### Git

* [Pro Git](https://git-scm.com/book/en/v2)
* [MIT 6.NULL - git](https://missing.csail.mit.edu/2020/version-control/)
* [Learn Git Branching](https://learngitbranching.js.org/?locale=en_US)


#### VCS(version control system)
##### Git的理念
* distributed version control system (DVCS) 
  * VCS: 简单的管理，例如RCS
  * CVCS(centralized VCS)
  * DVCS: 每个clone都是一个backup，能处理不同的remote
* 操作模式
  *  本地操作、集中下载和上传
  *  The  Platonic  ideal  is  that  each  commit should compile and should move steadily towards more and more tests passing. 
*  ugly interface and beautiful design -> bottom-up地理解git

This [XKCD comic](https://xkcd.com/1597/) captures Git’s reputation:

<img src="git/git.png" alt="git" style="zoom:100%;" />

##### Git's data model
* Snapshots: 文件是blob，文件夹是tree，snapshot是top-level tree 
* Modeling history: relating snapshots
  * a history is a directed acyclic graph (DAG) of snapshots
  * 一个snapshot可能有多个parent，比如merge
  * snapshot被称作commit，是Git的精髓所在，不像其它VCS针对每个文件储存changes
* Data model, as pseudocode
* Objects and content-addressing 
  *  id由SHA-1 hash生成，40个十六进制字符
  * `git cat-file -p` 显示对象信息
* References：照顾可读性，和对象不同，它是mutable的
  * `master`表示主分支的最近commit
  * `HEAD`表示“where we currently are”
* repositories = objects + references 
```python
// a file is a bunch of bytes
type blob = array<byte>

// a directory contains named files and directories
type tree = map<string, tree | blob>

// a commit has parents, metadata, and the top-level tree
type commit = struct {
	parent: array<commit>
	author: string
	message: string
	snapshot: tree
}

type object = blob | tree | commit
objects = map<string, object>
def store(object):
    id = sha1(object) 
    objects[id] = object
def load(id):
    return objects[id]

references = map<string, string>
def update_reference(name, id):
    references[name] = id
def read_reference(name):
    return references[name]
def load_reference(name_or_id):
    if name_or_id in references:
        return load(references[name_or_id])
    else:
        return load(name_or_id)

```
##### Staging area
<img src="git/areas.png" alt="areas" style="zoom:80%;" />

* staging area也称作index
* 和`git add`相关
  * Git tracks changes to a developer’s codebase, but it’s necessary to stage and take a snapshot of the changes to include them  in the project’s history. `git add` performs staging, the first part  of that two-step process. Any changes that are staged will become a part of the next snapshot and a part of the project’s history. Staging and  committing separately gives developers complete control over the history of their project without changing how they code and work. 
* 意义在于摆脱snapshot和当前状态的绝对联系，使commit操作更灵活
* checkout在working directory之间切换；reset从staging area回复到working directory



#### Git command-line interface

##### Basics

- `git help <command>`: get help for a git command
  - `git command -h`: concise help 
  - [freenode](https://freenode.net/) 的`#git`和`#github`频道寻求帮助
- `git init`: creates a new git repo, with data stored in the `.git` directory
- `git status`: tells you what's going on
  - `git status -s`: 左列是staging area，右列是working tree 
- `git add <filename>`: adds files to staging area
  - 对于同一文件，add之后有新改动要重新add
  - `-p` 添加每个变化前，都会要求确认。对于同一个文件的多处变化，可以实现分次提交
- `git commit`: creates a new commit
  - Write [good commit messages](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html) and Even more reasons to write [good commit messages](https://chris.beams.io/posts/git-commit/): 大写开头，祈使句，简短
  - saves the snapshot to the project history and completes the  change-tracking process. In short, a commit functions like taking a photo. Anything that’s been staged with `git add` will become a part of the snapshot with `git commit`.
  - `git commit -am "m"`可以先add再commit，**commit的文件都是tracked状态**
  - `-v`显示diff信息
- `git log`: shows a flattened log of history
  - `git log --pretty=format:"%h %s" --all --graph --decorate --no-merges`: visualizes history as a DAG
  - `git shortlog/ git log --oneline`: 只显示标题
  - `-p`显示全部信息，`-3`显示三条
  - `--stat`显示统计信息
  - `-S function_name --since=2.weeks --before="2008-11-01" --grep --author --committer --no-merges`
  - `--all`显示所有branch信息, 或者指定特定的branch
- `git diff`: 比较working directory和staging area
  - `git diff --staged'：比较staging area和last commit
  - `git diff <filename>`: show differences since the last commit
  - `git diff <revision> <filename>`: shows differences in a file between snapshots
  - `git difftool`，图形界面
  - Diff without dots / with two dots / with three dots: https://matthew-brett.github.io/pydagogue/git_diff_dots.html
  - `git diff > tmp.diff` 编辑器可显示syntax
- `git checkout <revision>`: updates HEAD and current branch
  - `gco <revision> $file_path ` 用于回退特定文件
- `git revert`
  - `-m 1或2` 对于有多个上游commit的commit，用来指定具体revert到哪一个commit
- `git rm file`
  - `git rm --cached`，只删除index，不删除working tree
  - `git rm log/\*.log` ，通配符，注意要加`\`，Git有自己的文件名拓展
- `git tag -l "v1.8.5*"`: `-l`是为了通配符匹配
  - Annotated Tags: `git tag -a v1.4 (<commit>) -m "my version 1.4"`
  - Lightweight Tags: `git tag v1.4-lw`
  - `git push origin <tag>/--tags`，需要单独push, `--follow-tags`只push annotated tags
```shell
git tag -d v1.4-lw
git push origin :refs/tags/v1.4-lw
git push origin --delete <tagname>

