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Introduction of Git

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  • Linus Torvalds: linux kernel project 發起人
    速度受限於網路,
    Linus Torvalds use diff patch an tar to maintain Linux at first
    Then he use BitKeeper -everyone has a reposity
    其實需求是源自於一種對大型開源碼專案的需求
    Fully distributed – 為了要實現開發者local端的自我版本管理
    143MB的metadate還不包含歷史改變資料
  • 幾乎所有指令執行都不依賴網路
  • commit object is simple: it specifies the top-level tree for the snapshot (把某一特定時間點的根目錄資訊儲存下來)
    A "tag" is a way to mark a specific commit as special in some way. Tag無法被改變, 能對所有種類的object作標記
  • 圖中的箭頭只有示意的效果, 真正git的資料裡,只看的到很多整齊擺放的object檔案, 使用者並無法分辨哪些檔案是tree,哪些檔案是blob...
  • After cloning repo, everything is tracked.
    三種狀態是依照使用者對檔案修改的程度來分類
  • What things commited is in staged area not workspace
    如果git add之後懷疑程式有問題或是需要修改, 可在對程式進行測試的修改, 確定無誤即可下commit指令(用例子解釋,改code,add,加printk,commit)
  • 每依次commit,只是增加一個commit object指出root tree跟parent而已,就像是增加一個pointer指著目前的時間點,cheakout某個版本的code就像是移動指向時間的pointer
  • SHA-1就是ADDRESS, BRANCH就是POINTER,把POINTER移到哪,看到的東西就是蛇麼
    GIT適用BRANCH來TRACE COMMIT的歷史資料
    The HEAD ref is special in that it actually points to another ref. It is a pointer to the currently active branch.
  • Soft可以拿來壓縮commit次數
    git reset --soft HEAD^^
    git commit -m “Fix version”
  • reset說穿了只是移動HEAD的指針
    Soft可以拿來壓縮commit次數
    git reset --soft HEAD^^
    git commit -m “Fix version”
  • soft可以拿來壓縮commit次數
  • soft可以拿來壓縮commit次數
  • Clone下來會有一個叫做original的remote
    對應到的就是clone的網址
  • All your history is stored in the Git Directory; the working directory is simply a temporary checkout place where you can modify the files until your next commit.
  • Creating a new branch is as quick and simple as writing 41 bytes to a file (40 characters
    and a newline). 放在.git/refs/heads裡
  • Git的用意是讓使用者在local端方便的開發程式, 並把自己的貢獻開放出來, or push到主要的server, 對git來說, user local control也是同等的重要
    Svn則是一個嚴謹的版本控管, 並不care開發者在local端的改變, 重要的是紀錄server上的code是如何改變的
    Git允許較多開發者的想法, 而svn是力求簡單一制
    在svn做的改變跟每一個人都有關, 而git只與有ref到你的人有關
  • Transcript

    • 1. Wayne Chen 2012.1.6 Git
    • 2. Why Git? Linus Torvalds hate CVS & SVN ● Speed ● Strong support for non-linear development (thousands of parallel branches) ● Fully distributed ● Data size – SVN occupied 143MB of a 140MB project ● Perl, Eclipse, Qt, Ruby on Rails, Android...
    • 3. Key concept ● Nearly every operation is local ● Not by file name but the hash value ● The three states ● Snapshots, not differences ● Branch is cheap
    • 4. Nearly every operation is local ● After cloning a repository, you saved all of the history. ● No network requirement. – Except for: clone, pull, push, and fetch.
    • 5. Not by file name but the hash value ● Git use four types of objects to store the whole information, and each of these objects have an unique 20 bytes SHA-1 key to identify it. ● What if two identical files with inconsistent file names?
    • 6. root/ README lib/ mylib.rb tricks.rb inc/
    • 7. The three states ● First, the files that were in the last codebase are called tracked. If not, those are called untracked. ● All of the tracked files can be divided into modified, staged, and committed. – Modified (Working directory): You have just changed a file without doing anything to put it into Git database. – Staged (index): You have put a modified file into Git cache area. – Committed (objects): The data is safely stored in local Git database after taking a snapshot. ● What is committed is what is currently in the index, not what is in your working directory.
