Git基本概念及操作（3）
如果使用传统的如CC开发的话，刚开始进行GIT开发可能不是太适应。这个主要是有些概念不一样。比喻在CC中，我们一般是围绕一个主分支进行开发，对一个文件来说，在主分支上会生成不同的版本。同样，我们在每一个版本下面创立新的次分支，在次分支上也会生出很多版本。最后合到主分支，产生下一个版本。那么在GIT中是如何实现这些关联呢？GIT中同样有分支、版本概念。但是没有Configspec概念。tag概念同LABEL概念类似。当然这些概念都同GIT中是如何管理文件版本相关的。首先我们来看GIT是如何将文件对象化管理的，前面我们说GIT同其它版本管理系统不一样是GIT每个版本都不是保存变更，而是全保存。那么如果全保存的话，显然会带来相当大的硬盘开销，其弊端非常明显，那么GIT是怎么消除这个弊端的呢？GIT利用了HASH算法，我们知道在目前已知道的算法中，HASH（SHA-1）算法产生的唯一性还是非常强的。也就是说虽然在工作区域是一个普通文件，但是在仓库中保存的是一个HASH值，由这个HASH值来表示文件，自然空间节省很多。GIT中将这个HASH值称之为对象。这些对象通常是由提交版本、子目录、文件的HASH值组成。对每一个对象通常按类型、大小和内容进行管理。其中最主要的是类型分为三种：
1）blob 这种类型对象是用来存储文件数据，GIT中表现为一个HASH值，如下图所示：
在以前GIT版本中会有一个单独的目录保存这些文件生成的HASH值通常是.git/objects/fc现在不在有这个fc目录，直接取生成的HASH的前两个值作目录。如下所示，注意在未提交之前，目录是不诞生对象。
[root@wrlinux3 mygit]# git add . [root@wrlinux3 mygit]# ls -latr .git/objects/ total 28 drwxr-xr-x. 2 root root 4096 Apr 28 13:34 pack drwxr-xr-x. 2 root root 4096 Apr 28 13:34 info drwxr-xr-x. 2 root root 4096 Apr 28 13:35 f8 drwxr-xr-x. 2 root root 4096 Apr 28 13:35 d2 drwxr-xr-x. 2 root root 4096 Apr 28 13:35 52 drwxr-xr-x. 7 root root 4096 Apr 28 13:35 .. drwxr-xr-x. 7 root root 4096 Apr 28 13:35 . [root@wrlinux3 mygit]# git hash-object README 52c897fedf9f8728e953a149b7be3b5829a07b1c [root@wrlinux3 mygit]# git hash-object main.c f8b643afbf2c84dc03b777743d3e53a22045cf49 [root@wrlinux3 mygit]# git hash-object testdir/test.c d22409509cd2f0a420e0cb6355008a9676656961
[root@wrlinux3 mygit]# git show d224 int test() { return 0; } [root@wrlinux3 mygit]# git show f8b6 int main() { return 0; } [root@wrlinux3 mygit]# git show 52c8 new readme
[root@wrlinux3 mygit]# git cat-file -t 52c8 blob [root@wrlinux3 mygit]# git cat-file -t f8b6 blob [root@wrlinux3 mygit]# git cat-file -t d224 blob
2）tree 是一个simple 对象，它的内容就是一个指针表，指针指向的就是这个目录下的所有文件及子目录对象。也就是说这个对象记录了当前目录的元信息。如下图所示：
正如前面所说，tree也是一串HASH值，这串HASH存储的是元信息。用来表示目录层次关系的。这和一般的SCM系统中是不一样，一般的SCM中目录同文件是一样的保存为版本对象的。
[root@wrlinux3 mygit]# git commit -a -m "init commit" [master (root-commit) 4cfc524] init commit Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
3 files changed, 9 insertions(+) create mode 100644 README create mode 100644 main.c create mode 100644 testdir/test.c [root@wrlinux3 mygit]# ls -latr .git/objects/ total 40 drwxr-xr-x. 2 root root 4096 Apr 28 13:34 pack drwxr-xr-x. 2 root root 4096 Apr 28 13:34 info drwxr-xr-x. 2 root root 4096 Apr 28 13:35 f8 drwxr-xr-x. 2 root root 4096 Apr 28 13:35 d2 drwxr-xr-x. 2 root root 4096 Apr 28 13:35 52 drwxr-xr-x. 2 root root 4096 Apr 28 14:12 bc drwxr-xr-x. 2 root root 4096 Apr 28 14:12 a8 drwxr-xr-x. 2 root root 4096 Apr 28 14:12 4c drwxr-xr-x. 10 root root 4096 Apr 28 14:12 . drwxr-xr-x. 8 root root 4096 Apr 28 14:12 .. [root@wrlinux3 mygit]# ls -latr .git/objects/4c/fc5245433ac0b9277892ccb6e7bd4347aa01ec -r--r--r--. 1 root root 133 Apr 28 14:12 .git/objects/4c/fc5245433ac0b9277892ccb6e7bd4347aa01ec [root@wrlinux3 mygit]# ls -latr .git/objects/a8/8383ae64b09e71666adc12f4bf4760857f8c55 -r--r--r--. 1 root root 115 Apr 28 14:12 .git/objects/a8/8383ae64b09e71666adc12f4bf4760857f8c55 [root@wrlinux3 mygit]# ls -latr .git/objects/bc/a9ca7434467d22451fc370375dbab1a8930433 -r--r--r--. 1 root root 51 Apr 28 14:12 .git/objects/bc/a9ca7434467d22451fc370375dbab1a8930433 [root@wrlinux3 mygit]# git cat-file -t bca9 tree [root@wrlinux3 mygit]# git cat-file -t a883 tree [root@wrlinux3 mygit]# git cat-file -t 4cfc commit [root@wrlinux3 mygit]# git ls-tree bcag fatal: Not a valid object name bcag [root@wrlinux3 mygit]# git ls-tree bca9 100644 blob d22409509cd2f0a420e0cb6355008a9676656961 test.c [root@wrlinux3 mygit]# git ls-tree a883 100644 blob 52c897fedf9f8728e953a149b7be3b5829a07b1c README 100644 blob f8b643afbf2c84dc03b777743d3e53a22045cf49 main.c 040000 tree bca9ca7434467d22451fc370375dbab1a8930433 testdir [root@wrlinux3 mygit]#
