在项目开发中，特别是web前端开发中，有非常多的开源第三方library，我们希望引用他们，同时也希望能够方便地保持这些第三方
开源repo的更新。另外一方面如果我们自己在开发一个网站的项目，这个项目一般分为前端和后端两个相对独立的子项目，特别是前端的repo可能在不同的项目中共享，那么这时，你就可能希望将项目分开为前端和后端两个repo，如何管理这种情况呢？一个比较好的方案就是使用git的submodule功能。
假设我们的父repo在prepo目录，sumodule newtestrepo希望放在prepo/submods/newtestrepo这个目录,首先我们cd submods目录，
1. 在submods目录下执行：git submodule add https://github.com/cnweibo/newtestrepo.git 这个命令将在prepo目录下创建.gitmodules文件以及在prepo/submods/目录下创建newtestrepo目录用于保存newtestrepo内容。.gitmodules文件包含以下内容：
[submodule "submods/newtestrepo"] path = submods/newtestrepo url = https://github.com/cnweibo/newtestrepo.git
注意：在1.8版本git之前，上述命令必须在preop的root目录下执行！
2.执行git status，则发现有两个变更需要commit
(newcnweibo_branch)*$ git status On branch newcnweibo_branch Your branch is up-to-date with 'origin/newcnweibo_branch'. Changes to be committed: (use "git reset HEAD <file>..." to unstage) new file: ../.gitmodules new file: newtestrepo
3.执行git commit将增加submodule newtestrepo这个commit做一下提交
(newcnweibo_branch)*$ git commit -m "intro newtestrepo submodule" [newcnweibo_branch 2bb87a3] intro newtestrepo submodule 2 files changed, 4 insertions(+) create mode 100644 .gitmodules create mode 160000 submods/newtestrepo
4. git push 将上述修改放到preop的中央库中去以便其他teammember使用
5. 如何在prepo这个项目中修改submodule呢？git checkout master 进入newtestrepo的master branch, 修改文件，commit，随后git push，注意这里的push是将submodule push到中央库中。注意：submodule/newtestrepo目录下不再有.git目录，而只有一个.git文件，这一点很是奇妙！
在我们的submodule的目录中，你查看一下.git这个文件（不是文件夹哦！）的内容实际上指示列出来一个引用gitdir指向prepo的.git/modules/xxxdir/xxxrepo
(detached*)$ cat .git gitdir: ../../.git/modules/submods/newtestrepo
6. 注意，这时如果我们到prepo的目录中，git status发现我们又有了两个没有commit的commit。
随后我们需要将prepo也做push以便将上述两个commits递交.
之所以在submodule中修改并且push后还要在prepo中push是因为我们的父repo其实是引用了子module的一个snapshot，子module修改后，父repo并没有修改对版本的引用，因此需要commit来反映这个变化。
7.当我们另外一个teammember clone这个prepo时，注意，prepo里面所包含的submodule本身目录都为空！
*$ pwd /home/cabox/workspace/testgit *$ git clone https://github.com/cnweibo/githubtest.git Cloning into 'githubtest'... remote: Counting objects: 53, done. remote: Total 53 (delta 0), reused 0 (delta 0), pack-reused 53 Unpacking objects: 100% (53/53), done. Checking connectivity... done. *$ ls githubtest *$ cd githubtest/ (newcnweibo_branch)$ ls localconfig newweibobranchfile readmegithub submods (newcnweibo_branch)$ git status On branch newcnweibo_branch Your branch is up-to-date with 'origin/newcnweibo_branch'. nothing to commit, working directory clean (newcnweibo_branch)$ cd submods/ (newcnweibo_branch)$ ls newtestrepo (newcnweibo_branch)$ cd newtestrepo/ (newcnweibo_branch)$ ls (newcnweibo_branch)$ ls -la total 8 drwxrwxr-x 2 cabox cabox 4096 May 30 01:08 . drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 .. (newcnweibo_branch)$
但是我们的prepo: githubtest本身却有一个文件.gitmodules,它列出了在prepo中是如何引用sub module的
(newcnweibo_branch)$ pwd /home/cabox/workspace/testgit/githubtest (newcnweibo_branch)$ ls -la total 32 drwxrwxr-x 4 cabox cabox 4096 May 30 01:08 . drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 .. drwxrwxr-x 8 cabox cabox 4096 May 30 01:11 .git -rw-rw-r-- 1 cabox cabox 108 May 30 01:08 .gitmodules -rw-rw-r-- 1 cabox cabox 1747 May 30 01:08 localconfig -rw-rw-r-- 1 cabox cabox 66 May 30 01:08 newweibobranchfile -rw-rw-r-- 1 cabox cabox 185 May 30 01:08 readmegithub drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 submods (newcnweibo_branch)$ cat .gitmodules [submodule "submods/newtestrepo"] path = submods/newtestrepo url = git@github.com:cnweibo/newtestrepo.git
