Lecture 3: Basic concepts

---

Remark

• You are not intended to memorize any commands or low-level details.
• The goal is to learn the basic concepts:
  • hash sums, blobs, trees, commits, references, branches, ...
• Understanding these concepts helps to understand what the commands actually do!

---

What is Git?

• Git is a distributed VCS:
  • Does not rely on a server-client model.
  • Instead, everyone has a full copy of the entire project (repository).
    • Complete history, metadata, etc.
  • People can work completely independently.
  • An (optional) server is used only to distribute changes.

---

Why use Git?

• It is popular.
  • Many project already use it, people know how to use it, people can tell you how to use it, ...
• Relies on hash sums:
  • Built-in data corruption detection.
  • Built-in security.
• Distributed.
• Fast, simple and flexible.
• Free and open-source.

---

How does Git store the history?

---

What is inside a repository?

$ mkdir repository && cd repository
$ git init
Initialized empty Git repository in .../repository/.git/
$ find

digraph {
  nodesep=0.2
  "repository/" -> ".git/"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/"
  "refs/" -> "tags/"
}

---

Most directories are empty and the files are not that interesting:

$ cat .git/config 
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true
$ cat .git/HEAD 
ref: refs/heads/master
$ cat .git/description 
Unnamed repository; edit this file 'description' to name the
repository.

---

Let's add some content:

$ echo "This file is very interesting" > file.txt
$ git add file.txt
$ git commit -m "This is the first commit"
[master (root-commit) 23b3ed5] This is the first commit
 1 file changed, 1 insertion(+)
 create mode 100644 file.txt
$ find

digraph { 
  nodesep=0.04
  "file.txt" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "logs/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "master" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "index" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "23" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "1a" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "09" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
}

---

Working tree

• Everything inside repository/ is a part of the working tree (or the workspace).
  • .git/ is not included.
  • At the moment, the working tree contains just one file, file.txt.
  • Working tree is just a regular directory.
• The git add and git commit commands tell Git to care about file.txt.
  • More on that later...

---

Objects

• Git stores files etc as objects:
  • Objects are stored under .git/objects/.
• Git uses content-based addressing.
  • A hash sum is computed from the content of the object.
  • The hash "uniquely" identifies the object.
  • Two objects with identical contents have the same hash and are stored only once.

---

• We can compute the hash manually:

$ git hash-object file.txt
09c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a

• We can find the corresponding object:

$ find
...
./.git/objects/09/c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a
...

• We can confirm that two files with identical contents have the same hash:

$ cp file.txt file2.txt 
$ git hash-object file.txt file2.txt
09c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a
09c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a

---

• Note that we do not have to use the entire hash:

git cat-file -p 09c78e6e
This file is very interesting

• We only need to use as many characters as is required to uniquely identify the object.
  • 7-8 is enough in most cases.
  • 12 in larger projects.
• If more characters is required, an error message is printed.

---

• Objects cannot (and should not) be accessed directly:

$ hexdump -C ./.git/objects/09/c78e6e97*
00000000  78 01 4b ca c9 4f 52 30  ....  |x.K..OR06`...,VH|
00000010  cb cc 49 55 00 d2 65 a9  ....  |..IU..e.E...y%.E|
00000020  a9 c5 25 99 79 e9 5c 00  ....  |..%.y.\..I.3|
0000002c

• However, we can observe the type and the content of an object:

$ git cat-file -t 09c78e6e
blob
$ git cat-file -p 09c78e6e
This file is very interesting

---

• It is also important to realize that the object stays even when the file is removed:

$ rm file.txt
$ find
....
./.git/objects/09/c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a
....
$ git cat-file -p 09c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a
This file is very interesting

• We can restore the file from the object:

$ git restore file.txt
$ cat file.txt 
This file is very interesting

---

Let's take a second look at the repository:

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
}

What are these two other objects?

---

Trees

• Let's investigate one of the remaining objects:

$ git cat-file -t 1a098a06
tree
$ git cat-file -p 1a098a06
100644 blob 09c78e6e971ce9e3d69e75b....    file.txt

• We can see that the type of the object is tree:
  • A tree stores pointers to
    • files (blobs) and
    • other trees,
  • Trees are used to represent directory structures.

