git实战案例之分支操作

0. 背景

在GitHub 上有一个代码库，但是我想在一台从来没有操作过的电脑上，想拉取这个Github 的代码，那该怎么操作呢？

1. 主要步骤

git init
这个步骤的主要功能是，初始化当前的文件夹为一git 文件，方便后期被追踪。

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git init
Reinitialized existing Git repository in D:/Java_Project/dayProgram/.git/

因为笔者的当前文件夹已经初始化过了，所以再次执行这个命令的时候，就出现了Reinitialized existing....
同时，可以看到在每行记录下末尾都标志了一个(master)，这个标志用于表示当前的分支

git remote add origin xxxx
这个命令是将当前的这个文件夹添加到远程指定的项目上。【但是我的疑问是，指定了这个项目，但是会指定一个具体的分支么？还是说默认指向 mster 分支？】

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git remote add origin https://github.com/LittleLawson/DayProgram.git
fatal: remote origin already exists.

因为我的这个文件已经指向了这个项目地址，所以这里会报错：remote origin already exists!!!

git branch
查看当前分支，有多种不同的用法
- git branch -r 查看远程分支

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git branch -r
  origin/master

git branch -a 查看所有分支

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git branch -a
* master
  remotes/origin/master

git checkout
创建新的分支。
创建新的分支的操作常用于提交代码，这样就会避免多个时刻提交代码带来的冲突。
如下，我有一些修改的代码，需要提交，那么我就需要创建一个新的分支，就命名成Develop，需要注意语法的格式，如下：git checkout -b [new branch name] origin/[existing branch]

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git checkout -b Develop origin/master
Switched to a new branch 'Develop'
A       .idea/vcs.xml
M       .idea/workspace.xml
M       src/main/java/multi/thread/consume/Consumer.java
M       src/main/java/multi/thread/consume/ConsumerOne.java
M       src/main/java/multi/thread/entrance/Main.java
M       src/main/java/multi/thread/produce/Producer.java
M       src/main/java/multi/thread/produce/ProducerOne.java
M       src/main/java/multi/thread/threadpool/ConsumeThread.java
M       src/main/java/multi/thread/threadpool/ProduceThread.java
M       target/classes/multi/thread/consume/ConsumerOne.class
M       target/classes/multi/thread/entrance/Main.class
M       target/classes/multi/thread/produce/Producer.class
M       target/classes/multi/thread/produce/ProducerOne.class
M       target/classes/multi/thread/threadpool/ConsumeThread.class
M       target/classes/multi/thread/threadpool/ProduceThread.class
Branch 'Develop' set up to track remote branch 'master' from 'origin'.

可以看到当前的 branch 从 master 切到了 develop。Branch 'Develop' set up to track remote branch 'master' from 'origin'.这句话的意思就是，Develop这个分支开始跟踪远程的master分支。
再次检查分支：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git branch
* Develop
  master

可以看到这次当前分支从master 移到了Develop了。但是通过git branch -r查看远程分支，看到还是只有一个master分支

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git branch -r
  origin/master

查看当前分支状态：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git status
On branch Develop
Your branch is up to date with 'origin/master'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        new file:   .idea/vcs.xml
        modified:   .idea/workspace.xml
        modified:   src/main/java/multi/thread/consume/Consumer.java
        modified:   src/main/java/multi/thread/consume/ConsumerOne.java
        modified:   src/main/java/multi/thread/entrance/Main.java
        modified:   src/main/java/multi/thread/produce/Producer.java
        modified:   src/main/java/multi/thread/produce/ProducerOne.java
        modified:   src/main/java/multi/thread/threadpool/ConsumeThread.java
        modified:   src/main/java/multi/thread/threadpool/ProduceThread.java
        modified:   target/classes/multi/thread/consume/ConsumerOne.class
        modified:   target/classes/multi/thread/entrance/Main.class
        modified:   target/classes/multi/thread/produce/Producer.class
        modified:   target/classes/multi/thread/produce/ProducerOne.class
        modified:   target/classes/multi/thread/threadpool/ConsumeThread.class
        modified:   target/classes/multi/thread/threadpool/ProduceThread.class

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:   .idea/workspace.xml

注意到，这里的Develop分支里面的状态为空，所以需要添加东西进去：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git add -A
warning: LF will be replaced by CRLF in .idea/workspace.xml.
The file will have its original line endings in your working directory

