1. 假定原作者的名称叫gittest, 我叫uniquejava.
原作者新建了一个项目叫nocrazy。 于是路径为gittest/nocrazy
做了两次提交
提交一：
a.txt
this is a
提交二：b.txt
this is b.
2. 我cyper某天fork了这个项目，于是有origin指向uniquejava/nocrazy, 并且新增一个upstream指向原作者gittest/nocrazy
$ git clone https://git.oschina.net/uniquejava/nocrazy.git
$ cd nocrazy
➜ nocrazy git:(master) $ git remote add upstream https://git.oschina.net/gittest/nocrazy.git
➜ nocrazy git:(master) $ git remote -v
origin https://git.oschina.net/uniquejava/nocrazy.git (fetch)
origin https://git.oschina.net/uniquejava/nocrazy.git (push)
upstream https://git.oschina.net/gittest/nocrazy.git (fetch)
upstream https://git.oschina.net/gittest/nocrazy.git (push)
➜ nocrazy git:(master) $
3. gittest正常提交了一个新版本
c.txt
this is c
git log --oneline
e63667b this is c
11f5850 this is b
1cbdb56 this is a
4. Cyper想把fork同步到和gittest一模一样，怎么办？
办法一(思考未试验）：把自己的fork删除重新fork? 然后呢在本地git pull origin master
办法二：
git fetch upstream
git checkout master
git merge upstream/master
方法二需要push才能到origin么。(yes)
➜ nocrazy git:(master) $ ll
total 16
drwxr-xr-x 13 cyper staff 442B Aug 26 05:11 .git/
-rw-r--r-- 1 cyper staff 10B Aug 26 05:07 a.txt
-rw-r--r-- 1 cyper staff 10B Aug 26 05:07 b.txt
➜ nocrazy git:(master) $ git fetch upstream
remote: Counting objects: 4, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://git.oschina.net/gittest/nocrazy
* [new branch] master -> upstream/master
➜ nocrazy git:(master) $ git diff ..upstream/master
diff --git a/c.txt b/c.txt
new file mode 100644
index 0000000..5da75cf
--- /dev/null
+++ b/c.txt
@@ -0,0 +1 @@
+this is c
使用以上git diff命令可以看到author新增了一个c.txt文件。
➜ nocrazy git:(master) $ git merge upstream/master
Updating 11f5850..e63667b
Fast-forward
c.txt | 1 +
1 file changed, 1 insertion(+)
create mode 100644 c.txt
➜ nocrazy git:(master) $ git status
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
(use "git push" to publish your local commits)
nothing to commit, working directory clean
➜ nocrazy git:(master) $ git fetch upstream
➜ nocrazy git:(master) $ git merge upstream/master
Already up-to-date.
➜ nocrazy git:(master) $ git diff ..origin/master
diff --git a/c.txt b/c.txt
deleted file mode 100644
index 5da75cf..0000000
--- a/c.txt
+++ /dev/null
@@ -1 +0,0 @@
-this is c
通过以上可以看到git merge只是把upstream上的代码拉到(本地工作目录+本地仓库)，需要我们push origin
➜ nocrazy git:(master) $ git push origin master
Username for 'https://git.oschina.net': uniquejava
Password for 'https://uniquejava@git.oschina.net':
Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 292 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To https://git.oschina.net/uniquejava/nocrazy.git
11f5850..e63667b master -> master
➜ nocrazy git:(master) $ ll
total 24
drwxr-xr-x 14 cyper staff 476B Aug 26 05:21 .git/
-rw-r--r-- 1 cyper staff 10B Aug 26 05:07 a.txt
-rw-r--r-- 1 cyper staff 10B Aug 26 05:07 b.txt
-rw-r--r-- 1 cyper staff 10B Aug 26 05:14 c.txt
到这里， 步骤4验证结束。。。成功的将fork上的代码更新到最新版。
5. 假设上一步成功执行， 效果应该和我重新fork一遍最新的项目是一样的。
6. 重复3:即gitetst提交了一个新版本
d.txt
this is d.