---

In this case, the tree has one level and one blob:

digraph {
  first_blob [label="blob 09c78e6e...\nThis file is very interesting" shape=box]

  "tree 1a098a06b...\nblob 09c78e6e.... file.txt" -> first_blob
}

---

Let's take a third look at the repository:

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
}

Just one object remains...

---

Commits

• Let's investigate the last object:

$ git cat-file -t 23b3ed5b
commit
$ git cat-file -p 23b3ed5b
tree 1a098a06bf0bcae9695238d9d5cb96345c00cacf
author Mirko Myllykoski <....@gmail.com> 1600867851 +0200
committer Mirko Myllykoski <....@gmail.com> 1600867851 +0200

This is the first commit

• The type of the object is commit. It contains
  • a pointer to a tree,
  • an author and a committer (+time), and
  • a commit message

---

A commit stores the state of the project in a given point of time.

---

In this case, the commit points to a tree that has one level and one blob:

digraph {
  first_blob [label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  file [label="file.txt\nThis file is very interesting" shape=box]

  "commit 23b3ed5b1...\ntree 1a098a06b\nMirko Myll...\nThis is the first commit" -> "tree 1a098a06b...\nblob 09c78e6e.... file.txt" -> first_blob
  
  "metadata" -> "repository/" -> file
}

---

In a more general case, the associated tree can contain several levels and multiple blobs:

digraph {
  file1 [label="file1.txt" shape=box]
  file2 [label="file2.txt" shape=box]
  file3 [label="file3.txt" shape=box]
  file4 [label="file4.txt" shape=box]

  blob1 [label="blob 1" shape=box]
  blob2 [label="blob 2" shape=box]
  blob3 [label="blob 3" shape=box]
  blob4 [label="blob 4" shape=box]

  "commit 1" -> "tree 1"
  "tree 1" -> blob1
  "tree 1" -> blob2
  "tree 1" -> "tree 2"
  "tree 2" -> blob3
  "tree 2" -> blob4
  
  "metadata" -> "repository/"
  "repository/" -> file1
  "repository/" -> file2
  "repository/" -> "directory/"
  "directory/" -> file3
  "directory/" -> file4
}

---

Working with Git

---

Let's see what else we can find...

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
}

---

HEAD and other references

• HEAD points (indirectly) to 23b3ed5b1:

$ cat ./.git/HEAD
ref: refs/heads/master
$ cat .git/refs/heads/master
23b3ed5b16095bb84b18d06734fdd614c8982841

digraph {
  rankdir=LR
  
  "HEAD" [shape=plaintext]
  "master" [shape=plaintext]
  
  first_blob [label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  "HEAD" -> "master" -> "commit\n23b3ed5b1..." -> "tree\n1a098a06b..." -> first_blob
}

---

• HEAD and master are references.
  • A reference points to commits and another reference.
• HEAD determines "most recent" commit.
  • Many commands act on the current HEAD.
  • More on this later
• master is the current branch (more later).

---

• You can create a reference yourself:

$ git tag first
$ find

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  "first" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
  "tags/" -> "first"
}

$ git rev-parse first
23b3ed5b16095bb84b18d06734fdd614c8982841

digraph {
  rankdir=LR
  
  "first" [shape=plaintext]
  
  first_blob [label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  "first" -> "commit\n23b3ed5b1..." -> "tree\n1a098a06b..." -> first_blob
}

---

Index (staging area)

Let's repeat some of the earlier steps:

$ echo "More content" >> file.txt
$ git add file.txt
$ find

digraph { 
  nodesep=0.04
  "file.txt" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "3b" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "23ff0c41..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  "first" [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "3b" -> "23ff0c41..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
  "tags/" -> "first"
}

$ git cat-file -p 3b23ff0c
This file is very interesting
More content

---

• The git add command creates a blob that correspond to the update file.txt file.
  • No other object are created yet.
• The command also adds the file to the index.
• The index will become the next commit.
  • Contains a representation of the tree object.

