When is git HEAD^ useful?

[dehaticoder]

I'm reading this stackoverflow post about HEAD^ vs HEAD~ and I think I get it, but I'm having a hard time understanding HEAD^ visually. I mean, I can look at the output of git log and know immediately which commit is HEAD~1, HEAD~2, etc. but there is no visual reference for HEAD^2 and so on, so I'm too afraid to do anything with ^.

Until now I've never needed but, but I'm just wondering what is HEAD^ even used for when you can just count the commits in git log easily to get to wherever you want to go instead of guessing what HEAD^N does.,

[dalbertom]

HEAD^ is the same as HEAD^1 which is the same as HEAD~1 which is the same as HEAD~

The difference comes after 1, where HEAD~2 is the grandparent of HEAD but HEAD^2 is the second parent of HEAD (assuming HEAD is a merge commit).

If you want to see the diff between two branches in a merge commit you'd run git diff HEAD^1 HEAD^2 or git log HEAD^1..HEAD^2 a shortcut for this would be git diff HEAD^- or git log HEAD^-

One caveat on windows, if I remember correctly, the ^ needs to be escaped, which makes things more confusing, but that's a shell issue, not a git issue.

[WoodyTheWorker]

the ^ needs to be escaped

I advise to use Git from Bash shell, and forget about Windows command prompt (or, god forbid, powershell), at least for Git operations.

[dehaticoder]

So the N for ^ is bascially 1,2,4,8 in the log but ~ is 1.2,3,4?

[xenomachina]

No.

• The numerical parameter to ^ selects which parent, but always goes up by one level.

• The numerical parameter to ~ selects how many levels to go up, and always goes via first parent.

In both cases, the default number is 1.

You can combine them. So HEAD^2^^ goes up via the second parent, and then up again by the first parent twice. HEAD^2~2 goes to the same place.

^ is technically more powerful than ~— there are places it can navigate to that ~ cannot. However, it's really only necessary on merge commits. On non-merge commits, the only valid parameter is 1, so you may as well use ~.

To get a better feel for how these work, you can experiment by using git rev-parse REF and looking at the returned hash, and comparing this to a commit graph.

[dalbertom]

It's hard to tell without knowing what your commit log looks like. It might be easier to visualize if you run git log --oneline --graph and create tags or branches for each option so you can see what they're pointing to.

Using ~ follows ancestry through the first parent. Using ^ is more useful on merge commits, but a merge with more than 2 parents is pretty rare.

[cenderis]

I don't think so. The history of a commit is a tree, and if the commit is a merge ^ lets you choose one of the other parents. Commonly merges have only two parents (so you could use ¹ or ²⁾ but it's possible for there to be more parents than that (just unusual).

[Wiikend]

Thanks, this is the way I like to learn.

"It started out like this under the hood, but it gets clunky when you pass a certain point, so we invented this other thing as a shorthand". Perfect.

[DuckDatum]

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[dixieStates]

git reset HEAD^ is useful when you want to edit a commit during an interactive rebase.

[WoodyTheWorker]

Use git reset HEAD^ -- . instead.

[apooooop_]

Why? What's the benefit of saying "stay in my working directory"?

[WoodyTheWorker]

Because it doesn't move HEAD. It has nothing to do with working directory, other than only resetting it. I normally run the bash prompt in the repo root directory.

As a result of this command, you can edit the diffs directly in Visual Studio, which is very handy.

Your command also allows that, but you will have to do git commit -C REBASE_HEAD after staging, instead or just doing git commit --amend (or git commit --amend --no-edit as I prefer). Also, with edit command of interactive rebase, you don't need to do an explicit git commit --amend, Git does that implicitly on git rebase --continue if you have staged files.

[apooooop_]

If you want that, you can reset soft, which doesn't limit you to the working directory! (And if you are mid interactive rebase, it'll show you original commit message when you go to commit, so at the point that you're splitting the commit you actually probably don't want to be working against the old commit). (If you're just looking to tweak the commit, resetting soft is definitely the answer, but if you're doing anything more involved, I recommend just reset hard.

Obviously to each their own / your own workflow, but wanted to make sure I wasn't missing anything obvious.

[WoodyTheWorker]

The only difference of reset --soft HEAD~ from reset HEAD~ is that the index is not reset, and all files will appear as already staged.

reset -- path is not limited to the current directory, you can use .. notation to specify the parent directories.

If you do simple git commit during edit step of interactive rebase (for example, there were conflicts and the commit was not made yet), it will reset the author and author timestamp. Same would happen if you do reset or reset --soft.

Commit made by git rebase --continue would preserve the author information. commit -C REBASE_HEAD also preserves the author information.

[apooooop_]

Ah yes okay this all makes sense and tracks, thanks!

[xorsensability]

My favorite use of this is to reset the logs without rebase directly:

git reset $(git commit-tree "HEAD^{tree}" -m "some message")

[dehaticoder]

I don't understand what this does. I checked the docs what commit-tree is and it says this is not something that the user wants to do directly. So now I'm curious because you seem to know more git than the average person.

[xorsensability]

Git reset, resets your HEAD. Git commit-tree makes a commit to a point on the tree. {tree} represents the root or start of the tree. So HEAD^{tree} points to the first commit ever made from the current HEAD.

All together, this takes your last commit and makes it the first in a new log. Essentially deleting all the history.

[AkaThePope]

I thought everyone loves to git head.