Re: About detached heads

[Jakub Narebski <jnareb@gmail.com>]

"Geoff Russell" <geoffrey.russell@gmail.com> writes:

> This should be simple! I have a series of commits:
> 
>            1---2---3---4---5
> 
> I want to go back to 3 but not branch, so I want
> 
>            1---2---3---4---5---3
> 
> ?
> 
>          git checkout 3...
> 
> gets me the commit on a detached head, but I don't know how to put this back
> as the HEAD.

Lets check what git does in each of scenarios. Let's assume that
current branch is named 'master'.

At beginning we have:

   1---2---3---4---5    <--- master <--- HEAD

HEAD contents is "ref: refs/heads/master"

1. Now, "git checkout 3...", which is equivalent to "git checkout 3",
detaches HEAD because commit '3' is not a head (is not a branch), so
we have:

   1---2---3---4---5    <--- master
           ^
            \ 
             \-------------- HEAD

HEAD contents is "<sha1 of 3>"


2. If we did "git reset --hard 3" we would rewind the history,
resulting in the following situation:

   1---2---3           <--- master <--- HEAD
            \           
             \-4---5   <... master@{1}, ORIG_HEAD, HEAD@{1}
              
and now commits 4 and 5 are referenced only by reflogs, and by the
(temporary) "last position of HEAD" reference named ORIG_HEAD.


3. Now, if you have published 1..5 history you would not want
(usually) to rewind published branch. If you do the following:

  $ git revert --no-commit 5
  $ git revert 4

you would get the following:

   1---2---3---4---5---(5^-1 4^-1 => 3)  <--- master <--- HEAD

git-revert applies reversal of changes in given commit, in the 
"patch -R" ("patch --reverse") sense. Using '--no-commit' option
allows to squash reverting two commits into one commit. The ordering
of reverting ensures that there are no merge conflicts.


4. Or you can just put the _contents_ of revision 3 into your working
tree, either using plumbing command git-read-tree, or by checking out
or resetting to top tree: "git checkout 3^{tree}", or 
"git checkout 3 -- .", or equivalent git-reset invocation.

This way you would get exactly

   1---2---3---4---5---3   <--- master <--- HEAD

but the relation of 5---3 parentage is unclear: you would have to
explain it in the commit mesage.

HTH
-- 
Jakub Narebski
Poland
ShadeHawk on #git