---

The index is a binary file:

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "3b" [fontsize=9 width=0.03 height=0.02]
  "23ff0c41..." [fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  "first" [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "3b" -> "23ff0c41..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
  "tags/" -> "first"
}

---

We can now turn the index to the next commit:

$ git commit -m "This is the second commit"
[master d3c6c63] This is the second commit
 1 file changed, 1 insertion(+)
$ find

digraph { 
  nodesep=0.04
  "file.txt" [fontsize=9 width=0.03 height=0.02]
  "logs/" [fontsize=9 width=0.03 height=0.02]
  "master" [fontsize=9 width=0.03 height=0.02]
  "COMMIT_EDITMSG" [fontsize=9 width=0.03 height=0.02]
  "index" [fontsize=9 width=0.03 height=0.02]
  "23" [fontsize=9 width=0.03 height=0.02]
  "b3ed5b16..." [fontsize=9 width=0.03 height=0.02]
  "1a" [fontsize=9 width=0.03 height=0.02]
  "098a06bf..." [fontsize=9 width=0.03 height=0.02]
  "09" [fontsize=9 width=0.03 height=0.02]
  "c78e6e97..." [fontsize=9 width=0.03 height=0.02]
  "3b" [fontsize=9 width=0.03 height=0.02]
  "23ff0c41..." [fontsize=9 width=0.03 height=0.02]
  "22/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "b5208beb..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "d3/" [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "c6c635fb..." [fontcolor=red fontsize=9 width=0.03 height=0.02]
  "repository/" [fontsize=9 width=0.03 height=0.02]
  ".git/" [fontsize=9 width=0.03 height=0.02]
  "branches/" [fontsize=9 width=0.03 height=0.02]
  "hooks/" [fontsize=9 width=0.03 height=0.02]
  "info/" [fontsize=9 width=0.03 height=0.02]
  "objects/" [fontsize=9 width=0.03 height=0.02]
  "refs/" [fontsize=9 width=0.03 height=0.02]
  "config" [fontsize=9 width=0.03 height=0.02]
  "description" [fontsize=9 width=0.03 height=0.02]
  "HEAD" [fontsize=9 width=0.03 height=0.02]
  "pack/" [fontsize=9 width=0.03 height=0.02]
  "heads/" [fontsize=9 width=0.03 height=0.02]
  "tags/" [fontsize=9 width=0.03 height=0.02]
  "info/ " [fontsize=9 width=0.03 height=0.02]
  "first" [fontsize=9 width=0.03 height=0.02]
  
  "repository/" -> ".git/"
  "repository/" -> "file.txt"
  ".git/" -> "branches/"
  ".git/" -> "hooks/"
  ".git/" -> "info/ "
  ".git/" -> "logs/"
  ".git/" -> "objects/"
  ".git/" -> "refs/"
  ".git/" -> "COMMIT_EDITMSG"
  ".git/" -> "config"
  ".git/" -> "description"
  ".git/" -> "HEAD"
  ".git/" -> "index"
  "objects/" -> "23" -> "b3ed5b16..."
  "objects/" -> "1a" -> "098a06bf..."
  "objects/" -> "09" -> "c78e6e97..."
  "objects/" -> "3b" -> "23ff0c41..."
  "objects/" -> "22/" -> "b5208beb..."
  "objects/" -> "d3/" -> "c6c635fb..."
  "objects/" -> "info/"
  "objects/" -> "pack/"
  "refs/" -> "heads/" -> "master"
  "refs/" -> "tags/"
  "tags/" -> "first"
}

---

• Just as before, we have a tree object that describes the directory structure:

$ git cat-file -p 22b5208b
100644 blob 3b23ff0c411faf22c9253ed0....    file.txt

• And a commit, that describes the state of the repository:

$ git cat-file -p d3c6c635
tree 22b5208bebacfcf745691f799b08df492b2a7da9
parent 23b3ed5b16095bb84b18d06734fdd614c8982841
author Mirko Myllykoski <mirko...> 1601228824 +0200
committer Mirko Myllykoski <mirko....> 1601228824 +0200

This is the second commit

---

Parent

• The major difference is that the commit contains a pointer to a parent:

parent 23b3ed5b16095bb84b18d06734fdd614c8982841

• The parent pointer points to the previous commit:

digraph {
  rankdir=LR
  
  second_commit [fontsize=10 label="commit d3c6c635...\ntree 22b5208b\nparent 23b3ed5b1\nMirko Myll..\nThis is the second commit"]
  first_commit [fontsize=10 label="commit 23b3ed5b1...\ntree 1a098a06b\nMirko Myll...\nThis is the first commit"]
  
