git pull --rebase的正確使用

在多人使用同一個遠程分支合做開發的時候，極可能出現 push 代碼的時候出現如下問題：

$ git push origin master

# 結果以下
To github.com:hello/demo.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'git@github.com:hello/demo.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.
複製代碼

很明顯此時遠程分支有新的 commit 未同步到本地，沒法推送。正常狀況下咱們會執行如下操做：

$ git pull origin master

# 結果以下
remote: Counting objects: 2, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 2 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (2/2), done.
From github.com:hello/demo
   3824da0..f318a05  master     -> origin/master
Merge made by the 'recursive' strategy.
 one.md | 2 ++
 1 file changed, 2 insertions(+)
 
$ git push origin master

# 結果以下
Enumerating objects: 10, done.
Counting objects: 100% (8/8), done.
Delta compression using up to 8 threads
Compressing objects: 100% (8/8), done.
Writing objects: 100% (8/8), 1.24 KiB | 1.24 MiB/s, done.
Total 8 (delta 5), reused 0 (delta 0)
To github.com:hello/demo.git
   f318a05..1aefef1  master -> master
複製代碼

確實 push 成功了，可是此時用 git log 查看如下提交記錄：

$ git log

# 結果以下
commit 1aefef1a2bedbd3ebd82db8dcf802011a35a9888 (HEAD -> master, origin/master, origin/HEAD)
Merge: 24cfa5c f318a05
Author: hello <hello@qq.com>
Date:   Tue Jul 23 09:53:47 2019 +0800

    Merge branch 'master' of github.com:hello/demo

commit 24cfa5c3ad271e85ff0e64793bf2bcc9d700c233
Author: hello <hello@qq.com>
Date:   Tue Jul 23 09:50:06 2019 +0800

    feat: 新功能提交

commit f318a05b1a4cbc0a6cf8d7dc7d3fb99cbafb0363
Author: world <world@qq.com>
Date:   Tue Jul 23 09:48:20 2019 +0800

    feat: 其餘功能提交

...
複製代碼

你會發現多出了一條 merge commit，這個 commit 就是在執行 git pull origin master 的時候自動生成的。若是多人屢次如此操做，那麼提交記錄就會出現不少條這種自從生成的 merge commit，很是難看。

要解決以上問題，再也不出現自動生成的 merge commit，那麼只要在執行 git pull origin master 的時候帶上 --rebase 便可：

$ git pull --rebase origin master

$ git push origin master
複製代碼

項目示例

如今經過一個示例項目來示範以上命令的用法。項目（demo）的結構以下：

# 在 demo 目錄下執行如下命令

$ ls

# 結果以下
one.md  two.md

$ cat one.md

# 結果以下
hello one

$ cat two.md

#結果以下
hello two
複製代碼

A、B兩位開發同窗使用一個遠程分支（master）合做開發。A同窗修改了 one.md 文件，提交了一個 commit 而且已經推送到遠程分支 master：

$ cat one.md

# 結果以下
hello one

new feature for one

$ git push origin master

# 結果以下
Enumerating objects: 4, done.
Counting objects: 100% (4/4), done.
Delta compression using up to 8 threads
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 1.24 KiB | 1.24 MiB/s, done.
Total 4 (delta 3), reused 0 (delta 0)
To github.com:hello/demo.git
   f318a05..1aefef1  master -> master
複製代碼

此時B同窗不知情，也並未將遠程分支的更新拉到本地。B同窗修改了 two.md 文件，也提交了一個 commit 並嘗試推送到遠程分支 master：

$ cat two.md

# 結果以下
hello two

new feature for two

$ git push origin master

# 結果以下
To github.com:hello/demo.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'git@github.com:hello/demo.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.
複製代碼

發現報錯，沒法推送。此時執行如下命令：

$ git pull --rebase

$ git push origin master
複製代碼

便可成功推送。

注意事項

• 執行 git pull --rebase 的時候必須保持本地目錄乾淨。即：不能存在狀態爲 modified 的文件。（存在Untracked files是不要緊的）
• 若是出現衝突，能夠選擇手動解決衝突後繼續 rebase，也能夠放棄本次 rebase

執行 git pull --rebase 的時候必須保持本地目錄乾淨

1. 有 modified 狀態的文件

本地有受版本控制的文件改動的時候，執行以上命令：

$ git pull --rebase

# 結果以下
error: cannot pull with rebase: You have unstaged changes.
error: please commit or stash them.
複製代碼

會出現以上報錯，沒法操做。

此時查看如下文件狀態：

$ git status

# 結果以下
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   two.md

no changes added to commit (use "git add" and/or "git commit -a")
複製代碼

就是由於本地有文件改動還沒有提交形成的。此時有兩種作法：

• 若是本次修改已經完成，則能夠先提交（commit）一下
• 若是本次修改還沒有完成，則能夠先貯藏：git stash

作了以上一種操做以後再嘗試 git pull --rebase，不會再報錯。

若是作了 git stash 操做，此時能夠經過 git stash pop 回到以前的工做狀態。

2. 有 Untracked files

查看本地文件狀態：

$ git status

# 結果以下
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	three.md

no changes added to commit (use "git add" and/or "git commit -a")
複製代碼

若是是以上這種狀況，直接執行 git pull --rebase 是不會有問題的。

若是出現衝突，能夠選擇手動解決衝突後繼續 rebase，也能夠放棄本次 rebase

在上面的示例項目中，若是 A、B 同窗修改了同一個文件。那麼頗有可能會出現衝突，當 B 同窗來執行命令的時候會出現以下情況：

$ git pull --rebase

# 結果以下
remote: Counting objects: 6, done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 6 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (6/6), done.
From gitlab.lrts.me:fed/gitlab-merge
   93a1a93..960b5fc  master     -> origin/master
First, rewinding head to replay your work on top of it...
Applying: feat: 其餘功能提交
Using index info to reconstruct a base tree...
M	one.md
Falling back to patching base and 3-way merge...
Auto-merging one.md
CONFLICT (content): Merge conflict in one.md
error: Failed to merge in the changes.
Patch failed at 0001 feat：其餘功能提交
hint: Use 'git am --show-current-patch' to see the failed patch

Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".
複製代碼

這種狀況下，能夠手動打開 one.md 文件解決衝突，而後再執行：

$ git add one.md

$ git rebase --continue
複製代碼

解決問題。

也能夠用 git rebase --abort 放棄本次 rebase 操做。

總結

多人基於同一個遠程分支開發的時候，若是想要順利 push 又不自動生成 merge commit，建議在每次提交都按照以下順序操做：

$ git stash

$ git pull --rebase

$ git push

$ git stash pop
複製代碼