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Git - Reset

You should have previously read about branches.

NOTE reset is really an advanced command, and for many users it's not something they need (or should) be running. That said, it's a fairly powerful and useful command in the hands of someone who understands the risks.

Checkout

Let's reorientate ourselves.

> git log --graph --oneline --decorate master

* 41ce7fa (HEAD, master) Third commit
* 672e562 Second commit
* 406bb3b Initial commit

Right, so currently HEAD is pointing to master.

> cat .git/HEAD

ref: refs/heads/master

> cat .git/refs/heads/master

41ce7faa5b8b2cf480d44c3b12f0545fed9683c3

Previously we saw that checkout changes the contents of the HEAD file (and updates files in the working directory, but that's less important).

Reset

Imagine that you want to "undo" a commit on your current branch. Why, might you ask? There might be a number of reasons.

  • You've just added a commit with a spelling mistake
  • You've added a file by mistake

Given what we (now) know about Git, there is something dodgy we could do - update the master ref with the previous commit.

echo "672e562c008009ec391e11a1fa3574af39428ae1" > .git/refs/heads/master

> git log --graph --oneline --decorate master

* 672e562 (HEAD, master) Second commit
* 406bb3b Initial commit

And that's exactly what reset does. Let's go back to where we were.

> git reset 41ce7faa5b8b2cf480d44c3b12f0545fed9683c3
Unstaged changes after reset:
M       file.txt

> cat .git/refs/heads/master
41ce7faa5b8b2cf480d44c3b12f0545fed9683c3

However, Git has told us something interesting after running reset. Something about our files being "unstaged".

It's important to note that by default reset doesn't modify any of your working files, only the ref. This isn't any different than if we had tweaked the ref file manually.

What do you expect file.txt to contain?

Hard

So how do we update the files at the same time?

> git reset --hard 672e562c008009ec391e11a1fa3574af39428ae1
HEAD is now at 672e562 Second commit

WARNING reset --hard can be very dangerous, it updates files on disk without prompting. If you have unsaved/uncommitted changes Git will quite happily undo those changes without so much as a "Do you wish to continue (y/n)".

Next

Check out the section on reflog to know what happens to commits that you undo.