7. 在6之前，Cyper因为要修复defect #1, 已经开始基于c.txt做修改。就没有来得及及时同步
cyper修改this is c变成this is c_cyper
然后正常的add, commit ， （Cyper此时是否可以push?)
➜ nocrazy git:(master) ✗ $ git commit -m "[nocrazy-1] fix c."
[master 77fd57f] [nocrazy-1] fix c.
1 file changed, 1 insertion(+), 1 deletion(-)
此时已经有了分歧。gittest上有新的提交， cyper也有新的提交。
cyper想基于最新的代码重新提交。
7-1 假定cyper没有push
他想先更新fork，然后基于 author最新的代码去做提交
git fetch upstream
git checkout master
git merge upstream/master 这一步是用git rebase upstream/master还是用git merge upstream/master
=====
➜ nocrazy git:(master) $ git rebase origin/master(这步只是想验证base到当前base的情况，无效果）
Current branch master is up to date.
➜ nocrazy git:(master) $ git fetch upstream
remote: Counting objects: 4, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://git.oschina.net/gittest/nocrazy
e63667b..380a8dd master -> upstream/master
➜ nocrazy git:(master) $ git rebase upstream/master
First, rewinding head to replay your work on top of it...
Applying: [nocrazy-1] fix c.
➜ nocrazy git:(master) $ git status
On branch master
Your branch is ahead of 'origin/master' by 2 commits.
(use "git push" to publish your local commits)
nothing to commit, working directory clean
➜ nocrazy git:(master) $
以上这步做了什么？？？原来git rebase upstream/master把upstream上的代码拉到(本地工作目录+本地仓库)，并且自动做了合并， 所以你的本了一下子多出来两个commit需要push, 因此有Your branch is ahead of 'origin/master' by 2 commits.
➜ nocrazy git:(master) $ git log --oneline
8530647 [nocrazy-1] fix c.
380a8dd this is d
e63667b this is c
11f5850 this is b
1cbdb56 this is a
现在只需做push origin。。。就行了！
➜ nocrazy git:(master) $ git push origin master
Username for 'https://git.oschina.net': uniquejava
Password for 'https://uniquejava@git.oschina.net':
Counting objects: 6, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), 566 bytes | 0 bytes/s, done.
Total 6 (delta 1), reused 0 (delta 0)
To https://git.oschina.net/uniquejava/nocrazy.git
e63667b..8530647 master -> master
所以在发pull request 之前不要去做push . (如果在rebase之前就做了push，需要怎么补救？？？）
到目前为止，cyper的远程仓库中是最新的代码，而原作者的代码不新，因为他的代码没有包含你的修改
8. 发pull request
8-1， 假设gittest在你发pull request前，没有提交任何代码。。
过程很简单。
接下来gittest review，他点击某个代码变动行的前面加了一个comment: 少了一个标点符号 。。
如果你此时使用git fetch upstream会没有任何输出
然后你
➜ nocrazy git:(master) $ git rebase upstream/master
Current branch master is up to date.
这是因为你当前没有任何commit是需要rebase的, 因为所有commit都已经push到了origin. 除非。。
与时同时， gittest正常添加了一个新文件
e.txt
this is e.
你重新fetch,
➜ nocrazy git:(master) $ git fetch upstream
remote: Counting objects: 5, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://git.oschina.net/gittest/nocrazy
380a8dd..fd2e641 master -> upstream/master
➜ nocrazy git:(master) $ git diff ..upstream/master
diff --git a/c.txt b/c.txt
index 3af12b3..5da75cf 100644
--- a/c.txt
+++ b/c.txt
@@ -1 +1 @@
-this is c_cyper
+this is c
diff --git a/e.txt b/e.txt
new file mode 100644
index 0000000..5641475
--- /dev/null
+++ b/e.txt
@@ -0,0 +1 @@
可以看到，当前你的本地仓库和upstream的区别是。。你改了c文件而upstream加了e文件。。
你重新rebase
➜ nocrazy git:(master) $ git rebase upstream/master
First, rewinding head to replay your work on top of it...
