Lecture 3: Basic concepts

--- title: "Lecture 3: Basic concepts" tags: Lecture, Birgitte, day 2 description: "What are the basic concepts of Git?" --- Introduction to Git --- Fall 2023 # Lecture 3: Basic concepts   ![TOC](https://www.hpc2n.umu.se/sites/default/files/umu-logo-left-se.png =200x) ![](https://www.hpc2n.umu.se/sites/default/files/hpc2n-logo-text5.png =200x) ![](https://www.hpc2n.umu.se/sites/default/files/conferences-courses/2023/Uppsala_Universitet-logo-2E2D20E6B3-seeklogo.com.png =100x) ![](https://www.hpc2n.umu.se/sites/default/files/conferences-courses/2023/naiss-narrow.jpg =200x) <small>Slides: https://hackmd.io/@git-fall-2023/L3-concepts#/</small> --- ## 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? ```shell $ mkdir repository && cd repository $ git init Initialized empty Git repository in .../repository/.git/ $ find ``` ```graphviz 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: ```shell $ 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: ```shell $ 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 ``` ```graphviz 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: ```shell $ git hash-object file.txt 09c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a ``` - We can find the corresponding object:  ```shell $ find ... ./.git/objects/09/c78e6e971ce9e3d69e75bcb3ffd5de05b0d59a ... ```  - We can confirm that two files with identical contents have the same hash:  ```shell $ 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: ```shell 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: ```shell $ 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:  ```shell $ 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: ```shell $ 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:  ```shell $ git restore file.txt $ cat file.txt This file is very interesting ```  --- Let's take a second look at the repository: ```graphviz 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: ```shell $ 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: ```graphviz 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: ```graphviz 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: ```shell $ 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: ```graphviz 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: ```graphviz 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... ```graphviz 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`: ```shell $ cat ./.git/HEAD ref: refs/heads/master $ cat .git/refs/heads/master 23b3ed5b16095bb84b18d06734fdd614c8982841 ``` ```graphviz 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: ```shell $ git tag first $ find ``` ```graphviz 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" } ```  ```shell $ git rev-parse first 23b3ed5b16095bb84b18d06734fdd614c8982841 ```  ```graphviz 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: ```shell $ echo "More content" >> file.txt $ git add file.txt $ find ```  ```graphviz 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" } ```  ```shell $ 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: ```graphviz 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: ```shell $ git commit -m "This is the second commit" [master d3c6c63] This is the second commit 1 file changed, 1 insertion(+) $ find ``` ```graphviz 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: ```shell $ git cat-file -p 22b5208b 100644 blob 3b23ff0c411faf22c9253ed0.... file.txt ``` - And a commit, that describes the state of the repository:  ```shell $ 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:  ```graphviz 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**): ```graphviz 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: ```graphviz 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`: ```shell $ cat .git/HEAD ref: refs/heads/master $ cat .git/refs/heads/master d3c6c635fb44c7084797d47050bff7961853c19b ``` ```graphviz 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: ```shell $ 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 ``` ```graphviz 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: ```shell $ 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:  ```shell $ 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**: ```graphviz 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: ```graphviz 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**: ```shell $ 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 ``` ```graphviz 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: ```shell $ 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... ```shell $ git add file.txt $ git merge --continue [master f0d7298] Merge branch 'second_branch' ``` --- The created commit has **two** parents: ```shell $ 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: ```graphviz 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] } } } ``` ```graphviz 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: ```shell $ git checkout 23b3ed5b1 .... HEAD is now at 23b3ed5 This is the first commit $ cat file.txt This file is very interesting ``` ```graphviz 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] } } } ``` ```graphviz 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.

Syntax	Example	Reference
# Header	Header	基本排版
- Unordered List	Unordered List
1. Ordered List	Ordered List
- [ ] Todo List	Todo List
> Blockquote	Blockquote
Bold font	Bold font
Italics font	Italics font
~~Strikethrough~~	~~Strikethrough~~
19^th^	19^th
H~2~O	H₂O
++Inserted text++	Inserted text
==Marked text==	Marked text
[link text](https:// "title")	Link
![image alt](https:// "title")	Image
`Code`	`Code`	在筆記中貼入程式碼
```javascript var i = 0; ```	`var i = 0;`
:smile:		Emoji list
{%youtube youtube_id %}	Externals
$L^aT_eX$	L^aT_eX
:::info This is a alert area. :::	This is a alert area.