# How to make a commit with plumbing commands

### 1. Setup Git repo
- Make empty folder called `git-plumbing`. Git is not initialised!
- Make `.git` directory where all the Git system will reside. Git is not initialised!
- Git repository is a collection of object and a system for naming those objects (_refs_). Let's make `objects` and `refs` subfolders.
- Let's make `.git/refs/heads` which keep track of branches. Git is not initialised!
- The last missing piece to initialise a Gitrepo is a `HEAD` file with content:
```
ref: refs/heads/master
```

### 2. Hash data into a blob
- Now we can create a hash of some data and generate a loose object (**blob**) out of it:
```bash
$ echo "In Xanadu did Kubla Kahn" | git hash-object --stdin -w
bf8febed0fafd7638ad2799f5925a9383039f58e
```

- I can see the actual content of the blob with:
```bash
$ git cat-file -p bf8feb # -p is for pretty-print
In Xanadu did Kubla Kahn
```

- I can find out what kind of Git object is with:
```bash
$ git cat-file -t bf8feb # -t is for type
blob
```

- I can find out the size of Git object with:
```bash
$ git cat-file -s bf8feb # -s is for size in bytes
25
```

- Git status doesn't detect anything yet

### 3. Growing a tree
- Let's put the object in the staging area (index). 100644 is a magic number for permissions and I also have to specify to which file the content belongs to.
```bash
$ git update-index --add --cacheinfo 100644 bf8febed0fafd7638ad2799f5925a9383039f58e coleridge.txt
```
- We need to represent a directory structure. Git doesn't care about single files. We need a tree:
```bash
$ git write-tree
de31c78155075dcebcc969331b213d5ed81c6636

$ git cat-file -t de31c7
tree

$ git cat-file -p de31c7 # printing content of tree
100644 blob bf8febed0fafd7638ad2799f5925a9383039f58e    coleridge.txt
```

### 4. The first commit
- Git status doesn't care at all about my tree. I need to make a commit. The commit is the snapshot:
```bash
$ git commit-tree de31c7 -m "Initial commit" f915575e03beebcef20778b99ae7aba5d00c0e9a # this is the commit hash

$ git cat-file -p f91557 # equivalent to git log --format=raw
tree de31c78155075dcebcc969331b213d5ed81c6636
author Simone Stefani <s.stefani95@gmail.com> 1518941735 +0100
committer Simone Stefani <s.stefani95@gmail.com> 1518941735 +0100

Initial commit
```

- Git status is still not happy since the `HEAD` ref has not been adjusted. The ref folder doesn't have info about our commit. Let's add it:
```bash
$ echo f915575e03beebcef20778b99ae7aba5d00c0e9a > .git/refs/heads/master

$ git log
commit f915575e03beebcef20778b99ae7aba5d00c0e9a (HEAD -> master)
Author: Simone Stefani <s.stefani95@gmail.com>
Date:   Sun Feb 18 09:15:35 2018 +0100

    Initial commit
```

- Let's checkout the file. The `--` is a way for Git to apply the first part of the command only to some objects. We simply ask Git to take out the file `coleridge.txt` and put it into the working directory.
```bash
$ git checkout -- coleridge.txt
```
- Now status is happy and we can see the file in the folder