git checkout [commit] [path]

Currently, I am curious to find out what Git will do for us behind the scene if we run the 'git checkout [commit] [path]' 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:

Run the following commands to keep changing a file and make three commits

    $ echo 'v1' > file.txt

$ git add .

$ git commit -m 'v1'

[master (root-commit) 1491536] v1 1 file changed, 1 insertion(+) create mode 100644 file.txt

    $ echo 'v2' > file.txt

$ git add .

$ git commit -m 'v2'

    [master 9a8fa1e] v2

     1 file changed, 1 insertion(+), 1 deletion(-)

    $ echo 'v3' > file.txt

$ git add .

$ git commit -m 'v3'

[master d0769a6] v3 1 file changed, 1 insertion(+), 1 deletion(-)

Check the git status.

    $ git status

On branch master nothing to commit, working tree clean

It is all synced (the three trees below keep the v3 version of file.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

│   └── master ├── objects

│   ├── 14 │   │   └── 91536a26a35b6a0037cfa9ccbac70fecdb77d0 (git commit object for v1) │   ├── 25 │   │   └── 7330bfc3b1cce445c8bf7202d88eca9ffff025 (git tree object for v2) │   ├── 29 │   │   └── ef827e8a45b1039d908884aae4490157bcb2b4 (git blob object for v3) │   ├── 62 │   │   └── 6799f0f85326a8c1fc522db584e86cdfccd51f (git blob object for v1) │   ├── 8c │   │   └── 1384d825dbbe41309b7dc18ee7991a9085c46e (git blob object for v2) │   ├── 9a │   │   └── 8fa1eee2b0d2841ffa1e5515935421c3d419e1 (git commit object for v2) │   ├── a7 │   │   └── 949a02518883287c308f0b47e2a4934b111cc3 (git tree object for v3) │   ├── cc │   │   └── 5fda52eb3ed07cce6357ac11e392cd2dbf6d16 (git tree object for v1) │   ├── d0 │   │   └── 769a6fa5bcf4c05f667eb635dd84c1174be895 (git commit object for v3)

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

│   └── master └── tags 21 directories, 31 files

Check more info.

    $ cat .git/HEAD

ref: refs/heads/master

$ cat .git/refs/heads/master

d0769a6fa5bcf4c05f667eb635dd84c1174be895

$ git ls-files -s

100644 29ef827e8a45b1039d908884aae4490157bcb2b4 0 file.txt (v3 version)

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

* d0769a6 (HEAD -> master) v3 * 9a8fa1e v2 * 1491536 v1

Step 2:

Let's run the following commands to edit the file.txt as the v4 version and update the Index.

    $ echo 'v4' > file.txt

$ git add .

Check the git status.

    $ git status

On branch master Changes to be committed: (use "git restore --staged <file>..." to unstage) modified: file.txt

Now, both the Index and the working directory keep the v4 version of file.txt.

Then run the 'git checkout [commit][path]' command.

    $ git checkout HEAD file.txt

    Updated 1 path from a7949a0

Check the git status.

    $ git status

On branch master nothing to commit, working tree clean

The [path] both in the Index and the working directory got reset to v3 based on the [commit] which is HEAD in this case.

After running the 'git checkout[commit] [path]' command, we revert the commands from

    $ echo 'v4' > file.txt

$ git add .

to

None