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    Git Rebase

     

     

    在上一節我們看到了,多人在同一個分支上協作時,很容易出現沖突。即使沒有沖突,后push的童鞋不得不先pull,在本地合并,然后才能push成功。

     

    每次合并再push后,分支變成了這樣:

     

    $ git log --graph --pretty=oneline --abbrev-commit
    * d1be385 (HEAD -> master, origin/master) init hello
    *   e5e69f1 Merge branch 'dev'
    |\  
    | *   57c53ab (origin/dev, dev) fix env conflict
    | |\  
    | | * 7a5e5dd add env
    | * | 7bd91f1 add new env
    | |/  
    * |   12a631b merged bug fix 101
    |\ \  
    | * | 4c805e2 fix bug 101
    |/ /  
    * |   e1e9c68 merge with no-ff
    |\ \  
    | |/  
    | * f52c633 add merge
    |/  
    *   cf810e4 conflict fixed

     

    總之看上去很亂,有強迫癥的童鞋會問:為什么Git的提交歷史不能是一條干凈的直線?

     

    其實是可以做到的!

     

    Git有一種稱為rebase的操作,有人把它翻譯成“變基”。

     

    先不要隨意展開想象。我們還是從實際問題出發,看看怎么把分叉的提交變成直線。

     

    在和遠程分支同步后,我們對hello.py這個文件做了兩次提交。用git log命令看看:

     

    $ git log --graph --pretty=oneline --abbrev-commit
    * 582d922 (HEAD -> master) add author
    * 8875536 add comment
    * d1be385 (origin/master) init hello
    *   e5e69f1 Merge branch 'dev'
    |\  
    | *   57c53ab (origin/dev, dev) fix env conflict
    | |\  
    | | * 7a5e5dd add env
    | * | 7bd91f1 add new env
    ...

     

    注意到Git用(HEAD -> master)和(origin/master)標識出當前分支的HEAD和遠程origin的位置分別是582d922 add author和d1be385 init hello,本地分支比遠程分支快兩個提交。

     

    現在我們嘗試推送本地分支:

     

    $ git push origin master
    To github.com:michaelliao/learngit.git
     ! [rejected]        master -> master (fetch first)
    error: failed to push some refs to 'git@github.com:michaelliao/learngit.git'
    hint: Updates were rejected because the remote contains work that you do
    hint: not have locally. This is usually caused by another repository pushing
    hint: to the same ref. You may want to first integrate the remote changes
    hint: (e.g., 'git pull ...') before pushing again.
    hint: See the 'Note about fast-forwards' in 'git push --help' for details.

     

    很不幸,失敗了,這說明有人先于我們推送了遠程分支。按照經驗,先pull一下:

     

    $ git pull
    remote: Counting objects: 3, done.
    remote: Compressing objects: 100% (1/1), done.
    remote: Total 3 (delta 1), reused 3 (delta 1), pack-reused 0
    Unpacking objects: 100% (3/3), done.
    From github.com:michaelliao/learngit
       d1be385..f005ed4  master     -> origin/master
     * [new tag]         v1.0       -> v1.0
    Auto-merging hello.py
    Merge made by the 'recursive' strategy.
     hello.py | 1 +
     1 file changed, 1 insertion(+)

     

    再用git status看看狀態:

     

    $ git status
    On branch master
    Your branch is ahead of 'origin/master' by 3 commits.
      (use "git push" to publish your local commits)
    
    nothing to commit, working tree clean

     

    加上剛才合并的提交,現在我們本地分支比遠程分支超前3個提交。

     

    用git log看看:

     

    $ git log --graph --pretty=oneline --abbrev-commit
    *   e0ea545 (HEAD -> master) Merge branch 'master' of github.com:michaelliao/learngit
    |\  
    | * f005ed4 (origin/master) set exit=1
    * | 582d922 add author
    * | 8875536 add comment
    |/  
    * d1be385 init hello
    ...

     

    對強迫癥童鞋來說,現在事情有點不對頭,提交歷史分叉了。如果現在把本地分支push到遠程,有沒有問題?

     

    有!

     

    什么問題?

     

    不好看!

     

    有沒有解決方法?

     

    有!

     

    這個時候,rebase就派上了用場。我們輸入命令git rebase試試:

     

    $ git rebase
    First, rewinding head to replay your work on top of it...
    Applying: add comment
    Using index info to reconstruct a base tree...
    M	hello.py
    Falling back to patching base and 3-way merge...
    Auto-merging hello.py
    Applying: add author
    Using index info to reconstruct a base tree...
    M	hello.py
    Falling back to patching base and 3-way merge...
    Auto-merging hello.py

     

    輸出了一大堆操作,到底是啥效果?再用git log看看:

     

    $ git log --graph --pretty=oneline --abbrev-commit
    * 7e61ed4 (HEAD -> master) add author
    * 3611cfe add comment
    * f005ed4 (origin/master) set exit=1
    * d1be385 init hello
    ...

     

    原本分叉的提交現在變成一條直線了!這種神奇的操作是怎么實現的?其實原理非常簡單。我們注意觀察,發現Git把我們本地的提交“挪動”了位置,放到了f005ed4 (origin/master) set exit=1之后,這樣,整個提交歷史就成了一條直線。rebase操作前后,最終的提交內容是一致的,但是,我們本地的commit修改內容已經變化了,它們的修改不再基于d1be385 init hello,而是基于f005ed4 (origin/master) set exit=1,但最后的提交7e61ed4內容是一致的。

     

    這就是rebase操作的特點:把分叉的提交歷史“整理”成一條直線,看上去更直觀。缺點是本地的分叉提交已經被修改過了。

     

    最后,通過push操作把本地分支推送到遠程:

     

    Mac:~/learngit michael$ git push origin master
    Counting objects: 6, done.
    Delta compression using up to 4 threads.
    Compressing objects: 100% (5/5), done.
    Writing objects: 100% (6/6), 576 bytes | 576.00 KiB/s, done.
    Total 6 (delta 2), reused 0 (delta 0)
    remote: Resolving deltas: 100% (2/2), completed with 1 local object.
    To github.com:michaelliao/learngit.git
       f005ed4..7e61ed4  master -> master

     

    再用git log看看效果:

     

    $ git log --graph --pretty=oneline --abbrev-commit
    * 7e61ed4 (HEAD -> master, origin/master) add author
    * 3611cfe add comment
    * f005ed4 set exit=1
    * d1be385 init hello
    ...

     

    遠程分支的提交歷史也是一條直線。

     

    小結

     

    ⒈ rebase操作可以把本地未push的分叉提交歷史整理成直線;

     

    ⒉ rebase的目的是使得我們在查看歷史提交的變化時更容易,因為分叉的提交需要三方對比。

     

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