    • 8. The three states Working Directory Working DirectoryModified Staged Committed Staging area (index) Git repository (objects) git add git commit git checkout
    • 9. Snapshots, not differences ● Opposite to other VCS, Git is more like a mini file system. File 2 File 3 File N File 2 File 3 File N File N-1 File 4 File 1 File 2 File N-1 Time Line One snapshot All of the tracked files
    • 10. Branch is cheap ● Git is an addressable file system, and branch is a pointer. – Create a branch is just as storing a 20 bytes file. ● Not trace the file, Git trace the commit. ● HEAD file pointing to the branch you’re on.
    • 11. In a common case An engineer is doing his normal job, – Work on a project. – Create a branch in the current codebase he works on. – Work on this branch to implement a new feature. At this moment, he receives a phone call from a customer in mad who asks him to fix a terrible issue, – Revert back to original production branch. – Create a branch to add the fix. – After the solution tested OK, merge the fix back. – Switch back to the branch he worked at first. – Merge new feature to production branch.
    • 12. Get a repository ● First of all, you should get a repository. $ git init • Create an empty repository in your working folder. • After the first commit, git start to track files. $ git clone [url] • Establish a working folder and create a .git/ inside. • git pull the whole history data from sever. • git checkout the newest code to your workspace.
    • 13. Add some modifications ● Modified your files, then you can use git add to stage files. – git add -A: for any tracked or untracked – git add -u: update tracked files – git add -i: interactive select – Git also handle binary. – Flexible: .gitignore ● In git root, in directory of a project, or commit it.
    • 14. Commit change ● After doing commit, the change you make is safe. – If go wrong, please use $ git commit –amend – It is not a good habit to use $ git commit -a workspace stage area HEAD branch git diff git diff HEAD git diff --cached The better way is, Be used to $ git add after making any change. If the work is done, - $ git diff to check missed - $ git diff -–cached to check commit - $ git commit -m
    • 15. Step One, create branch ● Create a branch to work on new feature, C0 C1 C2 master new $ git checkout -b new It's the shorthand of: $ git branch new $ git checkout new
    • 16. Step Two, commit something ● Commit something on this new feature, C1 C2 C3 master new After the completion of some functions, he like to do one commit, $ git commit -a -m ’add a new api’ It's the shorthand of: $ git add -A $ git commit -m ’add a new api’ C0
    • 17. Step Three, receive an urgent issue ● Revert to production version, and create a branch for this urgent issue, C1 C2 C3 master new For working on issue, you have to save current update, and rollback to stable production branch. $ git checkout master and then create a new branch: $ git checkout -b ’issue’ after fix it, $ git commit -a -m ’fixed the issue’ C4 issue
    • 18. Step Four, test fix OK ● After passing test, merge the fix to master branch, C1 C2 C3 master new After the solution tested OK, merge it back to original master branch, $ git checkout master $ git merge issue Merge with directly upstream is called fast forward. C4 issue
    • 19. Step Five, finish the feature ● Delete the issue branch and switch back to the branch of new feature, C1 C2 C3 new Delete existed branch, $ git branch -d issue Switch back to work-in-progress branch, and finish it. $ git checkout new $ git commit -a -m ’finish it’ C4 master C5
    • 20. The whole commit steps C0 C1 C2 C4 C3 C5 C6 Checkout C3 and merge C5 After doing: $ git checkout master $ git merge new master new
    • 21. Undo ● Git provides some of the mechanisms for developers to regret their mind. – The latest commits are no longer needed. – A specific commit is better to be rolled back. – If you like to undo your modifications or give up the data in stage area. ● Reset, revert, and checkout is easy to be misused.
    • 22. Undo – use Reset ● Reset – Let developer able to reset commit status, stage area, or workspace. ● $ git reset --soft HEAD~N – Reset commit status to the latest Nth commit without changing any files. ● $ git reset HEAD~N – Reset commit status with undo git add command. ex. $ git reset HEAD play.c ● $ git reset --hard HEAD~N – Not only commits, but also stage area and files. objects Working directory index master HEAD soft none hard
    • 23. Undo – use Checkout ● Checkout – Move the HEAD pointer and checkout code. ● $ git checkout [file] – Checkout staged file to cover the real one. ● $ git checkout [branch_name] – Use the specific commit version to clean stage area and workspace. – 'reset' changes the SHA-1 key of branch, but 'checkout' just moves the HEAD.
    • 24. Undo – use Revert ● Revert – Rollback files with creating a rollback commit. – Reset is back and revert is forward. ● $ git revert HEAD~N – Create a new commit which revert the latest Nth commit. ● $ git revert SHA-1
    • 25. Rescue mechanism ● Git store every move of HEAD, so don't worry. – $ git reflog show master – $ git reset -- hard master@{N} ● But there are still some dangerous events, do not easily use it. – $ git reset -hard – $ git checkout HEAD Branch is cheap!