Applying: [nocrazy-1] fix c.
➜ nocrazy git:(master) $ git status
On branch master
Your branch and 'origin/master' have diverged,
and have 2 and 1 different commit each, respectively.
(use "git pull" to merge the remote branch into yours)
nothing to commit, working directory clean
➜ nocrazy git:(master) $
这时和第一次有所不一样,
➜ nocrazy git:(master) $ git log --oneline
8c4c4b6 [nocrazy-1] fix c.
fd2e641 this is e
380a8dd this is d
e63667b this is c
11f5850 this is b
1cbdb56 this is a
查看一下有什么不一样
➜ nocrazy git:(master) $ git diff ..origin/master
diff --git a/e.txt b/e.txt
deleted file mode 100644
index 5641475..0000000
--- a/e.txt
+++ /dev/null
@@ -1 +0,0 @@
-this is e
是因为你自己的远程仓库没有包含e文件。。。按照提示。 你需要做一次pull
但是pull会产生分支 。。
尝试一下然后立马撤销（http://stackoverflow.com/questions/1223354/undo-git-pull-how-to-bring-repos-to-old-state）
➜ nocrazy git:(master) $ git pull
Merge made by the 'recursive' strategy.
➜ nocrazy git:(master) $ git log --oneline
9d7a324 Merge branch 'master' of https://git.oschina.net/uniquejava/nocrazy
8c4c4b6 [nocrazy-1] fix c.
fd2e641 this is e
8530647 [nocrazy-1] fix c.
380a8dd this is d
e63667b this is c
11f5850 this is b
1cbdb56 this is a
➜ nocrazy git:(master) $ 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 directory clean
可见， git pull生成的commit history多么的混乱！(相对于当前的origin/master,
一下子多出了3条commit history)
9d7a324 Merge branch 'master' of https://git.oschina.net/uniquejava/nocrazy
8c4c4b6 [nocrazy-1] fix c.
fd2e641 this is e
赶紧撤销并换成git rebase
➜ nocrazy git:(master) $ git show HEAD
commit 9d7a324e5e562475b9585ed029817fc98d09a1d4
Merge: 8c4c4b6 8530647
Author: cyper <345343747@qq.com>
Date: Wed Aug 26 06:11:07 2015 +0800
Merge branch 'master' of https://git.oschina.net/uniquejava/nocrazy
原来。。HEAD并不是指向远程仓库的head..
➜ nocrazy git:(master) $ git show head
➜ nocrazy git:(master) $ git show HEAD
➜ nocrazy git:(master) $ git show master
➜ nocrazy git:(master) $ git show origin/master
。。。前三个信息都是指向本地仓库的头，是没一个commit id.最后一个是我想要的头，是我pull之前的commit id
所以有三种方法
git reset --hard HEAD^3
或git reset --hard origin/master
或者git reset --hard 8530647b
重做第7步。。
➜ nocrazy git:(master) $ git rebase upstream/master
First, rewinding head to replay your work on top of it...
Applying: [nocrazy-1] fix c.
➜ nocrazy git:(master) $ git status
On branch master
Your branch and 'origin/master' have diverged,
and have 2 and 1 different commit each, respectively.
(use "git pull" to merge the remote branch into yours)
nothing to commit, working directory clean
➜ nocrazy git:(master) $
现在因为本地仓库才是最新的。。你需要force push到origin而不需要理pull操作。。。
➜ nocrazy git:(master) $ git push origin master --force
Username for 'https://git.oschina.net': uniquejava
Password for 'https://uniquejava@git.oschina.net':
Counting objects: 6, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), 569 bytes | 0 bytes/s, done.
Total 6 (delta 1), reused 0 (delta 0)
To https://git.oschina.net/uniquejava/nocrazy.git
+ 8530647...3e23afc master -> master (forced update)
git rebase产生了完美的commit history:
3e23afc [nocrazy-1] fix c.
fd2e641 this is e
380a8dd this is d
e63667b this is c
11f5850 this is b
1cbdb56 this is a
来源：oschina
链接：https://my.oschina.net/u/113421/blog/497112