  second_blob [fontsize=10 label="blob 3b23ff0c\nThis file is very interesting\nMore content" shape=box]
  first_blob [fontsize=10 label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  "tree 22b5208b...\nblob 3b23ff0c file.txt" [fontsize=10]
  "tree 1a098a06b...\nblob 09c78e6e.... file.txt" [fontsize=10]
  
  second_commit -> "tree 22b5208b...\nblob 3b23ff0c file.txt" -> second_blob

  first_commit -> "tree 1a098a06b...\nblob 09c78e6e.... file.txt" -> first_blob
  
  second_commit -> first_commit
}

---

Commit tree

• Usually, we have a complete tree of commits (commit tree):

digraph {
  rankdir=LR
  "commit 1" -> "tree 1"
  "commit 2" -> "tree 2"
  "commit 2" -> "commit 1"
  "commit 3" -> "tree 3"
  "commit 3" -> "commit 2"
  "commit 4" -> "tree 4"
  "commit 4" -> "commit 3"
}

• Each commit represents the state of the repository at a given point of time.

---

• Each commit is allowed to have multiple parents:

digraph {
  rankdir=LR
  "commit 2" -> "commit 1"
  "commit 4" -> "commit 3"
  "commit 4" -> "commit 2"
}

• These parents appear when two (or more) branches are merged.
  • More on this later...

---

HEAD and other references (again)

• Let's investigate HEAD and master:

$ cat .git/HEAD 
ref: refs/heads/master
$ cat .git/refs/heads/master
d3c6c635fb44c7084797d47050bff7961853c19b

digraph {
  rankdir=LR
  
  "HEAD" [fontsize=12 shape=plaintext]
  "master" [fontsize=12 shape=plaintext]
  
  second_commit [fontsize=12 label="commit d3c6c635...\ntree 22b5208b\nparent 23b3ed5b1\nMirko Myll..\nThis is the second commit"]

  first_commit [fontsize=12 label="commit 23b3ed5b1...\ntree 1a098a06b\nMirko Myll...\nThis is the first commit"]
  
  second_commit -> first_commit
  
  "HEAD" -> "master" -> second_commit
  
  subgraph cluster_working_tree {
    label="Working tree"
    subgraph cluster_file {
      label="file.txt"
        "This file is very interesting\nMore content
" [fontsize=12 shape=plain]
    }
  }
}

• Remember, many Git commands act on the current HEAD.

---

• We can change the HEAD to something else:

$ git checkout 23b3ed5b
....
HEAD is now at 23b3ed5 This is the first commit
$ cat .git/HEAD 
23b3ed5b16095bb84b18d06734fdd614c8982841
$ cat file.txt 
This file is very interesting

digraph {
  rankdir=LR
  
  "HEAD" [shape=plaintext]
  "master" [shape=plaintext]
  
  second_commit [label="commit d3c6c635...\ntree 22b5208b\nparent 23b3ed5b1\nMirko Myll..\nThis is the second commit"]

  first_commit [label="commit 23b3ed5b1...\ntree 1a098a06b\nMirko Myll...\nThis is the first commit"]
  
  second_commit -> first_commit
  
  "HEAD" -> first_commit 
  "master" -> second_commit
  
  subgraph cluster_working_tree {
    label="Working tree"
    subgraph cluster_file {
      label="file.txt"
        "This file is very interesting
" [shape=plain]
    }
  }
}

---

Branches

• We can modify the working tree and create a new commit:

$ echo "Different content" >> file.txt 
$ git commit -a -m "This is the third commit"
[detached HEAD a118ae8] This is the third commit
 1 file changed, 1 insertion(+)

• Let's investigate the newly created commit:

$ git cat-file -p a118ae8c
tree 5fcc4f83fedf5a94cd773704bdb1ab2cdcadc6fd
parent 23b3ed5b16095bb84b18d06734fdd614c8982841
author Mirko Myllykoski <mirko....> 1601286412 +0200
committer Mirko Myllykoski <mirko....> 1601286412 +0200