3）commit 也是一种对象，但是这种对象存储是在提交这个点上的元信息，包括提交人、上次提交的版本对象指针及本次提交包含的树节点指针。如下图所示：
看到这里可能还不是很明白。那我们将面连接起来如下所示：
从这个图我们应该能够简单的明白了，在GIT中版本这种对象是在最高层，不像一般SCM中，对一个文件来说会记录不同的版本。而在GIT中不是这样的，它是对当下这个项目提交一个版本就进行一次快照。通俗的说就是一个版本包含当前快照下所有对象指针。这样从版本管理的角度来说，要管理的对象的是版本。不再是文件或者目录。这些东西统统可以缩成一个快照。如下图所示：
在实际上可以使用如下命令查看：
[root@wrlinux3 mygit]# git show -s --pretty=raw 4cfc commit 4cfc5245433ac0b9277892ccb6e7bd4347aa01ec tree a88383ae64b09e71666adc12f4bf4760857f8c55 author ROOT root <root@wrlinux3.nsn-nsn.net> 1335593557 +0800 committer ROOT root <root@wrlinux3.nsn-nsn.net> 1335593557 +0800
init commit [root@wrlinux3 mygit]#
通过上面的讲述我相信在Commit、Tree和Blob对象之三者的关系有了一个简单的认识，有人总结成了如下一张图，可以说是GIT实现的核心。
从这张图我们也就明白了前面所说了Commit对象作为GIT管理的核心。GIT的Commit对象它不是说针对单个文件，它是针对的整个项目的。另外还有一种对象，TAG对象。TAG对象是用来标记特定的Commit的，如下图所示，这里可能还不能够清楚解析，后面学习了分支之后会比较清楚的了解。
前面讲述了四种GIT对象，GIT对象是不可改变的（除了一些撤消操作之外），当然这里面的TAG对象有时候也可以称之为References引用，引用是可以移动，引用都是基本版本Commit对象的。因此从另外一个角度来说TAG也可以称之为引用。但实际上我们打了一个TAG之后是不能移动，他的特性与对象又一样。常用的引用主要分为分支、头、及远程分支。如下图所示：
从上图可以看出，HEAD,BRACH,REMOTE,TAG最下层都是Commit对象，也就是GIT管理的粒度是以Commit来决定。Commit相当于一个快照。我们可以从git目录查看到这种组织方式。
[root@wrlinux3 mygit]# cat .git/HEAD ref: refs/heads/master [root@wrlinux3 mygit]# ls -l .git/refs/ total 8 drwxr-xr-x. 2 root root 4096 Apr 28 14:12 heads drwxr-xr-x. 2 root root 4096 Apr 28 13:34 tags [root@wrlinux3 mygit]# ls -l .git/refs/tags/ total 0 [root@wrlinux3 mygit]# ls -l .git/refs/heads/ total 4 -rw-r--r--. 1 root root 41 Apr 28 14:12 master [root@wrlinux3 mygit]# cat .git/refs/heads/master 4cfc5245433ac0b9277892ccb6e7bd4347aa01ec [root@wrlinux3 mygit]# git cat-file -t 4cfc commit [root@wrlinux3 mygit]#
另外GIT也使用了优化算法，就是前面两个版本之间如果相同的对象没有发生改变的话，会直接采用链接的方式。如下图所示：
这样就可以省去很多对象的产生。前面我们对Reference对象没有特别强调，只是强调他们是可以随着分支移动。这一点与很多SCM系统不同，比喻说CC，分支拉出来就固定，分支上生长出版本。而GIT不同，GIT上的分支它是随着版本移动的。我们来研究如下：
1）分支 GIT中分支本质上是个指向Commit 对象的可变分支。默认情况下Git使用master作为分支的默认名字，每次一提交，master指针都会自动向前移动到下一个版本的commit对象上。当然你也可创建一个分支，使用命令git branch testing.如下图所示：
可以通过仓库显示如下：
[root@wrlinux3 mygit]# git branch testing [root@wrlinux3 mygit]# git branch * master testing
[root@wrlinux3 mygit]# cat .git/HEAD ref: refs/heads/master
[root@wrlinux3 mygit]# cat .git/refs/heads/master 4cfc5245433ac0b9277892ccb6e7bd4347aa01ec [root@wrlinux3 mygit]# cat .git/refs/heads/testing 4cfc5245433ac0b9277892ccb6e7bd4347aa01ec [root@wrlinux3 mygit]#
这时我们可以看master和testing都指向4cfc Commit对象，同时master前面带有*号,HEAD中指向master，表示当前工作分支还在master上，如果想要在testing上工作，就必须修改HEAD指针。使用git checkout testing就可以切换了。
[root@wrlinux3 mygit]# git checkout testing Switched to branch 'testing' [root@wrlinux3 mygit]# cat .git/HEAD ref: refs/heads/testing [root@wrlinux3 mygit]#
这时HEAD指针指向Testing分支了。
因为在GIT中创建和删除分支基本上不需要耗费计算机资源，因为分支在GIT表示的就是一个指向版本的指针（这个文件中保存的是版本对像的SHA值）。所以GIT是非常鼓励多建分支。那么很明显分支太多，也会带来版本合并的麻烦，所以一般来说建议尝试一个新功能在一个分支上开发完毕后，进行测试完成后就合并到主分支上再删除原来分支。下面我们围绕一个开发过程来进行分支的创建和合并。
对应的版本图如下：
对应的操作记录如下：
