如果你的專案中有很多第三方的lib，你希望使用它，並且也希望可能對該lib做修改並且貢獻到原始的專案中去，或者你的專案希望模組化，分為幾個repo單獨維護，那麼git subtree就是一個選擇。git subtree管理的子專案在父專案中作為一個完整的程式碼copy存在，並不包含歷史資訊。綜合考慮git subtree和git submodule的優缺點，一個可行的管理策略是：使用git subtree對父專案（在該父專案中可以臨時將子專案資料夾加入到tracking中）做split,將需要單獨存在的目錄形成一個子專案，並且push出來。隨後再父專案的開發中，將子專案目錄加入git ignore中，不做版本跟蹤，子專案目錄以另一個git repo存在（git clone子專案）。所有本地副專案中的修改如果需要分享的話，只在子專案repo中提交併且push，其他成員只需pull這個子專案repo。這個策略是一個折中。
git subtree是附屬於git的附加工具，預設情況下並不會安裝。https://github.com/git/git/tree/master/contrib/subtree。注意新版本的git中已經預設包含了subtree這個子命令
#git clone https://github.com/git
# cd git# make prefix=/usr/local/git install
# echo "export PATH=$PATH:/usr/local/git/bin" >> /etc/bashrc # source /etc/bashrc
# cd contrib/subtree
# makeprefix=/usr/local/git/bin
# makeprefix=/usr/local/git/bin install
# makeprefix=/usr/local/git/bin install-doc
# cp git-subtree/usr/local/git/bin :一旦將git-bustree copy到git/bin目錄下，執行git subtree 命令時，git就可以查詢並且呼叫到這個subtree子命令了！
就ok了！在本文中，我們假設我們工作在subtree-p這個專案中（放在github上的一個repo），而在該專案中，我們需要subtree-sub這個子模組（或者說是Lib）.
當前我們的subtree-p專案中，沒有引入任何subtree-sub這個模組，現在我們通過以下命令來實現：
(master)$ git remote add subtree-sub-remote https://github.com/cnweibo/subtree-sub.git (master)$ git remote -v origin https://github.com/cnweibo/subtree-p.git (fetch) origin https://github.com/cnweibo/subtree-p.git (push) subtree-sub-remote https://github.com/cnweibo/subtree-sub.git (fetch) subtree-sub-remote https://github.com/cnweibo/subtree-sub.git (push) (master)$ pwd /home/cabox/workspace/subtreetest/subtree-p (master)$ ls README.md (master)$ git subtree add --prefix=mysubtree subtree-sub-remote master git fetch subtree-sub-remote master warning: no common commits remote: Counting objects: 3, done. remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0 Unpacking objects: 100% (3/3), done. From https://github.com/cnweibo/subtree-sub * branch master -> FETCH_HEAD * [new branch] master -> subtree-sub-remote/master Added dir 'mysubtree' (master)$ ls README.md mysubtree (master)$ ls -la mysubtree/ total 12 drwxrwxr-x 2 cabox cabox 4096 May 31 02:14 . drwxrwxr-x 4 cabox cabox 4096 May 31 02:14 .. -rw-rw-r-- 1 cabox cabox 14 May 31 02:14 README.md
上面的一系列命令中，首先在subtree-p專案目錄中引入了subtree-sub這個子專案，它存放在目錄mysubtree中，和我們的subtree-sub-remote這個remote相對應。這時，subtree-p專案由於增加了新的目錄（subtree-sub子專案repo），因此需要commit和push.