This is the third commit

---

• First, the parent points to the first commit:

digraph {
  rankdir=LR
  
  third_commit [label="commit a118ae8c...\nparent 23b3ed5b1...\nThis is the third commit"]
  first_commit [label="commit 23b3ed5b1...\nThis is the first commit"]
  
  third_commit -> first_commit
}

---

• Second, the commit tree now has two branches:

digraph {
  rankdir=LR
  nodesep=0.2
  
  "HEAD" [fontsize=10 shape=plaintext]
  "master" [fontsize=10 shape=plaintext]
  
  third_commit [fontsize=10 label="commit a118ae8c...\nThis is the third commit"]
  second_commit [fontsize=10 label="commit d3c6c635...\nThis is the second commit"]
  first_commit [fontsize=10 label="commit 23b3ed5b1...\nThis is the first commit"]
  
  third_blob [fontsize=10 label="blob ea5f4b8e\nThis file is very interesting\nDifferent content" shape=box]
  second_blob [fontsize=10 label="blob 3b23ff0c\nThis file is very interesting\nMore content" shape=box]
  first_blob [fontsize=10 label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  third_commit -> third_blob [style=dashed]
  second_commit -> second_blob [style=dashed]
  first_commit -> first_blob [style=dashed]
  
  third_commit -> first_commit
  second_commit -> first_commit
  
  "HEAD" -> third_commit 
  "master" -> second_commit
  
  subgraph cluster_working_tree {
    nodesep=0.2
    fontsize=10
    label="Working tree"
    subgraph cluster_file {
      fontsize=10
      label="file.txt"
        "This file is very interesting\nDifferent content
" [fontsize=10 shape=plain]
    }
  }
}

---

We can give the second branch a name:

$ git checkout -b second_branch
Switched to a new branch 'second_branch'
$ cat .git/HEAD 
ref: refs/heads/second_branch
$ cat .git/refs/heads/second_branch
a118ae8cda10a8f0a966ab7b9158b4a6d3b48cfc

digraph {
  rankdir=LR
  nodesep=0.2
  
  "HEAD" [fontsize=10 shape=plaintext]
  "master" [fontsize=10 shape=plaintext]
  "second_branch" [fontsize=10 shape=plaintext]
  
  third_commit [fontsize=10 label="commit a118ae8c...\nThis is the third commit"]
  second_commit [fontsize=10 label="commit d3c6c635...\nThis is the second commit"]
  first_commit [fontsize=10 label="commit 23b3ed5b1...\nThis is the first commit"]
  
  third_blob [fontsize=10 label="blob ea5f4b8e\nThis file is very interesting\nDifferent content" shape=box]
  second_blob [fontsize=10 label="blob 3b23ff0c\nThis file is very interesting\nMore content" shape=box]
  first_blob [fontsize=10 label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  third_commit -> third_blob [style=dashed]
  second_commit -> second_blob [style=dashed]
  first_commit -> first_blob [style=dashed]
  
  third_commit -> first_commit
  second_commit -> first_commit
  
  "HEAD" -> third_commit
  "master" -> second_commit
  "second_branch" -> third_commit
  
  subgraph cluster_working_tree {
    fontsize=10
    label="Working tree"
    subgraph cluster_file {
      fontsize=10
      label="file.txt"
        "This file is very interesting\nDifferent content
" [fontsize=10 shape=plain]
    }
  }
}

---

Merging

We can merge the two branches together:

$ git checkout master
$ git merge --no-ff second_branch
Auto-merging file.txt
CONFLICT (content): Merge conflict in file.txt
Automatic merge failed; fix conflicts and then commit the
result.
$ vim file.txt

We fix some conflicts at this point...

$ git add file.txt
$ git merge --continue
[master f0d7298] Merge branch 'second_branch'

---

The created commit has two parents:

$ git cat-file -p f0d72989
tree f63f3a4c548f5065cee598bed4ae189bd2c099d8
parent d3c6c635fb44c7084797d47050bff7961853c19b
parent a118ae8cda10a8f0a966ab7b9158b4a6d3b48cfc
author Mirko Myllykoski <mirko....> 1601288485 +0200
committer Mirko Myllykoski <mirko....> 1601288485 +0200

Merge branch 'second_branch'

