How do I fix a Git detached head?

I was doing some work in my repository and noticed a file had local changes. I didn't want them anymore so I deleted the file, thinking I can just checkout a fresh copy. I wanted to do the Git equivalent of

svn up .

Using git pull didn't seem to work. Some random searching led me to a site where someone recommended doing

git checkout HEAD^ src/

(src is the directory containing the deleted file).

Now I find out I have a detached head. I have no idea what that is. How can I undo?

Detached head means you are no longer on a branch, you have checked out a single commit in the history (in this case the commit previous to HEAD, i.e. HEAD^).

If you want to delete your changes associated with the detached HEAD

You only need to checkout the branch you were on, e.g.

git checkout master

Next time you have changed a file and want to restore it to the state it is in the index, don't delete the file first, just do

git checkout -- path/to/foo

This will restore the file foo to the state it is in the index.

If you want to keep your changes associated with the detached HEAD

1. Run git branch tmp - this will save your changes in a new branch called tmp.
2. Run git checkout master
3. If you would like to incorporate the changes you made into master, run git merge tmp from the master branch. You should be on the master branch after running git checkout master.

If you have changed files you don't want to lose, you can push them. I have committed them in the detached mode and after that you can move to a temporary branch to integrate later in master.

git commit -m "....."
git branch my-temporary-work
git checkout master
git merge my-temporary-work

Extracted from:

What to do with commit made in a detached head

A solution without creating a temporary branch.

How to exit (“fix”) detached HEAD state when you already changed something in this mode and, optionally, want to save your changes:

1. Commit changes you want to keep. If you want to take over any of the changes you made in detached HEAD state, commit them. Like:

   git commit -a -m "your commit message"
   

2. Discard changes you do not want to keep. The hard reset will discard any uncommitted changes that you made in detached HEAD state:

   git reset --hard
   

   (Without this, step 3 would fail, complaining about modified uncommitted files in the detached HEAD.)

3. Check out your branch. Exit detached HEAD state by checking out the branch you worked on before, for example:

   git checkout master
   

4. Take over your commits. You can now take over the commits you made in detached HEAD state by cherry-picking, as shown in my answer to another question.

   git reflog
   git cherry-pick <hash1> <hash2> <hash3> …
   

Detached head means:

1. You are no longer on a branch,
2. You have checked out a single commit in the history

If you have no changes: you can switch to master by applying the following command

  git checkout master

If you have changes that you want to keep:

In case of a detached HEAD, commits work like normal, except no named branch gets updated. To get master branch updated with your committed changes, make a temporary branch where you are (this way the temporary branch will have all the committed changes you have made in the detached HEAD), then switch to the master branch and merge the temporary branch with the master.

git branch  temp
git checkout master
git merge temp