注意：在mysubtree這個子目錄中是沒有任何.git檔案的，它實際上是subtree-sub這個子專案repo的純程式碼copy！！ git subtree add命令本身不會像git submodule add一樣新增任何meta data@！！
(master)$ git s 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 (master)$ git push Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 5, done. Delta compression using up to 12 threads. Compressing objects: 100% (3/3), done. Writing objects: 100% (5/5), 528 bytes | 0 bytes/s, done. Total 5 (delta 0), reused 0 (delta 0) To https://github.com/cnweibo/subtree-p.git 67da25d..c1e6018 master -> master
這時，我們檢視在github上的subtree-p repo，你就可以看到這個父專案已經引入了mysubtree（代表了subtree-sub repo！）
現在我們在parent專案中對子專案做修改並且commit, push：
(master)$ echo "kidsit add subfile1 within parent" >subfile1 (master)*$ git s On branch master Your branch is up-to-date with 'origin/master'. Untracked files: (use "git add <file>..." to include in what will be committed) subfile1 nothing added to commit but untracked files present (use "git add" to track) (master)*$ git a . (master)*$ git ci "kidsit add subfile1 within parent" [master ed30a68] kidsit add subfile1 within parent 1 file changed, 1 insertion(+) create mode 100644 mysubtree/subfile1 (master)$ git s 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 (master)$ git push Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 4, done. Delta compression using up to 12 threads. Compressing objects: 100% (3/3), done. Writing objects: 100% (4/4), 398 bytes | 0 bytes/s, done. Total 4 (delta 0), reused 0 (delta 0) To https://github.com/cnweibo/subtree-p.git c1e6018..ed30a68 master -> master (master)$
這時我們再檢視github上的subtree-p我們就能看到我們對子專案的新的改動了：
我們來繼續模擬程式碼開發過程：修改subtree-p和subtree-sub後，統一一次性做一個提交：
(master)$ echo "kidsit change README for p" >README.md (master)*$ echo "kidsit add new subfileNew2 " >mysubtree/subfileNew2 (master)*$ git s On branch master Your branch is up-to-date with 'origin/master'. 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: README.md Untracked files: (use "git add <file>..." to include in what will be committed) mysubtree/subfileNew2 no changes added to commit (use "git add" and/or "git commit -a") (master)*$ git a . (master)*$ git ci "kidsit change p and s in ONE commit" [master a27d335] kidsit change p and s in ONE commit 2 files changed, 2 insertions(+), 2 deletions(-) create mode 100644 mysubtree/subfileNew2 (master)$ git push Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 5, done. Delta compression using up to 12 threads. Compressing objects: 100% (3/3), done. Writing objects: 100% (5/5), 461 bytes | 0 bytes/s, done. Total 5 (delta 0), reused 0 (delta 0) To https://github.com/cnweibo/subtree-p.git ed30a68..a27d335 master -> master (master)$
這時，我們要注意，我們再subtree-p專案中對父專案和子專案分別都做了修改，但是子專案本身的repo還沒有做過任何更新，它根本不知道你曾經做過修改，這時，如果你希望將你對子專案的修改貢獻出來，可以執行git subtree push命令
(master)$ git push Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 5, done. Delta compression using up to 12 threads. Compressing objects: 100% (3/3), done. Writing objects: 100% (5/5), 461 bytes | 0 bytes/s, done. Total 5 (delta 0), reused 0 (delta 0) To https://github.com/cnweibo/subtree-p.git ed30a68..a27d335 master -> master (master)$ git subtree push --prefix=mysubtree/ subtree-sub-remote master git push using: subtree-sub-remote master Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 6, done. Delta compression using up to 12 threads. Compressing objects: 100% (4/4), done. Writing objects: 100% (6/6), 563 bytes | 0 bytes/s, done. Total 6 (delta 1), reused 0 (delta 0) To https://github.com/cnweibo/subtree-sub.git 9a0ef93..001c5d8 001c5d86c4da43ab676aaeb49763d5353353159f -> master
上面我們分別對subtree-p這個父專案和subtree-sub這個子專案分別作了push操作分享（一般來說對子專案push是非常少見的動作，因為我們更多的是拿別人的lib來用而不是開發），這時如果我們去看github上的兩個專案repo的commit歷史，你會發現兩個repo上都有了同一個commit！！
正因為如此，我們最好是將父專案和子專案分別做commit，不要混淆在一起！這樣做的好處是git subtree push時，git會自動將對父專案和子專案的commit區分清楚的，對父專案的修改不會出現在子專案repo修改歷史中！
現在，我們假設另外一個人對subtree-sub這個repo獨立做了新的開發，有了一個commit,但是我們的subtree-p專案實際上對此毫不知情，我們怎麼能夠獲取別人對我們用到的子專案lib來更新呢？
在父專案目錄中執行git subtree pull命令：