```
- `git show <commit>/<tag>`
* `git mv file_from file_to`
```shell
mv README.md README
git rm README.md
git add README
```

##### Branching and merging

思考git的原理，commit的组成

branch的使用方式：1.Long-Running Branches 2.Topic Branches

branch的成本极低，instantaneous

- `git branch`: shows branches
  - `-v -r` verbose，显示远程分支, `-a`显示所有本地和远程
  - `--merged` 和`--no-merged`
- `git branch <name>`: creates a branch
  - `git branch <name> <commit>` 
  - `git branch -d` 删除branch 
- `git checkout -b <name>`: creates a branch and switches to it 
  - same as `git branch <name>; git checkout <name>`
  - `git checkout --` 切换到上一个分支
  - checkout需要所有changes已经commit，或者参考[Stashing and Cleaning](https://git-scm.com/book/en/v2/ch00/_git_stashing)
  - `git checkout <tag>`会进入detached HEAD状态，做的commit只属于这一个commit
- `git checkout large-mr-branch -- path/to/your/file.ext`
  - 拆MR

- `git checkout --track origin/serverfix`: 创建本地分支并track远程分支 
- `git merge <revision>`: merges into current branch, merge的不同策略如下：
  * Fast-forward，直接在当前commit上遍历即可
  * recursive strategy: three-way merge
  * 如果发生冲突，手动修改unmerged files然后add ，或者用mergetool
- `git mergetool`: use a fancy tool to help resolve merge conflicts
- `git rebase`: rebase set of patches onto a new base
  - `git rebase <basebranch> <topicbranch>`
  rebase命令找到共同的祖先节点，然后按顺序replay commits

```shell
git checkout experiment
git rebase master   # <=> git rebase master experiment
git checkout master
git merge experiment
```

`git rebase --onto master server client`

This basically says, “Take the `client` branch, figure out the patches since it diverged from the `server` branch, and replay these patches in the `client` branch as if it was based directly off the `master` branch instead.” It’s a bit complex, but the result is pretty cool.

Do not rebase commits that exist outside your repository and that people may have based work on. 会发生混淆，尤其是别人能重新把已经被rebase消除掉的commits push上来的时候，可能会fetch到存在冲突的experiment分支
* 如果遇到了这种问题，可以`git fetch & git rebase experiment`或直接`git pull --rebase`

