What is the meaning of - (single dash) in "git checkout - ..."?

Question

In my repository, I had some changes (let's say that all the edited files' paths started with abc_) that I wanted to discard, with the command:

git checkout -- abc_*

However, I made a typo and instead of two dashes I used only one:

git checkout - abc_*

Looks like instead of discarding the changes, the command added even more changes to be committed - in fact, it added a few hundred files whose names started with abc_ (but not all of them in the project!).

My question is: What have I just done? Or, more precisely: what does the dash stand for?

All the answers that I was able to find explained what two dashes (--) do, but I understand this. I also believe I understand how git checkout works, both with the branch/tree-ish argument or with file paths. I cannot find any information on the single dash meaning, however - dashes are also used to designate parameters (e.g. git checkout -b ...), and it makes searching for this command problematic.

Answer 1

As others already answered, git checkout - checkouts the last branch. Check the release notes:

@{-1} is a way to refer to the last branch you were on.
This is accepted not only where an object name is expected, but anywhere a branch name is expected and acts as if you typed the branch name. E.g. "git branch --track mybranch @{-1}", "git merge @{-1}", and "git rev-parse --symbolic-full-name @{-1}" would work as expected.

In order to demonstrate what happened, let's try this example:

$ (master) echo 'Hello from master' > test
$ (master) git commit -am 'Hello from master'

Now let's create another branch, branch_1 and modify the "test" file:

$ (master) git checkout branch_1
$ (branch_1) echo 'Hello from branch_1' > test
$ (branch_1) git commit -am 'Hello from branch_1'

If you now run git checkout -, you'll get back to master:

$ (branch_1) git checkout -
$ (master)

Finally, running git checkout - test will checkout the test file from the branch_1 branch:

$ (master) git checkout - test
Updated 1 path from e2bcb78
$ git diff --cached
@@ -1 +1 @@
-Hello from master
+Hello from branch_1

Answer 2

-- is a special argument that tells Git that the arguments that follow it are paths; those before it are something else (command options, remote names, branch names, tag names etc.)

- in git checkout - is the branch name. - is a alias of "@{-1}" which represents the name of the previous current branch (the branch that was the current branch before the last git checkout command that was used to change the branch).

Answer 3

The single dash here means the previous active branch or detached HEAD.

Case 1:

git checkout master
git checkout dev

# here - means master
git checkout -

# here - means dev
git checkout -

Case 2:

# detached HEAD
git checkout refs/heads/master
# back to master
git checkout master

# detached HEAD again
git checkout -

# master again
git checkout -

So git checkout - abc_* means to overwrite paths (abc_*) in the working tree by replacing with the contents in the previous active branch or detached HEAD. See git checkout