提交到本地缓存：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git commit -m "develop"
[Develop e178879] develop
 15 files changed, 286 insertions(+), 170 deletions(-)
 create mode 100644 .idea/vcs.xml
 rewrite target/classes/multi/thread/consume/ConsumerOne.class (91%)
 rewrite target/classes/multi/thread/entrance/Main.class (75%)
 rewrite target/classes/multi/thread/produce/ProducerOne.class (90%)
 rewrite target/classes/multi/thread/threadpool/ConsumeThread.class (100%)
 rewrite target/classes/multi/thread/threadpool/ProduceThread.class (100%)

git push origin Develop:master //推送本地的Develop(冒号前面的)分支到远程origin的master(冒号后面的)分支(没有会自动创建)

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git push origin Develop:master
Enumerating objects: 64, done.
Counting objects: 100% (64/64), done.
Delta compression using up to 4 threads
Compressing objects: 100% (26/26), done.
Writing objects: 100% (35/35), 7.28 KiB | 621.00 KiB/s, done.
Total 35 (delta 13), reused 0 (delta 0)
remote: Resolving deltas: 100% (13/13), completed with 12 local objects.
To https://github.com/LittleLawson/DayProgram.git
   d7717da..e178879  Develop -> master

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git push origin --delete Develop
error: unable to delete 'Develop': remote ref does not exist
error: failed to push some refs to 'https://github.com/LittleLawson/DayProgram.git'

因为是将本地的Develop分支推送到远端的master分支，而不是创建一个新的Develop分支，所以远端并不存在一个新的Develop分支，所以删除远端Develop分支会报remote ref does not exist。同时使用如下命令查看的时候，仍然只有一个远端分支：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git branch -r
  origin/master

git branch -d Develop
删除分支

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git branch -d Develop
error: Cannot delete branch 'Develop' checked out at 'D:/Java_Project/dayProgram'

但是你会看到这个命令执行却报错了，原因是：我在当前分支删除这个分支，肯定是不可以的，我需要切换到其它分支再执行这个操作。

git checkout xxx
切换分支

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (Develop)
$ git checkout master
Switched to branch 'master'
Your branch is behind 'origin/master' by 1 commit, and can be fast-forwarded.
  (use "git pull" to update your local branch)

因为刚才推送了一个版本到远端的master上，所以这里会报一个hint，意思就是说：你的分支落后于远端的origin/master的一次commit，（但是）可以使用fast-forwarded这个方法去更新。（使用git pull去更新你的本地分支）。

解决上述的提示有两种方法，分别如下：

git merge [branch]
这里的git merge Develop的功能就是将Develop分支合并到 master上。

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git merge Develop
Updating d7717da..e178879
Fast-forward
 .idea/vcs.xml                                      |   6 +
 .idea/workspace.xml                                | 357 +++++++++++++---
 src/main/java/multi/thread/consume/Consumer.java   |   2 +-
 .../java/multi/thread/consume/ConsumerOne.java     |  31 +-
 src/main/java/multi/thread/entrance/Main.java      |  17 +-
 src/main/java/multi/thread/produce/Producer.java   |   4 +-
 .../java/multi/thread/produce/ProducerOne.java     |  29 +-
 .../multi/thread/threadpool/ConsumeThread.java     |   5 +-
 .../multi/thread/threadpool/ProduceThread.java     |   5 +-
 .../classes/multi/thread/consume/ConsumerOne.class | Bin 1561 -> 1927 byt
 target/classes/multi/thread/entrance/Main.class    | Bin 1395 -> 1432 byt
 target/classes/multi/thread/produce/Producer.class | Bin 1017 -> 1016 byt
 .../classes/multi/thread/produce/ProducerOne.class | Bin 1482 -> 1842 byt
 .../multi/thread/threadpool/ConsumeThread.class    | Bin 706 -> 589 bytes
 .../multi/thread/threadpool/ProduceThread.class    | Bin 731 -> 590 bytes
 15 files changed, 286 insertions(+), 170 deletions(-)
 create mode 100644 .idea/vcs.xml

可以看到在合并的过程中，输出了一堆的Fast-forward信息。
在合并Develop分支到master之后，就可以开始删除Develop分支了：

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git branch -d Develop
Deleted branch Develop (was e178879).

LittleLawson@DESKTOP-PA2BQ2D MINGW64 /d/Java_Project/dayProgram (master)
$ git branch
* master