总结：`rebase`和`filter-branch`本质上是在tell storys，呈现给读者可读性更强的历史记录



##### Remotes

tracking branch的缩写：`@{u}或@{upstream}`

- `git remote`: list remotes
  - `-v` 
  
- `git remote add <name> <url>`: add a remote
  - name本质上是reference 
  
- `git remote show/rename/rm <remote>`
  - `git ls-remote <remote>` 
  
- 如何更新fork仓库的代码
  
  - ```shell
    git remote -v
    git remote add upstream git@github.com:xxx/yyy.git
    git fetch upstream
    git merge upstream/master
    ```
  
- `git push <remote> <local branch>:<remote branch>`: send objects to remote, and update remote reference
  * `git push origin lab1:lab1`
  * `git push --set-upstream origin my-branch`，本地关联远程分支，用来省略上面一行的分支标注
  * `git push origin --delete my-branch` 删除远程分支

- `git branch --set-upstream-to=<remote>/<remote branch>`: set up correspondence between local and remote branch
  - 相当于 `-u`

- `git fetch <remote>`: retrieve objects/references from a remote
  - `git fetch origin` 
```shell
git fetch origin master:tmp
git diff tmp
git merge tmp
git branch -d tmp
```
```shell
git fetch --all
git branch -vv
git push origin --delete serverfix
```

- `git pull`: same as `git fetch; git merge` 

  `git pull --rebase`  and `git rebase --continue` [实用的冲突处理策略](https://juejin.im/post/5d3685146fb9a07ed064f11b)

- `git clone`: download repository from remote
  - 在最后可加文件夹名参数 
  - `-o` 修改origin name

fetch只能得到远程新分支的引用，如果想得到实体：
* `git merge origin/serverfix`
* `git checkout -b serverfix origin/serverfix`

##### Undo
- `git commit --amend`: edit a commit's contents/message, 可把staging area加到上一次commit里
  - 可指定文件
- `git reset HEAD <file>`: unstage a file 
  * `git reset HEAD~ `或 `git reset HEAD~1` 撤销上次的commit（会回复到modified状态） 
  * `git reset --hard`
    *  回到上次commit的版本，配合`git pull/push`（如果file是working directory内的，会很危险）
  * [Github如何回退敏感信息](https://help.github.com/en/github/authenticating-to-github/removing-sensitive-data-from-a-repository) 
```shell
# 切到被别人push --force过的分支版本
git fetch --all
git reset --hard origin/dev

# 回退remote敏感信息
git log
git reset --hard XXXXXXXX
git push origin HEAD --force 
```
- `git checkout -- <file>`: discard changes （很危险的指令）
- `git clean -fd` 删掉unstaged文件

##### Advanced Git

* `git diff > patch && git apply patch`

- `git cat-file -p`: 显示对象信息
  * 40位Hash值，前2位是文件夹，后38位是文件名
  * 存在`.git/objects/`中 
  * [理解git常用命令原理](http://www.cppblog.com/kevinlynx/archive/2014/09/09/208257.html)
- `git config`: Git is [highly customizable](https://git-scm.com/docs/git-config)
  - `/etc/gitconfig`对应`--system` 
  - `~/.gitconfig`或`~/.config/git/config`对应 `--global`
  - `path/.git/config`对应`--local`
  - `git config --list (--show-origin)`显示所有config
  - 设置Identity，见下面Github部分
- `git clone --depth=1`: shallow clone, without entire version history
- `git add -p`: interactive staging
- `git rebase -i`: interactive rebasing
- `git blame`: show who last edited which line
  - `git blame -L :collection _config.yml`
- `git stash`: temporarily remove modifications to working directory
	- `git stash pop [--index][stash@{id}]`
	    - `git stash pop` 恢复最新的进度到工作区
	    - `git stash pop --index` 恢复最新的进度到工作区和暂存区
	    - `git stash pop stash@{1}` 恢复指定的进度到工作区。stash_id是通过git stash list命令得到的。通过git stash pop命令恢复进度后，会删除当前进度
	
	- `git stash save --include-untracked`
	  
	- `git stash apply stash@{n}`
	  
	- `git stash list`
	  
	- `git stash show -p | git apply -R`
	
- `git cherry-pick <commit>`: 将指定的提交（的变更）应用到当前分支。
  - [利用它只pull request一个特定的commit](https://www.iteye.com/blog/bucketli-2442195)
  - `git cherry-pick commit1..commit2`：应用从 commit1 (不含) 到 commit2 (含) 的所有提交。
  - **处理合并提交 (Merge Commit)**: 直接 `cherry-pick` 一个合并提交会失败，因为 Git 不知道应该采用哪个父分支的变更。
    - **解决方案**: 使用 `-m` (mainline) 选项指定“主线”。
    1.  **查看父提交**: `git show <merge-commit-hash>`，输出中会包含 `Merge: <parent-1-hash> <parent-2-hash>`。
    2.  **确定主线**: 父提交1通常是**被合并的目标分支** (如 `master`)，父提交2是**来源分支**。我们通常想应用来源分支的变更。
    3.  **执行命令**: `git cherry-pick -m 1 <merge-commit-hash>`。此命令意为“将该合并提交与它的第一个父提交进行比较，然后将差异应用到当前分支”。
  - **注意**: 操作前最好确保不在“分离头指针”(`detached HEAD`)状态，可以先 `git checkout -b <new-branch-name>` 创建一个新分支，否则 `cherry-pick` 产生的新提交可能会丢失。
- `git bisect`: binary search history (e.g. for regressions)
- [`git ls-files`](https://git-scm.com/docs/git-ls-files)
- `git submodule add <url> /path`: 添加子模块。
- `git submodule update`: 更新子模块，使其与主项目的记录匹配。
    * `--init`: 初始化尚未克隆的子模块。
    * `--recursive`: 递归更新所有嵌套的子模块。
    * **常用组合**: `git submodule update --init --recursive` 是克隆一个带子模块的仓库后，最常用的初始化命令。
    * `--remote`: **拉取最新代码**。此选项会忽略主项目记录的 commit SHA-1，转而检出子模块远程跟踪分支的最新 commit。这对于始终希望使用子模块最新版本的场景非常有用。
    * `--force`: **强制覆盖**。此选项会强制检出子模块，丢弃所有本地的修改。这在需要将子模块重置为干净状态时非常有用，但操作具有破坏性，需谨慎使用。例如：`git submodule update --force --recursive`。
- `git submodule deinit -f .`: 反初始化子模块。
- **其他技巧与注意事项**:
    * 移除submodule：https://stackoverflow.com/questions/1260748/how-do-i-remove-a-submodule/36593218#36593218
    * 如果报错 `already exists in the index`，用 `git rm -r --cached /path` 解决此问题。
    * `submodule` 很适合和[dotfiles](https://github.com/huangrt01/dotfiles)搭配，但如果用在项目里可能[出现问题](https://codingkilledthecat.wordpress.com/2012/04/28/why-your-company-shouldnt-use-git-submodules/)，尤其是需要 commit 模块代码的时候。
    * [使用时可能遇到的坑的集合](https://blog.csdn.net/a13271785989/article/details/42777793)
    * commit 时有坑，需要先 commit 子模块，再 commit 主体，参考：https://stackoverflow.com/questions/8488887/git-error-changes-not-staged-for-commit
- `.gitignore`: [specify](https://git-scm.com/docs/gitignore) intentionally untracked files to ignore
  - `.gitignore_global`，[我的设定](https://github.com/huangrt01/dotfiles/blob/master/gitignore_global)

