

b135500a8511bfa6f8a75d219a0d7e3951bedf78

$ git log --oneline --decorate --graph

* b135500 (HEAD -> hotfix) fig bug in hotfix branch | * eef89eb (master) second commit from master branch |/ * 7353d8d first commit

It is a diverge graph, not a linear graph.

NOTE: We can use 'git checkout -b xxx' as a shortcut to create the xxx branch and switch to it directly.