How to move HEAD to other branch in Git without checking out files or updating any refs?

Question

I usually do it by echo ref: refs/heads/new_branch > .git/HEAD. How to do it properly?

Some use cases:

1. You imported source code from some other location, now want to commit it into Git (but not into current branch).
2. Your .git is just a symlink and now you are accessing it from other workdir (I already know about git-new-workdir to do it properly)

Therefore you want to choose manually what parent the next commit should have and what ref should be updated by it.

Answer 1

The git plumbing command for this is

git symbolic-ref HEAD refs/heads/existing_branch

You cannot go into detached head state (it needs a ref, not a SHA). However you can switch to an nonexisting ref (aka. unknown branch). To prevent that, perhaps use following git alias:

git config --global alias.switch '!f() { git show-ref --heads --verify "refs/heads/$1" && git symbolic-ref -m "switch to branch $1 not touching workdir" HEAD "refs/heads/$1"; }; f'

Then you can use it like git switch existing_branch. Please note, that you need option -m to see an entry in git reflog, this is usually what you want.

Notes:

• git reflog HEAD shows the ref log of HEAD (you will see the comment given at option -m again). The full file (which includes before-after SHAs in case you want to find some lost commit) is stored at $(git rev-parse --git-dir)/logs/HEAD (for HEAD)
• git show-ref --heads lists all refs you can use on the RHS. As shown in the alias you can use it with --verify to check if an argument is a proper (existing) ref (aka. branch).
• Beware: git symbolic-ref can point to any other ref like refs/tags/ or refs/remotes/ or even inside packed refs. This is probably not what you want, hence the alias restricts this to refs/heads/.

Answer 2

git reset --soft $branch_to_make_HEAD

man git reset docs on the --soft option:

Does not touch the index file nor the working tree at all (but resets the head to <commit>, just like all modes do). This leaves all your changed files "Changes to be committed", as git status would put it.

Answer 3

git checkout -B existing_or_new_branch # HEAD by default
git reset       existing_branch@{1}    # or anywhere else for a new_branch