```
# ignore all .a files
*.a

# but do track lib.a, even though you're ignoring .a files above
!lib.a

# only ignore the TODO file in the current directory, not subdir/TODO
# /TODO

# ignore all files in any directory named build
# build/

# ignore doc/notes.txt, but not doc/server/arch.txt
# doc/*.txt

# ignore all .pdf files in the doc/ directory and any of its subdirectories
# doc/**/*.pdf
```

- Git and GPG: my security [notes](https://github.com/huangrt01/CS-Notes/blob/master/Notes/Output/Security-Privacy-Cryptography.md)

#### Miscellaneous
- **GUIs**: there are many [GUI clients](https://git-scm.com/downloads/guis)
out there for Git. We personally don't use them and use the command-line
interface instead.
- **Shell integration**: it's super handy to have a Git status as part of your
shell prompt ([zsh](https://github.com/olivierverdier/zsh-git-prompt),
[bash](https://github.com/magicmonty/bash-git-prompt)). Often included in
frameworks like [Oh My Zsh](https://github.com/ohmyzsh/ohmyzsh).
- **Editor integration**: similarly to the above, handy integrations with many
  features. [fugitive.vim](https://github.com/tpope/vim-fugitive) is the standard
  one for Vim.
  - `:Gblame`, `:Gbrowse`很方便，需要装[rhubarb.vim](https://github.com/tpope/vim-rhubarb)
  - `:Git`直接打开`hub`
- **Workflows**: we taught you the data model, plus some basic commands; we
didn't tell you what practices to follow when working on big projects (and
there are [many](https://nvie.com/posts/a-successful-git-branching-model/)
[different](https://www.endoflineblog.com/gitflow-considered-harmful)
[approaches](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow)).
- **GitHub**: Git is not GitHub. GitHub has a specific way of contributing code
to other projects, called [pull
requests](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests).
- **Other Git providers**: GitHub is not special: there are many Git repository
  hosts, like [GitLab](https://about.gitlab.com/) and
  [BitBucket](https://bitbucket.org/).
  - 自建 / 内网场景的选项与对比（Gitea、Forgejo、GitLab）见 [Software-Engineering：开发协作工具链](./Software-Engineering.md#开发协作工具链github--gitlab--gitea--sonarcloud--1password--confluence--jira--netlify)。

#### 和Github联动
* GitHub is a Git hosting repository that provides developers with tools to ship better code through command line features, issues (threaded discussions), pull requests, code review, or the use of a collection of free and for-purchase apps in the GitHub Marketplace. 
* [用SSH连GitHub](https://help.github.com/en/github/authenticating-to-github/connecting-to-github-with-ssh)
```shell
git config --global user.name "huangrt01"
git config --global user.email huangrt01@163.com

