HEADWhen we make commits to a Git repo, it tracks each of those commits in a log that you can visit. Let’s take a look at that now.
You can get the commit log by typing git log.
Let’s say I’m in a repo with a single commit where I’ve just added a file with the commit message “Added”.
$ git logproduces:
commit 5a02fede3007edf55d18e2f9ee3e57979535e8f2 (HEAD -> main)
Author: User Name <user@example.com>
Date: Thu Feb 1 09:24:52 2024 -0800
AddedIf I make another commit, we get a longer log:
commit 5e8cb52cb813a371a11f75050ac2d7b9e15e4751 (HEAD -> main)
Author: User Name <user@example.com>
Date: Thu Feb 1 12:36:13 2024 -0800
More output
commit 5a02fede3007edf55d18e2f9ee3e57979535e8f2
Author: User Name <user@example.com>
Date: Thu Feb 1 09:24:52 2024 -0800
AddedNotice that the most recent commit entry is at the top of the output.
There are a few things to notice in the log:
Also, we have those huge hex14 numbers after the word commit.
This is the commit ID or commit hash. This is universally unique number that you can use to identify a particular commit.
Normally you don’t need to know this, but it can be useful for going back in time or keeping track of commits in multi-developer projects.
We also see a bit at the top that says (HEAD -> main). What’s that about?
HEAD ReferenceWe’ve seen that each commit has a unique and unwieldy identifier like this:
5a02fede3007edf55d18e2f9ee3e57979535e8f2
Luckily, there are a few ways to refer to commits with more human symbolic names.
HEAD is one of these references. It indicates which branch or commit you’re looking at right now in your project subdirectory. Remember how we said you could go look at previous commits? The way you do that is by moving HEAD to them.
We haven’t talked about branches yet, but the
HEADnormally refers to a branch. By default, it’s themainbranch. But since we’re getting ahead of ourselves, I’m going to just keep saying thatHEADrefers to a commit, even though it usually does it indirectly via a branch.So this is a bit of a lie, but I hope you forgive me.
Some terminology: the files in your git subdirectory you’re looking at right now is referred to as your working tree. The working tree is the files as they appear at the commit pointed to by HEAD, plus any uncommitted changes you might have made.
So if you switch HEAD to another commit, the files in your working tree will be updated to reflect that.
Okay then, how do we know which commit HEAD is referring to? Well, it’s right there at the top of the log:
commit 5e8cb52cb813a371a11f75050ac2d7b9e15e4751 (HEAD -> main)
Author: User Name <user@example.com>
Date: Thu Feb 1 12:36:13 2024 -0800
More outputWe see HEAD right there on the first line, indicating that HEAD is referring to commit with ID:
5e8cb52cb813a371a11f75050ac2d7b9e15e4751
Again, that’s a bit of a lie. The HEAD -> main means that HEAD is actually referring to the main branch, and that main is referring to the commit. HEAD is therefore indirectly referring to the commit. More on that later.
HEADHere’s my full Git log:
commit 5e8cb52cb813a371a11f75050ac2d7b9e15e4751 (HEAD -> main)
Author: User Name <user@example.com>
Date: Thu Feb 1 12:36:13 2024 -0800
More output
commit 5a02fede3007edf55d18e2f9ee3e57979535e8f2
Author: User Name <user@example.com>
Date: Thu Feb 1 09:24:52 2024 -0800
AddedIf I look at the files, I’ll see the changes indicated by the “More output” commit. But let’s say I want to go back in time to the previous commit and see what the files looked like then. How would I do that?
Maybe there were some changes that existed back in an earlier commit that had since been removed, and you wanted to look at them, for example.
I can use the git checkout command to make that happen.
Let’s checkout the first commit, the one with ID 5a02fede3007edf55d18e2f9ee3e57979535e8f2.
Now, I could say:
$ git checkout 5a02fede3007edf55d18e2f9ee3e57979535e8f2and that would work, but the rule is that you must specific at least 4 unique digits of the ID, so I could have also done this:
$ git checkout 5a02for the same result.
And that result is:
Note: switching to '5a02'.
You are in 'detached HEAD' state. You can look around, make
experimental changes and commit them, and you can discard any
commits you make in this state without impacting any branches by
switching back to a branch.
