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Git - Reflog

You should have previously read about branches.

For the most part Git tries to keep you safe and make sure that nothing bad happens to your branches/commits. However, there are cases where you may accidentally "lose" a commit.

Not to fear! Git has a nifty way to find those missing commits.

Lost

At one point the graph looked something like this.

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

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

The HEAD is referencing master, which is at a different commit to hello.

Let's checkout hello.

> git checkout hello

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

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

And now for something a little crazy.

# Delete the master branch - weeeeeeee
> rm .git/refs/heads/master

> git log --graph --oneline --decorate hello

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

Oh no, where did it go? How do we get it back?!?

Warning

In many cases Git will actually try to prevent this ever happening. There are checks and warnings on the relevant Git commands that will stop you shooting yourself in the foot.

This goes without saying but it is always recommended to use Git commands rather than modifying the contents of the .git directory.

We manually deleted the master branch by deleting a file, can you work out how you would "normally" delete a branch?

Found

It's hard to imagine that just by deleting the ref file that we actually deleted the commit itself. The commit message has to be stored separately at least.

Does the commit still exist? Let's check by using the commit hash...

> git log --graph --oneline --decorate 41ce7fa

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

Phew! So it's still there. But what happens if we didn't know, or forgot the hash?

Ladies and gentlemen, I give you the...

Reflog

> git reflog HEAD

672e562 HEAD@{0}: checkout: moving from master to hello
41ce7fa HEAD@{1}: commit: Third commit
672e562 HEAD@{2}: commit: Second commit
406bb3b HEAD@{3}: commit (initial): Initial commit

Whenever you run commit or checkout commands, Git logs the the old and new commits. So for every change to your HEAD (and refs) Git actually tracks what happened. The reflog.

A (very bad) anology is a bank keeping a transaction log of changes to your account. Or at least you hope they do!

My advice - don't leave home without git reflog.

Can you re-create the master branch on that commit?

Do you think the log keeps growing forever? If you're interested read more at git reflog --help and git gc --help.

If you're interested in how that is actually stored in .git:

> cat .git/logs/HEAD

0000000000000000000000000000000000000000 406bb3bffbd02113ad5e2fecb4da364c01b20bb3 Charles O'Farrell <[email protected]> 1460853329 +1000   commit (initial): Initial commit
406bb3bffbd02113ad5e2fecb4da364c01b20bb3 672e562c008009ec391e11a1fa3574af39428ae1 Charles O'Farrell <[email protected]> 1460854615 +1000   commit: Second commit
672e562c008009ec391e11a1fa3574af39428ae1 41ce7faa5b8b2cf480d44c3b12f0545fed9683c3 Charles O'Farrell <[email protected]> 1460856405 +1000   commit: Third commit
41ce7faa5b8b2cf480d44c3b12f0545fed9683c3 672e562c008009ec391e11a1fa3574af39428ae1 Charles O'Farrell <[email protected]> 1460859487 +1000   checkout: moving from master to hello