What's the difference between HEAD^ and HEAD~ in Git?

https://stackoverflow.com/questions/2221658/whats-the-difference-between-head-and-head-in-git

Rules of thumb

• Use ~ most of the time — to go back a number of generations, usually what you want
• Use ^ on merge commits — because they have two or more (immediate) parents

Mnemonics:

• Tilde ~ is almost linear in appearance and wants to go backward in a straight line
• Caret ^ suggests an interesting segment of a tree or a fork in the road

Tilde

The “Specifying Revisions” section of the git rev-parse documentation defines ~ as

<rev>~<n>, e.g. master~3
A suffix ~<n> to a revision parameter means the commit object that is the nth generation ancestor of the named commit object, following only the first parents. [For example,] <rev>~3 is equivalent to <rev>^^^ which is equivalent to <rev>^1^1^1 …

You can get to parents of any commit, not just HEAD. You can also move back through generations: for example, master~2 means the grandparent of the tip of the master branch, favoring the first parent on merge commits.

Caret

Git history is nonlinear: a directed acyclic graph (DAG) or tree. For a commit with only one parent, rev~ and rev^ mean the same thing. The caret selector becomes useful with merge commits because each one is the child of two or more parents — and strains language borrowed from biology.

HEAD^ means the first immediate parent of the tip of the current branch. HEAD^ is short for HEAD^1, and you can also address HEAD^2 and so on as appropriate. The same section of the git rev-parse documentation defines it as

<rev>^, e.g. HEAD^, v1.5.1^0
A suffix ^ to a revision parameter means the first parent of that commit object. ^<n> means the nth parent ([e.g.] <rev>^ is equivalent to <rev>^1). As a special rule, <rev>^0 means the commit itself and is used when <rev> is the object name of a tag object that refers to a commit object.

http://www.paulboxley.com/blog/2011/06/git-caret-and-tilde

I spent a little bit of time playing with Git today, specifically the way that the ^ (caret) and ~ (tilde) work and thought I'd document it here in case I forget.

The short version

If you want a deeper explanation skip down to "The long version".

ref~ is shorthand for ref~1 and means the commit's first parent. ref~2 means the commit's first parent's first parent. ref~3 means the commit's first parent's first parent's first parent. And so on.

ref^ is shorthand for ref^1 and means the commit's first parent. But where the two differ is that ref^2 means the commit's second parent (remember, commits can have two parents when they are a merge).

The ^ and ~ operators can be combined.

Here's a diagram showing how to reference various commits using HEAD as the starting point.

HEAD~1^2    commit的 first parent的second parent

The long version

I've created a dummy repository with several commits in it.

$ git log --graph --oneline
* 8329384 Seventh commit
*   f5717b0 Merge branch 'my_branch'
|  
| * 956c87d Fourth commit on a branch
* | a8fe411 Sixth commit
|/  
* c7c2590 Third commit on a branch
* 86362ff Second commit on a branch
* 748855b First commit on a branch
* 1855b25 Fifth commit
* 67cf3a7 Fourth commit
* ea29778 Third commit
* 28c25b1 Second commit
* cd00b76 First commit

Starting at the bottom, the early commits were made straight onto master.

The commits starting at 748855b and moving up to c7c2590 were made on a branch and merged into master, but no changes had been made on master in the mean time.

The commits a8fe411 and 956c87d were made on separate branches at the same time. They were merged together in commit f5717b0.

Finally, 8329384 was committed straight onto master.

We can use git show to look at individual commits.

You'll already know that HEAD points to the tip of the current branch:

$ git show --oneline HEAD
8329384 Seventh commit

Putting the caret symbol (^) next to a commit means the parent of that commit. So the following will show the parent of HEAD:

$ git show --oneline HEAD^
f5717b0 Merge branch 'my_branch'
...

 HEAD^ is shorthand for saying HEAD^1, which literally means show me parent 1 of that commit. You can also say HEAD^2 but in this instance it won't make any sense:

$ git show --oneline HEAD^2
fatal: ambiguous argument 'HEAD^2': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

Because HEAD only has 1 parent.

But f5717b0, the point where the two branches were merged, has two parents, one on master and one on the branch:

$ git show --oneline f5717b0^1
a8fe411 Sixth commit
...

$ git show --oneline f5717b0^2
956c87d Fourth commit on a branch
...

The tilde symbol (~) works in a similar way. In fact HEAD~ will reference the same commit as HEAD^:

Again, HEAD~ is shorthand for HEAD~1, but here this means the first ancestor of HEAD – HEAD~2 is not the second parent of HEAD but the grandparent of HEAD:

$ git show --oneline HEAD~1
f5717b0 Merge branch 'my_branch'
...

$ git show --oneline HEAD~2
a8fe411 Sixth commit
...

$ git show --oneline HEAD~3
c7c2590 Third commit on a branch

As you can see, 956c87d Fourth commit on a branch is not visible when using the tilde operator. This is because the tilde operator always presumes you want to view the first parent's parent.

To access the second parent's parent the tilde and caret symbols can be combined:

$ git show --oneline HEAD~1^1
a8fe411 Sixth commit
...

$ git show --oneline HEAD~1^2
956c87d Fourth commit on a branch
...

In this way you should be able to reference any commit in your repository's history.