git merge vs git rebase

branches status before merge or rebase

branches status after git merge

branches status after git rebase

git merge and git rebase test scripts

git-merge-test.sh测试脚本

[ -a git-merge-test.git ] && rm -rf git-merge-test.git
[ -a git-merge-logs ] && rm -rf git-merge-logs

git --bare init git-merge-test.git
git clone git-merge-test.git git-merge-logs

cd git-merge-logs/
echo "14:59:+0800" > v01-master.txt
git add -A && git commit -m "v01-master"
echo "15:02:+0800" > v02-master.txt
git add -A && git commit -m "v02-master"
echo "15:03:+0800" > v03-master.txt
git add -A && git commit -m "v03-master"
git push origin master

git checkout -b feature
echo "15:04:+0800" > v04-feature.txt
git add -A && git commit -m "v04-feature"
echo "15:06:+0800" > v05-feature.txt
git add -A && git commit -m "v05-feature"
echo "15:09:+0800" > v06-feature.txt
git add -A && git commit -m "v06-feature"

git checkout master
echo "15:10:+0800" > v07-master.txt
git add -A && git commit -m "v07-master"
echo "15:12:+0800" > v08-master.txt
git add -A && git commit -m "v08-master"

git checkout feature
echo "15:15:+0800" > v09_feature.txt
git add -A && git commit -m "v09_feature"

git checkout master
# This creates a new "merge commit" in the feature branch that ties together the histories of both branches
git merge feature -m "merge feature into master"

git push

git log --decorate --graph --oneline --stat --all

执行结果

git merge简要说明

1. git merge会产生一次新的commit版本，并且此版本会有2个或者更多个父提交，可以通过HEAD^1，HEAD^2，HEAD^n方法来引用此提交的父提交。
2. git merge是非破坏性的(non-destructive)操作，没有负作用，并且被合并的这些分支都可以git push发布到remote公共仓库中。
3. 由于每次合并会产生一次新的提交历史，如果主分支非常活跃，就会产生很多这种合并分支的提交历史，会对项目本身的提交历史产生污染。

git-rebase-test.sh测试脚本

[ -a git-rebase-test.git ] && rm -rf git-rebase-test.git
[ -a git-rebase-logs ] && rm -rf git-rebase-logs

git --bare init git-rebase-test.git
git clone git-rebase-test.git git-rebase-logs

cd git-rebase-logs/
echo "14:59:+0800" > v01-master.txt
git add -A && git commit -m "v01-master"
echo "15:02:+0800" > v02-master.txt
git add -A && git commit -m "v02-master"
echo "15:03:+0800" > v03-master.txt
git add -A && git commit -m "v03-master"
git push origin master

git checkout -b feature
echo "15:04:+0800" > v04-feature.txt
git add -A && git commit -m "v04-feature"
echo "15:06:+0800" > v05-feature.txt
git add -A && git commit -m "v05-feature"
echo "15:09:+0800" > v06-feature.txt
git add -A && git commit -m "v06-feature"

git checkout master
echo "15:10:+0800" > v07-master.txt
git add -A && git commit -m "v07-master"
echo "15:12:+0800" > v08-master.txt
git add -A && git commit -m "v08-master"

git checkout feature
# Instead of using a merge commit, rebasing re-writes the project history by creating brand new commits for each commit in the original branch.
# The golden rule of git rebase is to never use it on public branches.
git rebase master
echo "15:15:+0800" > v09_feature.txt
git add -A && git commit -m "v09_feature"

git checkout master
git merge feature # -m "rebasing the feature branch onto master, and merge the feature branch does not create new commit."

git push

git log --decorate --graph --oneline --stat --all

# Don't push temporary feature branch to remote, and can delete this temporary branch.
git branch -d feature

# Interactive Rebasing to change history of commits
# git rebase -i master

执行结果

git rebase简要说明

1. git rebase 会产生一个修改过的很干净的项目提交历史，例如在feature分支上git rebase master，会将分支上的提交添加到master分支的最顶端(by creating brand new commits for each commit in origin branch)。
2. git rebase操作有负面效果，影响项目提交历史的安全性(safety)和可追溯性(traceability)。
3. git rebase -i master可以交互式压缩提交(squash)或者忽略指定提交。
4. git rebase使用黄金守则: NEVER USE IT ON PUBLIC BRANCHES.
5. 不要将主分支rebase到特性分支上(don't rebased master onto your feature branch: git rebase feature)。
6. 其他开发者提交的新特性和修改(如patch)应该创建一个临时分支apply这些修改，再使用git merge合并修改到主分支上，而不是使用git rebase方式操作，因为rebase操作修改了提交历史，很难追溯哪些提交新增了这些特性和修改。

在没有完全理解什么时候使用git rebase时，尽量在特性分支上再开个临时分支出来进行rebase操作，如下所示：

 git checkout feature
# [Create new branch from feature branch for rebasing]
 git checkout -b temporary-branch
 git rebase -i master
# [Clean up the history]
# pick ba7da2b v04-feature¬
# pick 5f45d15 v05-feature¬
# pick 43df101 v06-feature¬
# pick be55443 v09_feature¬
# Rebase 01642c6..be55443 onto 01642c6 (       4 TODO item(s))¬
#¬
# Commands:¬
# p, pick = use commit¬
# r, reword = use commit, but edit the commit message¬
# e, edit = use commit, but stop for amending¬
# s, squash = use commit, but meld into previous commit¬
# f, fixup = like "squash", but discard this commit's log message¬
# x, exec = run command (the rest of the line) using shell¬
#¬
# These lines can be re-ordered; they are executed from top to bottom.¬
#¬
# If you remove a line here THAT COMMIT WILL BE LOST.¬
#¬
# However, if you remove everything, the rebase will be aborted.¬
#¬
# Note that empty commits are commented out¬
 git checkout master
 git merge temporary-branch
 git branch -d temporary-branch

References

1. Merging vs. Rebasing