    • 26. Git communication Local Repository Remote name: v_a Remote name: v_b Remote name: v_c Remote Repository git://a... Remote Repository git://b... Remote Repository git://c... push pull push pull push pull add remote If a local repository is exist, you can add a remote (identify remote name and remote repository url) to git pull or git fetch data from remote repository, and use git push to put your contribution on it. If a local repository is exist, you can add a remote (identify remote name and remote repository url) to git pull or git fetch data from remote repository, and use git push to put your contribution on it. What you obtained is the latest branch, and the whole history. What you obtained is the latest branch, and the whole history. Remote name is used to identify the project resource, but tag and branch are used to identify the timing of the project snapshot. Remote name is used to identify the project resource, but tag and branch are used to identify the timing of the project snapshot.
    • 27. Look inside .git ● Git --- The stupid content tracker – Use compressed object which named as SHA-1 to store everything. – objects: Full objects (commits, trees, blobs, tags). – refs: Pointers to all of the branches and tags. – logs: A history of where your branches have been. – Current pointers
    • 28. Object folder ● objects: stores all of the commit, tag, tree, and blob objects. 00/ 6d/ 9b/ ac/ b0/ Info/ pack/ Loose objects Store the files named like: a9dca9a0fe0c031c996d308ab8a781ab7f358f which store the objects compressed by zlib. Packed objects Store the files named like: pack-a9dca9a0fe0c031c996d308ab8a781ab7f358f.ixd pack-a9dca9a0fe0c031c996d308ab8a781ab7f358f.pack .pack: The contents of all the objects that were removed from early loose objects. .idx: Offsets into the pack file. Totally 19 bytes
    • 29. Refs folder ● refs: stores all of the pointers. head/ remotes/ tag/ After creating a tag, a file named as tag is created here, and the content is the SHA-1 which tag point to. Otherwise, a tag object also created in the object folder. Each folders store the objects fetched from remote branch. Stores files named as each branch, and the contents are the SHA-1 which branch point to.
    • 30. Logs folder ● logs: every move will leave it's mark. Remember “$ git reflog show master”?
    • 31. Current status ● HEAD: points to the current active branch. ● ORIG_HEAD: stores the previous HEAD before doing git pull, git merge. – $ git reset --hard ORIG_HEAD ● FETCH_HEAD: record the branch you fetched. ● index: stores staged data. – The next proposed commit snapshot.
    • 32. Add and commit in low level $ git add ● Updates the index – Write to compressed file .git/index $ git commit ● Stores blobs for the changed files – Add a loose file to .git/object/ ● Writes out tree objects (.git/object) ● Writes commit objects that reference the top level tree ● Modified HEAD and branch pointer (.git/refs/heads & .git/logs/refs/heads) ● Store commit msg (.git/COMMIT_EDITMSG)
    • 33. Compete it with SVN again ● Centralized vs Distributed – SVN is one repo and lots of clients. GIT is a repo with lots of client repos. – Checkout working copy vs whole repository. ● Serial number or Lots of branch – Corporate work or distributed version control? ● Consistency vs Flexibility – SVN makes everyone working on the same thing. ● Ref: Please Stop Bugging Linus Torvalds About Subversion
    • 34. Repo ● There are over 160 projects involved in Android source. ● repo init - to set up clone script ● repo sync ● repo start - create local branch ● repo upload The definition of open: "mkdir android ; cd android ; repo init -u git://android.git.kernel.org/platform/manifest.git ; repo sync ; make" Andy Rubin repo init -u ssh://.../manifest.git -b xxx -m android.xml - Verify your SSH public key - Get repo - Get a manifest.xml - Clone the projects listed on manifest.xml - Identify specific project: repo sync kernel/linux - Transmit branch to Gerrit over an SSH connection - Gerrit reviews each commits. It is better to run git rebase -i before repo upload.
    • 35. Resource ● Install git – Cygwin or MsysGit ● Get repo – Curl -l -k http://android.git.kernel.org/repo ● Reference – Official: http://git-scm.com/ – Git Reference: http://gitref.org/ – Pro Git: http://progit.org/book/ – Repo: http://source.android.com/source/using-repo.html#init – Get project to practice: http://www.kernel.org/pub/ – How Linus Torvalds talk about GIT: http://www.youtube.com/watch?v=4XpnKHJAok8
    • 36. Thanks!