Point HEAD at a commit with no branch, understand why it is useful, and get back out safely.
You are on Level 15, the second-to-last level. In this one you learn about an often confusing Git concept with a morbid name: the detached HEAD state. You will find out what it is, when it occurs, why it is useful, and how to get out of it. Rest assured, at no point will your own head become detached.
As a keen observer, you probably noticed something in every scenario so far: the blue HEAD has always sat on top of a green marker. In scrapbook terms, you have only ever opened the book to the first page of a section.
HEAD is a moveable bookmark that always marks the page your scrapbook is currently open to, which is to say, which commit is currently checked out. That is the whole reason it exists.
If HEAD could only ever sit on a page with a marker, you could only ever open the book to those pages. You would never be able to look at the mementos on any other page, or at the code in any commit without a green marker on it.
What if you want to see what your files looked like at the time of the second commit in the chain? It has no marker, so how can you switch to it? git switch has a flag for exactly this:
git switch <commit> --detach
Instead of a branch name you give any old commit, and --detach tells Git it is OK to enter a detached HEAD state: HEAD pointing at a commit that has no branch. It is like opening the scrapbook to a page without a section marker.
Open the page one behind HEAD, the one with no marker:
git switch --detach HEAD^Notice how HEAD moved back one commit from where it was, and now sits on a commit with no green marker on it. You are officially in a detached HEAD state. (Does it hurt?)
The files on your table now reflect the commit HEAD points to: feature.html is gone from the table, because that page does not have it. This is useful because it lets you examine the code at any point in your history, not only at the points marked by branches.
Here is what detached looks like in the book: the bookmark is clipped straight onto a page, with no marker under it.
There is a reason to be careful here. If you change code while detached and commit it, Git will faithfully create the page. But there is no branch marker to move forward onto it. Switch back to an existing branch and you could lose track of that page: not deleted, but hard to find again, since no marker leads to it. To avoid that, plant a marker first: git switch -c <name> creates a branch right here and attaches HEAD to it.
You examined the code at this page and it looks good. Which command gets you out of the detached state and back to your latest work?
The graph appears here when the first command runs. The simulations are drawn by git-sim from a real repository.
The scrapbook picture appears as the story is told.
Level 15 complete
🎉 New rank unlocked
A detached HEAD occurs when HEAD is pointed at a commit with no branch. git switch <commit> --detach enters that state on purpose, and it is useful for seeing what your code looked like at any point in history. Be careful about creating commits there; create a branch first. Exiting is as simple as git switch <branch>. Great job. Only one level to go.
Get new levels and walkthroughs
New lessons and command walkthroughs, plus a Git trick or two. Your progress stays saved in this browser either way.
Free: the Git essentials cheat sheet
The 16 Git commands you'll use most, each with a picture of what it does to your repo, drawn by git-sim, on one printable sheet. Or pick your own 16. We'll email it to you, along with new levels and walkthroughs.