8.为了能够正常使用submodule,我们必须在新clone的prepo中执行 git submodule init命令，执行该命令时：git将通过检查.gitmodules文件中的
[submodule "submods/newtestrepo"] 这个section，为每一个submodule都正在prepo的.git/config文件中增加一条
[submodule "submods/newtestrepo"]
url = git@github.com:cnweibo/newtestrepo.git 信息！
(newcnweibo_branch)$ pwd /home/cabox/workspace/testgit/githubtest (newcnweibo_branch)$ git submodule init Submodule 'submods/newtestrepo' (git@github.com:cnweibo/newtestrepo.git) registered for path 'su bmods/newtestrepo' (newcnweibo_branch)$ ls -la total 32 drwxrwxr-x 4 cabox cabox 4096 May 30 01:08 . drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 .. drwxrwxr-x 8 cabox cabox 4096 May 30 01:17 .git -rw-rw-r-- 1 cabox cabox 108 May 30 01:08 .gitmodules -rw-rw-r-- 1 cabox cabox 1747 May 30 01:08 localconfig -rw-rw-r-- 1 cabox cabox 66 May 30 01:08 newweibobranchfile -rw-rw-r-- 1 cabox cabox 185 May 30 01:08 readmegithub drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 submods (newcnweibo_branch)$ cat .git/ HEAD config hooks/ info/ objects/ refs/ branches/ description index logs/ packed-refs (newcnweibo_branch)$ cat .git/config [core] repositoryformatversion = 0 filemode = true bare = false logallrefupdates = true [remote "origin"] url = https://github.com/cnweibo/githubtest.git fetch = +refs/heads/*:refs/remotes/origin/* [branch "newcnweibo_branch"] remote = origin merge = refs/heads/newcnweibo_branch [submodule "submods/newtestrepo"] url = git@github.com:cnweibo/newtestrepo.git (newcnweibo_branch)$
9.随后我们在prepo再执行git submodule update，这时git将通过Internet把所有的submodule clone进入我们的子目录中（由.gitmodules文件来决定放到哪个子目录中）
(newcnweibo_branch)$ pwd /home/cabox/workspace/testgit/githubtest (newcnweibo_branch)$ ls -la submods/newtestrepo/ total 8 drwxrwxr-x 2 cabox cabox 4096 May 30 01:08 . drwxrwxr-x 3 cabox cabox 4096 May 30 01:08 .. (newcnweibo_branch)$ git submodule update Cloning into 'submods/newtestrepo'... Warning: Permanently added 'github.com,192.30.252.129' (RSA) to the list of known hosts. Permission denied (publickey). fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists. Clone of 'git@github.com:cnweibo/newtestrepo.git' into submodule path 'submods/newtestrepo' failed $ ssh -T git@github.com Hi kidsit! You've successfully authenticated, but GitHub does not provide shell access. $ exit exit (newcnweibo_branch)*$ pwd /home/cabox/workspace/testgit/githubtest (newcnweibo_branch)*$ git submodule update Cloning into 'submods/newtestrepo'... Warning: Permanently added the RSA host key for IP address '192.30.252.130' to the list of known hosts. remote: Counting objects: 6, done. remote: Compressing objects: 100% (3/3), done. remote: Total 6 (delta 0), reused 3 (delta 0), pack-reused 0 Receiving objects: 100% (6/6), done. Checking connectivity... done. Submodule path 'submods/newtestrepo': checked out '6c495d0229fcd15fd7e0bb3cded878d0692988a5' (newcnweibo_branch)$ git submodule update^C (newcnweibo_branch)$ ls -la submods/newtestrepo/ .git README.md changefromprepo (newcnweibo_branch)$ ls -la submods/newtestrepo/ .git README.md changefromprepo (newcnweibo_branch)$ ls -la submods/newtestrepo/ total 20 drwxrwxr-x 2 cabox cabox 4096 May 30 01:32 . drwxrwxr-x 3 cabox cabox 4096 May 30 01:32 .. -rw-rw-r-- 1 cabox cabox 47 May 30 01:32 .git -rw-rw-r-- 1 cabox cabox 14 May 30 01:32 README.md -rw-rw-r-- 1 cabox cabox 29 May 30 01:32 changefromprepo (newcnweibo_branch)$ cat submods/newtestrepo/.git gitdir: ../../.git/modules/submods/newtestrepo