[root@wrlinux3 mygit]# git checkout -b iss53 Switched to a new branch 'iss53' [root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git commit -a -m 'add new variable to calculate the value on branch [iss53]' [iss53 2f1f64e] add new variable to calculate the value on branch [iss53] Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 5 insertions(+), 1 deletion(-) [root@wrlinux3 mygit]# git checkout master Switched to branch 'master' [root@wrlinux3 mygit]# git checkout -b 'hotfix' Switched to a new branch 'hotfix' [root@wrlinux3 mygit]# git branch * hotfix iss53 master testing [root@wrlinux3 mygit]# cd testdir/ [root@wrlinux3 testdir]# vi test.c [root@wrlinux3 testdir]# git commit -a -m 'fixed the issue" [root@wrlinux3 testdir]# cd .. > ls > ^C [root@wrlinux3 testdir]# cd .. [root@wrlinux3 mygit]# ls main.c README testdir [root@wrlinux3 mygit]# git commit -a -m 'fixed the issue" > ^C [root@wrlinux3 mygit]# git commit -a -m 'fixed the issue' [hotfix 85412f9] fixed the issue Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 4 insertions(+), 1 deletion(-) [root@wrlinux3 mygit]# git checkout master Switched to branch 'master' [root@wrlinux3 mygit]# git merge hotfix Updating 4cfc524..85412f9 Fast-forward testdir/test.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) [root@wrlinux3 mygit]# git branch -d hotfix Deleted branch hotfix (was 85412f9). [root@wrlinux3 mygit]# git branch iss53 * master testing [root@wrlinux3 mygit]# vi testdir/test.c [root@wrlinux3 mygit]# git checkout master Already on 'master' [root@wrlinux3 mygit]# git merge iss53 Merge made by the 'recursive' strategy. main.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) [root@wrlinux3 mygit]# git merge iss53 Already up-to-date. [root@wrlinux3 mygit]#
通过GIT进行合并，因为是基本文本方式合并，可能不与CC中图形化直观，但是一定要注意两点，一点是DIFF工具可改，二点是MERGE前的工作区应该是干净的。当提示一些CONFLIC时需要手工打这些文件进行修改。
[root@wrlinux3 mygit]# git mergetool No files need merging [root@wrlinux3 mygit]# git checkout testing Switched to branch 'testing' [root@wrlinux3 mygit]# ls main.c README testdir [root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git commit -a -m "test comments" [testing d2569cb] test comments Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 1 insertion(+) [root@wrlinux3 mygit]# git checkout master Switched to branch 'master' [root@wrlinux3 mygit]# git mergetool No files need merging [root@wrlinux3 mygit]# git merge testing Auto-merging main.c CONFLICT (content): Merge conflict in main.c Automatic merge failed; fix conflicts and then commit the result. [root@wrlinux3 mygit]# git status # On branch master # Unmerged paths: # (use "git add/rm <file>..." as appropriate to mark resolution) # # both modified: main.c # no changes added to commit (use "git add" and/or "git commit -a") [root@wrlinux3 mygit]# git merge testing error: 'merge' is not possible because you have unmerged files. hint: Fix them up in the work tree, hint: and then use 'git add/rm <file>' as hint: appropriate to mark resolution and make a commit, hint: or use 'git commit -a'. fatal: Exiting because of an unresolved conflict. [root@wrlinux3 mygit]# git status # On branch master # Unmerged paths: # (use "git add/rm <file>..." as appropriate to mark resolution) # # both modified: main.c # no changes added to commit (use "git add" and/or "git commit -a") [root@wrlinux3 mygit]# git commit -a -m "commit stat" [master 5e453fa] commit stat Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
[root@wrlinux3 mygit]# git st # On branch master nothing to commit (working directory clean)