If you want to create a new branch to retain commits you create,
you may do so (now or later) by using -c with the switch command.
Example:
git switch -c <new-branch-name>
Or undo this operation with:
git switch -
Turn off this advice by setting config variable advice.detachedHead
to false
HEAD is now at 5a02fed AddedLooks sort of scary, but look—Git is telling us how to undo the operation if we want, and so there’s really nothing to fear.
Let’s take a look around with git log:
commit 5a02fede3007edf55d18e2f9ee3e57979535e8f2 (HEAD)
Author: User <user@example.com>
Date: Thu Feb 1 09:24:52 2024 -0800
AddedThat’s all! Just one commit?! Where’s the second commit I made? Is it gone forever?!
No. Everything is fine.
When you have HEAD at a certain commit, you’re looking at the world as it looked at that snapshot in time. Future commits haven’t “happened” yet from this perspective. They are still out there, but you’ll have to change back to them by name.
Also, do you see anything different about that first line that reads (HEAD)? That’s right: no main to be seen.
That’s because the main branch is still looking at the latest commit, the one with the “More output” comment. So we don’t see it from this perspective.
Remember earlier when I said it was a bit of a lie to say that
HEADpoints to a commit? Well, detached head state is the case where it actually does. Detached head state is just what happens whenHEADis pointing to a commit instead of a branch. To reattach it, you have to change it to point to a branch again.
Let’s get back to the main branch. There are three options:
git switch -, just like the helpful message says.git switch maingit checkout mainGit replies:
Previous HEAD position was 5a02fed Added
Switched to branch 'main'And now if we git log, we see all our changes again:
commit 5e8cb52cb813a371a11f75050ac2d7b9e15e4751 (HEAD -> main)
Author: User Name <user@example.com>
Date: Thu Feb 1 12:36:13 2024 -0800
More output
commit 5a02fede3007edf55d18e2f9ee3e57979535e8f2
Author: User Name <user@example.com>
Date: Thu Feb 1 09:24:52 2024 -0800
Addedand our working tree will be updated to show the files as they are in the main commit.
git switchIn ye olden days, git checkout did a lot of things, and it still does. Because it does so much, the maintainers of Git have been trying to break some of that functionality into a new command, git switch.
We could redo the previous section by using just git switch instead of git checkout. Let’s try:
$ git switch 5a02and it says:
fatal: a branch is expected, got commit '5a02'
hint: If you want to detach HEAD at the commit, try again with the
--detach option.Hmmm! git switch is warning us that we’re about to go into detached head state, and is that what we really want? It’s not a crime or anything to do so, but it’s just letting us know that we’re not going to be on a branch any longer.
So we can override, just like it suggests:
$ git switch --detach 5a02
HEAD is now at 5a02fed AddedAll right! No big message about being detached, but we don’t need it because we know it’s detached since we specified.
And like before, we can get back to the main branch with either:
git switch -, switch to the previous stategit switch mainEasy.
HEADThere are a couple shortcuts to get to commits that are earlier than HEAD, like, “I want to switch to the 3rd commit before this one.”
Here’s a pretty useless example that we’ll start with:
$ git switch --detach HEADThis moves HEAD to where HEAD was. That is, it moves it nowhere. (Though it does have the effect of detaching it from the branch.)
But what if I wanted to move to the commit right before where HEAD is now? You can do it with caret notation like this:
$ git switch --detach HEAD^And that gets you to the previous commit.
What if you wanted to get to the third-previous commit? You can add more carets!
$ git switch --detach HEAD^^^Or the 10th previous commit!
$ git switch --detach HEAD^^^^^^^^^^Or the 100th previous commit!
$ git switch --detach HEAD^^^^^^^^^^^^^^^^^^^^forget thisTyping all these carets is wearing me out. Luckily there’s another shorthand we have at our disposal with tilde notation. The two following lines are equivalent:
$ git switch --detach HEAD^^^
$ git switch --detach HEAD~3After the tilde, you can just give the number of commits back you want to go. So back to my example:
$ git switch --detach HEAD~100 # Much easierAll this said, personally I usually just look at the log and go to a specific commit instead of counting back.
“But that’s just, like, my opinion, man.”
—The Dude