HEAD~4^2 meaning

Question

In a Udacity lesson covering relative commit references, it says:

^ indicates the parent commit, ~ indicates the first parent commit

The main difference between the ^ and the ~ is when a commit is created from a merge. A merge commit has two parents. With a merge commit, the ^ reference is used to indicate the first parent of the commit while ^2 indicates the second parent. The first parent is the branch you were on when you ran git merge while the second parent is the branch that was merged in.

According to the lesson, based on the following output of git log --graph --oneline, the commit with SHA f69811c is HEAD~4^2 relative to the (topmost, with the head pointer) commit 9ec05ca.

So HEAD~4 on its own means the first parent, while ^2 means it's also the second parent? Don't these things contradict each other? Any clarification appreciated.

Apteral

https://mcmap.net/q/12625/-what-39-s-the-difference-between-head-and-head-in-git/1030675 – Leveller

Instead of reading third-party tutorials, I recommend the Git docs themselves. ~n says chase a first-parent link that many times, default 1, ^n says chase the nth-parent link, default 1. – Cresida

Answer 1

X~n means: The nth ancestor of X.

X^ means: The parent of X. This is equivalent to X~1.

If X has more than one parent, one needs to distinguish between them when using the ^ notation. So X^1 would be the first parent, X^2 would be the second parent, and so on. X^ is equivalent to X^1 (and also equivalent to X~1).

In your example, starting from commit 9ec05ca, which is HEAD:

• db7e87a is HEAD~1 (or alternatively HEAD^).
• 796ddb0 is HEAD~2 (or alternatively HEAD^^).
• 1a56a81 is HEAD~4 (or alternatively HEAD^^^^, but nobody would use that).
• e014d91, being the first parent of 1a56a81, is HEAD~5, or HEAD~4^, or HEAD~4^1.
• f69811c, being the second parent of 1a56a81, is HEAD~4^2.

Reference

https://git-scm.com/docs/gitrevisions

Alric

Thank you, I'm not grasping your last two points. Is Udacity wrong in saying ~ means first parent? – Apteral

It's not wrong though a bit confusing. ~n means "select the nth generation ancestor, always following the first parents", while ^n means "among the previous generation, select the nth parent." – Backslide

Since e6c65a6 is on the branch that was merged into master, is it also a second parent of 1a56a81 (merge commit), in addition to f69811c? – Apteral

@NatalieCardot No, e6c65a6 is not a parent of 1a5681 at all, it is one of its grandparents (because it is the parent of f69811c). – Alric

Bonus question: What is the other grandparent of 1a5681? – Alric

1a5681's other grandparent is 209752a (plus, its great grandparent is 3772ab1). I think I finally understand :) – Apteral

Answer 2

It's not totally wrong, but it's a bit too short: since the "~" operator is performing a depth search (nth ancestor, which actually corresponds to the nth last commit on a single branch), its argument can't specify a parent if they are many of them. It hence assumes it should always follow the first one of them. This correlates with your other question, where we were discussing about parent order.

Climatology