前面讲了一些通用的分支操作，通过创建分支、合并分支并最终形成一个稳定的版本的过程，这个过程也是我们常见的SCM的管理职责，目前SCM人员需要制定主分支、开发分支、特性分支，通过管控这些分支来保证整个产品的质量的稳定。如下图所示：
上图中不同的开发分支，代表了产品成熟的不同的周期。通过维护这些分支的生命周期来维护产品的生命周期。这里还需要强调的我们前面所进行的操作都是向有操作，也就是MERGE操作，实际上还有一种操作叫REBASE操作，rebase比较复杂，轻易不要使用，因为rebase会将当前已提交的版本历史给删除掉了，这样的话，一旦别人根据这些开发的话会产生丢失版本情况。如下图所示：
对应的操作记录如下：
[root@wrlinux3 mygit]# git branch experiment [root@wrlinux3 mygit]# git branch experiment * master [root@wrlinux3 mygit]# git checkout -b experiment fatal: A branch named 'experiment' already exists. [root@wrlinux3 mygit]# git checkout -B experiment Switched to and reset branch 'experiment' [root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git commit -a -m 'commit on experiment' [experiment 774b7ee] commit on experiment Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 2 insertions(+), 3 deletions(-) [root@wrlinux3 mygit]# git checkout master Switched to branch 'master' [root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git commit -a -m 'commit on master' [master cbe0f99] commit on master Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 1 insertion(+), 2 deletions(-) [root@wrlinux3 mygit]# gitk [root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git commit -a -m 'commit on master more' [master 8f20d8c] commit on master more Committer: ROOT root <root@wrlinux3.nsn-nsn.net> Your name and email address were configured automatically based on your username and hostname. Please check that they are accurate. You can suppress this message by setting them explicitly:
git config --global user.name "Your Name" git config --global user.email you@example.com
After doing this, you may fix the identity used for this commit with:
git commit --amend --reset-author
1 file changed, 1 insertion(+) [root@wrlinux3 mygit]# git checkout experiment Switched to branch 'experiment' [root@wrlinux3 mygit]# git rebase master
It seems that there is already a rebase-apply directory, and I wonder if you are in the middle of another rebase. If that is the case, please try git rebase (--continue | --abort | --skip) If that is not the case, please rm -fr /work/bongos/mygit/.git/rebase-apply and run me again. I am stopping in case you still have something valuable there. [root@wrlinux3 mygit]# rm -fr /work/bongos/mygit/.git/rebase-apply [root@wrlinux3 mygit]# git rebase master First, rewinding head to replay your work on top of it... Applying: commit on experiment Using index info to reconstruct a base tree... Falling back to patching base and 3-way merge... Auto-merging main.c CONFLICT (content): Merge conflict in main.c Failed to merge in the changes. Patch failed at 0001 commit on experiment
When you have resolved this problem run "git rebase --continue". If you would prefer to skip this patch, instead run "git rebase --skip". To check out the original branch and stop rebasing run "git rebase --abort".
[root@wrlinux3 mygit]# vi main.c [root@wrlinux3 mygit]# git rebase --continue main.c: needs merge You must edit all merge conflicts and then mark them as resolved using git add [root@wrlinux3 mygit]# git add . [root@wrlinux3 mygit]# git rebase --continue Applying: commit on experiment [root@wrlinux3 mygit]# git st # On branch experiment nothing to commit (working directory clean) [root@wrlinux3 mygit]# gitk [root@wrlinux3 mygit]# git branch * experiment master [root@wrlinux3 mygit]#
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