---

Finally, the tree looks like follows:

digraph {
rankdir=LR
nodesep=0.03
    subgraph cluster_working_tree {
        label="Working tree"
        nodesep=0.1
        fontsize=9 width=0.03 height=0.02
        subgraph cluster_file {
           fontsize=9 width=0.03 height=0.02
           label="file.txt"
        "This file is very interesting\nMore content\nDifferent content" [fontsize=9 width=0.03 height=0.02 shape=plain]
    }
  }
  }

digraph {
  rankdir=LR
  nodesep=0.03
  
  "HEAD" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  "master" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  "second_branch" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  
  fourth_commit [fontsize=8 width=0.03 height=0.02 label="commit f0d72989...\nMerge branch 'second_branch'"]
  third_commit [fontsize=8 width=0.03 height=0.02 label="commit a118ae8c...\nThis is the third commit"]
  second_commit [fontsize=8 width=0.03 height=0.02 label="commit d3c6c635...\nThis is the second commit"]
  first_commit [fontsize=8 width=0.03 height=0.02 label="commit 23b3ed5b1...\nThis is the first commit"]
  
  fourth_blob [fontsize=8 width=0.03 height=0.02 label="blob e51364b9\nThis file is very interesting\nMore content\nDifferent content" shape=box]
  third_blob [fontsize=8 width=0.03 height=0.02 label="blob ea5f4b8e\nThis file is very interesting\nDifferent content" shape=box]
  second_blob [fontsize=8 width=0.03 height=0.02 label="blob 3b23ff0c\nThis file is very interesting\nMore content" shape=box]
  first_blob [fontsize=8 width=0.03 height=0.02 label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  fourth_commit -> fourth_blob [fontsize=8 width=0.03 height=0.02 style=dashed]
  third_commit -> third_blob [fontsize=8 width=0.03 height=0.02 style=dashed]
  second_commit -> second_blob [fontsize=8 width=0.03 height=0.02 style=dashed]
  first_commit -> first_blob [fontsize=8 width=0.03 height=0.02 style=dashed]
  
  fourth_commit -> second_commit
  fourth_commit -> third_commit
  third_commit -> first_commit
  second_commit -> first_commit
  
  "HEAD" -> fourth_commit
  "master" -> fourth_commit
  "second_branch" -> third_commit
  
}

---

Switching to a specific commit

We can always move back to any of the previous commits:

$ git checkout 23b3ed5b1
....
HEAD is now at 23b3ed5 This is the first commit
$ cat file.txt 
This file is very interesting

digraph {
rankdir=LR
nodesep=0.03
    subgraph cluster_working_tree {
    label="Working tree"
    fontsize=8 width=0.03 height=0.02 
    subgraph cluster_file {
      label="file.txt"
      fontsize=8 width=0.03 height=0.02 
        "This file is very interesting" [fontsize=8 width=0.03 height=0.02 shape=plain]
    }
  }
  }

digraph {
  rankdir=LR
  nodesep=0.2
  
  "HEAD" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  "master" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  "second_branch" [fontsize=8 width=0.03 height=0.02 shape=plaintext]
  
  fourth_commit [fontsize=8 width=0.03 height=0.02 label="commit f0d72989...\nMerge branch 'second_branch'"]
  third_commit [fontsize=8 width=0.03 height=0.02 label="commit a118ae8c...\nThis is the third commit"]
  second_commit [fontsize=8 width=0.03 height=0.02 label="commit d3c6c635...\nThis is the second commit"]
  first_commit [fontsize=8 width=0.03 height=0.02 label="commit 23b3ed5b1...\nThis is the first commit"]
  
  fourth_blob [fontsize=8 width=0.03 height=0.02 label="blob e51364b9\nThis file is very interesting\nMore content\nDifferent content" shape=box]
  third_blob [fontsize=8 width=0.03 height=0.02 label="blob ea5f4b8e\nThis file is very interesting\nDifferent content" shape=box]
  second_blob [fontsize=8 width=0.03 height=0.02 label="blob 3b23ff0c\nThis file is very interesting\nMore content" shape=box]
  first_blob [fontsize=8 width=0.03 height=0.02 label="blob 09c78e6e...\nThis file is very interesting" shape=box]
  
  fourth_commit -> fourth_blob [style=dashed]
  third_commit -> third_blob [style=dashed]
  second_commit -> second_blob [style=dashed]
  first_commit -> first_blob [style=dashed]
  
  fourth_commit -> second_commit
  fourth_commit -> third_commit
  third_commit -> first_commit
  second_commit -> first_commit
  
  "HEAD" -> first_commit
  "master" -> fourth_commit
  "second_branch" -> third_commit
  
}

---

The end.

An idea: Try to play with different commands. See what happens to the .git/ directory.