(master)$ git subtree pull --prefix=mysubtree -squash subtree-sub-remote master You need to run this command from the toplevel of the working tree. (master)$ kidsit -bash: kidsit: command not found (master)$ pwd /home/cabox/workspace/subtreetest/subtree-p/mysubtree (master)$ cd .. (master)$ ls README.md mysubtree (master)$ pwd /home/cabox/workspace/subtreetest/subtree-p (master)$ git subtree pull --prefix=mysubtree subtree-sub-remote master remote: Counting objects: 3, done. remote: Compressing objects: 100% (3/3), done. remote: Total 3 (delta 0), reused 3 (delta 0), pack-reused 0 Unpacking objects: 100% (3/3), done. From https://github.com/cnweibo/subtree-sub * branch master -> FETCH_HEAD 001c5d8..55689fc master -> subtree-sub-remote/master Merge made by the 'recursive' strategy. mysubtree/standardaloneChangeInsubtreesubRepo | 1 + 1 file changed, 1 insertion(+) create mode 100644 mysubtree/standardaloneChangeInsubtreesubRepo (master)$ git lg * 4d2a47a (HEAD -> master) Merge commit '55689fc679ec0eeaf0a7853cd348309dad99fb96' from cnweibo update as standard dev |\ | * 55689fc (subtree-sub-remote/master) new content as a standard repo dev env for subtree-sub by cnweibo | * 001c5d8 kidsit change p and s in ONE commit | * ef33027 kidsit add subfile1 within parent * | a27d335 (origin/master, origin/HEAD) kidsit change p and s in ONE commit * | ed30a68 kidsit add subfile1 within parent * | c1e6018 Add 'mysubtree/' from commit '9a0ef93b82b73fee89dd3d5123d441680c2a7e40' |\ \ | |/ | * 9a0ef93 Initial commit * 67da25d p 2 * f05da04 Create README.md (master)$ ls .git/ README.md mysubtree/ (master)$ ls mysubtree/ README.md subfile1 standardaloneChangeInsubtreesubRepo subfileNew2 (master)$ ls mysubtree/ README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2 (master)$ git push Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 8, done. Delta compression using up to 12 threads. Compressing objects: 100% (8/8), done. Writing objects: 100% (8/8), 1.06 KiB | 0 bytes/s, done. Total 8 (delta 1), reused 0 (delta 0) To https://github.com/cnweibo/subtree-p.git a27d335..4d2a47a master -> master (master)$
上述命令主要是在subtree-p專案中pull別的獨立專案對subtree-sub子模組的貢獻，隨後git push到主專案的repo中獨立引用那一份修改。
接下來，我們來看新的專案成員進到我們的subtree-p專案中，他又該如何呢？
他首先需要做的是git clone yourParentRep, 隨後git rm -r subModule, git add ,git commit.這時他的working directory中不再含有subModule的程式碼，他需要做的是手工建立git remote add --prefix=submodDirectory submodRemote master/v1.0,注意：在這裡可以指定一個tag來選擇相應的歷史版本！！！ git subtree add submodRemote xxxx.git master獲取subrep程式碼到你的submodDirectory目錄，這樣他自己clone的repo就有了和submodRepo的聯絡了，他也可以對父專案和子專案來做貢獻了。
請參考： https://lostechies.com/johnteague/2014/04/04/using-git-subtrees-to-split-a-repository/
git subtree方式管理的parent專案最終是包含所有子模組的完整程式碼的（可以不含子模組的歷史），同時支援對子專案的contribute。
在一個前臺可能被share，後臺可能從一個換成另外一個，這樣的場景就比較適合使用subtree來管理。
接下來我們再看另外一種情況： 在一個專案中，當我們完整做完穩定版本後，我們希望將public前端拿出來作為一個獨立的repo來管理，這樣前端和後端完全可以獨立開發，要實現這個，就需要使用git subtree split命令了：
(master)$ ls README.md mysubtree (master)$ git subtree split --prefix=mysubtree -b splittedbranch Created branch 'splittedbranch' 55689fc679ec0eeaf0a7853cd348309dad99fb96 (master)$ git s On branch master Your branch is up-to-date with 'origin/master'. nothing to commit, working directory clean (master)$ git branch -a * master splittedbranch remotes/origin/HEAD -> origin/master remotes/origin/master (master)$ ls README.md mysubtree (master)$ git remote add splittedRemote https://github.com/cnweibo/splitted.git (master)$ git checkout splittedbranch Switched to branch 'splittedbranch' (splittedbranch)$ ls README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2 (splittedbranch)$ pwd /home/cabox/workspace/subtreesplit/superrepo (splittedbranch)$ ls README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2 (splittedbranch)$ git remote -v origin https://github.com/cnweibo/subtree-p.git (fetch) origin https://github.com/cnweibo/subtree-p.git (push) splittedRemote https://github.com/cnweibo/splitted.git (fetch) splittedRemote https://github.com/cnweibo/splitted.git (push) (splittedbranch)$ git push splittedRemote splittedbranch:master Username for 'https://github.com': kidsit Password for 'https://kidsit@github.com': Counting objects: 15, done. Delta compression using up to 12 threads. Compressing objects: 100% (11/11), done. Writing objects: 100% (15/15), 1.43 KiB | 0 bytes/s, done. Total 15 (delta 2), reused 0 (delta 0) To https://github.com/cnweibo/splitted.git 2d00c85..703b820 splittedbranch -> master
上面的一串命令將parent專案的mysubtree目錄被splitted到splittedbranch上（當你checkout這個splittedbranch時，你看到的就只有這個目錄的檔案了！），隨後將該branch上的內容push到remote中，這就形成了獨立的repo了。隨後你要做的是在父專案中，將該子資料夾刪除，通過git subtree來引用這個共享的repo
http://blog.nwcadence.com/git-subtrees/
注意：git filter-branch 也可以實現類似的git subtree split的功能，將一個repo的部分目錄分割為一個新的repo.
具體可參考： https://help.github.com/articles/splitting-a-subfolder-out-into-a-new-repository/
另外一篇文章： http://log.pardus.de/2012/08/modular-git-with-git-subtree.html 可以參考更好的思路