Git Revert to a Good Commit

This post was originally published on my blog.

Today my co-worker asked me how to revert to an old and good commit on master ( after you accidentally merged dev branch to master ).
There are many ways to do that, for example we can use reset to discard all wrong commit to revert back, like undo when writing text.
You can find others way onthis topic on Stack Overflow
But the problem is too many options, we dont know which one is good, or even dont need to understand - it just works.

So i will introduce you my traditional way and extremely accurate.

First of all, after you did a commit and merge you should not delete commit with git reset (Undo) thus we want to repair it instead of delete it.

Next, in general, between any two commit in you git repository is a different line of codes, which you can always see wit the git diff command.
Revert an old commit is to delete these different.

Suppose we are in master branch and we want to revert back to commit 8398e5b.
So you first need to clone/checkout master branch to a new clean folder ( the directory where you type git status -u and you see nothing there.)

$ git status -u           # make sure it is clean
$ git checkout master     # be sure you are in master
$ git diff HEAD..8398e5b  > patch.diff

In the last command, the order is very important.
The commit we want to revert to must be in the end.
Then you apply this diff

$ patch -p1 < patch.diff
$ git status -u

Check if any new files have not been commited in the result of git status -u above, these are the files included in the old commit 8398e5b:

$ git add some/new/files.txt
$ git commit -m "Revert to 8398e5b, thanks to @kn"

To be sure, check the results:

$ git diff 8398e5b..

This time it should appear ... nothing, that is, you have completely restored your old commit. 🎉🎉🎉

This way is always successful, simple, you can even take advantage of a few things to adjust.
And above all, instead of having to understand a bunch of commands like git revert, git reset,... you just focus on understanding the problem of diff, diff, and diff