git checkout commit and delete newer commits

Question

I have made bad commits in the github and i want to do the following things:

1. checkout to an old commit from a different branch than master
2. make this commit my last commit(delete those after this)
3. make this branch as master branch and delete all the others branches
4. merge master branch to a new one

Is it possible to do this?

Answer 1

The key command is:

git checkout -B master aSHA1

If you know where you want to reset your master branch, that is the way to do it in one line.

From git checkout man page:

-B <new_branch> 

Creates the branch <new_branch> and start it at <start_point>;
if it already exists, then reset it to <start_point>. This is equivalent to running "git branch" with "-f";

Then you can force push it:

git push --force

That will reset origin/master (the master branch on the GitHub side) to your old SHA1.

---

Note:

With Git 2.40 (Q1 2023), "checkout -b/-B" is clarified, and explains how "git branch [-f]"^(man) is similar, but different in the documentation.

See commit fedb8ea (19 Jan 2023) by Junio C Hamano (gitster).
^{(Merged by Junio C Hamano -- gitster -- in commit 06f2b5f, 30 Jan 2023)}

checkout: document -b/-B to highlight the differences from "git branch"

The existing text read as if "git checkout -b/-B name"^(man) were equivalent to "git branch [-f] name"^(man), which clearly was not what we wanted to say.

git checkout now includes in its man page:

Creates the branch <new-branch>, start it at <start-point>; if it already exists, then reset it to <start-point>.
And then check the resulting branch out.
This is equivalent to running "git branch" with "-f" followed by "git checkout" of that branch;