# MacOs
ssh-keygen -t rsa -b 4096 -C "huangrt01@163.com" # 如果rsa已占用，可用ssh-keygen -t ed25519 -C "huangruiteng@xxx.com"
eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_rsa --apple-use-keychain # MacOS带
pbcopy < ~/.ssh/id_rsa.pub  # MacOS , Linux用xclip

上github添加SSH Key

ssh -T git@github.com
# ssh-keygen -y -f ~/.ssh/id_rsa

# Linux

```
Github被墙
* `sudo vim /etc/hosts`


```
# GitHub Start 
192.30.253.112 github.com 
192.30.253.119 gist.github.com
151.101.184.133 assets-cdn.github.com
151.101.184.133 raw.githubusercontent.com
151.101.184.133 gist.githubusercontent.com
151.101.184.133 cloud.githubusercontent.com
151.101.184.133 camo.githubusercontent.com
151.101.184.133 avatars0.githubusercontent.com
151.101.184.133 avatars1.githubusercontent.com
151.101.184.133 avatars2.githubusercontent.com
151.101.184.133 avatars3.githubusercontent.com
151.101.184.133 avatars4.githubusercontent.com
151.101.184.133 avatars5.githubusercontent.com
151.101.184.133 avatars6.githubusercontent.com
151.101.184.133 avatars7.githubusercontent.com
151.101.184.133 avatars8.githubusercontent.com
# GitHub End
```
* 改电信DNS: `101.226.4.6`, ` 218.30.118.6`




#### 坑!

* 如果设置ssh key后，git push仍然要求输入邮箱密码
  * `git remote -v`查看origin使用的是https还是ssh
  * 如果是https，替换成ssh即可 `git remote set-url origin git@github.com:huangrt01/XXX.git`

* `Failed to connect to github 443`这一问题的解决方案
  1. `git remote set-url origin git@github.com:huangrt01/XXX.git`, 先把连接方式由https改成ssh
  2. 再在`~/.ssh/config`中把ssh的端口22改成https端口443

```shell
Host github.com
	User huangrt01@163.com
	Hostname ssh.github.com
	PreferredAuthentications publickey
	IdentityFile ~/.ssh/id_rsa
	Port 443
```

* 建立仓库
```
git init
git remote add origin git@github.com:huangrt01/dotfiles.git
git pull --rebase origin master
git push --set-upstream origin master
```

#### Git Worktree

Git Worktree 允许在同一仓库的多个分支上同时工作，无需多次克隆仓库。

* 核心概念：
  * 一个 `.git` 目录可以关联多个工作目录（worktree）
  * 每个 worktree 对应一个不同的分支或 commit
  * 避免频繁切换分支的麻烦

* 常用命令：
  ```shell
  # 创建新的 worktree，关联到特定分支
  git worktree add ../feature-branch feature
  
  # 查看所有 worktree
  git worktree list
  
  # 删除 worktree
  git worktree remove ../feature-branch
  git worktree prune  # 清理已删除的 worktree
  ```

* 适用场景：
  * 同时开发多个功能分支
  * 紧急修复 bug 不打断当前开发
  * 对比不同分支的代码

#### Other Resources
- [Oh Shit, Git!?!](https://ohshitgit.com/) is a short guide on how to recover
from some common Git mistakes.
- [Git for Computer
Scientists](https://eagain.net/articles/git-for-computer-scientists/) is a
short explanation of Git's data model, with less pseudocode and more fancy
diagrams than these lecture notes.
- [Git from the Bottom Up](https://jwiegley.github.io/git-from-the-bottom-up/)
is a detailed explanation of Git's implementation details beyond just the data
model, for the curious.
- [How to explain git in simple
words](https://smusamashah.github.io/blog/2017/10/14/explain-git-in-simple-words)

* [git handbook](https://guides.github.com/introduction/git-handbook/)，里面有一些资源
* [完整doc文档](https://git-scm.com/docs)
* [resources to learn Git](https://try.github.io/)
* [如何fork一个私库](https://stackoverflow.com/questions/10065526/github-how-to-make-a-fork-of-public-repository-private)
* [tig](https://jonas.github.io/tig/doc/manual.html)：图形化git历史
  * >Tig是一个基于ncurses的git文本模式接口。它的功能主要是作为一个Git存储库浏览器，但也可以帮助在块级别上分段提交更改，并充当各种Git命令输出的分页器。
  * 先[安装ncurses](https://blog.csdn.net/weixin_40123831/article/details/82490687)
  * [使用指南](https://www.jianshu.com/p/d9f60c0abbf7)