How do we understand git checkout [dot]

Question

As my understanding, git checkout is about moving the head.

For example, git checkout <commit id> is to move the head to the <commit id>, git checkout <branch_name> is to move the head to the <branch_name>.

However, git checkout . is to discard the unstaged changes. it doesn't seem that there is any business about head.

So I can not understand why git uses the same key word checkout to do two totally no-relative things. Or git checkout . still works based on head?

Answer 1

As you've noticed, the checkout command is overloaded to mean two different things. I think git help checkout makes that fairly clear though:

git-checkout - Switch branches or restore working tree files

There are several forms of the command, the one you're asking about is:

git checkout [<tree-ish>] [--] <pathspec>...

In your case the <tree-ish> argument is omitted, and the -- argument to separate options and filenames is ommitted, and <pathspec> is . (i.e. the current directory). That form of the command is documented as:

Overwrite paths in the working tree by replacing with the contents in the index or in the <tree-ish> (most often a commit).

So since you didn't specify a <tree-ish> argument, the contents of the files in . are replaced with the contents from the index. That means discarding any changes in . that have not been added to the index yet.

You can think of it as "checkout these files from the repository", which can either mean from a commit, or from the index (which might contain changes that have been staged but not yet committed).