Git Rebase Issue

Hi,

I've encountered a situation using rebase for which I don't understand the results, even after reading the documentation. I'm currently working in my feature branch and then I want to squash commits, thus I use interactive rebase. After successfully completing the rebase, I end up in a detached HEAD state, rather than back on my branch, which is confusing. The command that is causing me to be in detached HEAD mode is: git rebase -i HEAD~2 HEAD From the documentation, I read that my second parameter (HEAD) is the <branch> parameter:

   git rebase [-i | --interactive] [options] [--exec <cmd>] [--onto <newbase>]

   [<upstream>] [<branch>]

   If <branch> is specified, git rebase will perform an automatic git checkout <branch> before doing anything else. Otherwise it remains on the current branch.
<branch> Working branch; defaults to HEAD.

   Upon completion, <branch> will be the current branch.

Here is a full example than can be used to easily repro the issue. Go to an empty folder. git init git echo text > file.txt git add . git commit -m "Add file.txt" git echo text2 > file.txt git commit -am "Modify file.txt" git echo text3 > file.txt git commit -am "Remodify file.txt"

Now the interesting part: $ git rebase -i HEAD~2 HEAD [detached HEAD 9178b93] Modify file 1 file changed, 1 insertion(+), 1 deletion(-) Successfully rebased and updated detached HEAD. 

From the documentation it says that <branch> (which is HEAD) will be checked out before doing anything and that upon completion, <branch> will be the current branch. However, this doesn't seem to happen. In fact, it seems more like the following is happening during the rebase:
1) detach HEAD
2) rebase
3) reattach to <branch>

If <branch> is HEAD, then is does nothing and remains detached. I find this behavior confusing since I would expect it to return to whatever HEAD was pointing to at the start of the command, such as my branch. Also, the documentation says that the <branch> parameter defaults to HEAD, so passing 'HEAD' explicitly should result in the same behavior as not passing it: <branch> Working branch; defaults to HEAD.

Pierre-Luc Loyer

Pierre-Luc Loyer <Pierre-Luc.Loyer@bhvr.com> writes:

You do not have to say "HEAD" for "<branch>", if you are rebasing the current branch. Either leave it unsaid, or name the branch.

Passing "HEAD" (or any of its variants that names the exact commit at the tip of the current branch, without using the name of the current branch, e.g. "master^0") as the "branch to rebase" is an advanced technique to use when you want to avoid messing with the branch itself. It is deliberate that the HEAD is left detached.

You'd (once you learn Git sufficently and got comfortable with working on a detached HEAD, that is) often find yourself doing things like this:

1. rebase temporarily on detached HEAD

    $ git rebase -i HEAD~2 HEAD ;# amend the tip two

2. double check the result and convince yourself that it is better
   than the original

    $ git diff master HEAD

    $ git log master.. >updated
    $ git log -2 master >original
    $ diff -u original updated

3-a. once satisfied, update the branch

    $ git checkout -B master

3-b. or abandon if the result is undesirable.

    $ git checkout master

Hi,

On Tue, Dec 22, 2015 at 6:53 PM, Pierre-Luc Loyer <Pierre-Luc.Loyer@bhvr.com> wrote:

You are right, it is probably a bug. I guess usually people just use "git rebase -i HEAD~2" or "git rebase -i master" and don't give the [<branch>] argument when using -i.

If you are interested in helping us debug this you might first want to check if older git versions behaved like this.

Thanks, Christian.