Re: user manual question

From: Björn Steinbrink <b.steinbrink@gmx.de>
Date: Dec 29, 2008, 17:18 UTC
Message-ID: <20081229171825.GB6200@atjola.homenet>
In-Reply-To: <gjauel$94s$4@ger.gmane.org>

On 2008.12.29 16:38:30 -0000, Zorba wrote:

Show 7 quoted lines

> so a detached HEAD is just a HEAD that is not sitting on a tip ?
> 
> i.e. if I do $ git reset --hard HEAD^
> 
> ...pointing HEAD to the previous committ
> 
> this is a detached HEAD

No. At least that won't turn an "attached" HEAD into a detached HEAD.

Attached HEAD: HEAD is a symbolic ref to another ref. Detached HEAD: HEAD is a non-symbolic ref to a commit.

So you can have: HEAD -> refs/heads/master -> commitA --> attached HEAD

Or: HEAD -> commitA --> detached HEAD

Note how the commit object is the same in both cases.

What you "git reset [--hard] HEAD^" does, depends on the kind of HEAD you have.

"git reset HEAD^" will (conceptually, implementation might differ):

Resolve HEAD^ to find a commit.

and

Resolve HEAD until it finds a non-symbolic ref, and update that ref to reference the commit it found.

Looking at the cases from above:

Attached case: HEAD -> refs/heads/master -> commitA

Resolve HEAD^: HEAD^ ==> refs/heads/master^ ==> commitA^ ==> commitA_parent

Find non-symbolic ref: HEAD (symbolic) ==> refs/heads/master (non-symbolic)

So refs/heads/master gets updated to reference commitA_parent.

Now the detached case: HEAD -> commitA

Resolve HEAD^: HEAD^ ==> commitA^ ==> commitA_parent

Find non-symbolic ref: HEAD (non-symbolic)

So HEAD gets updated to reference commitA_parent.

In other words, HEAD is what tells git what you have checked out. And that's either a branch head (attached HEAD, symbolic ref) or a commit (detached HEAD, non-symbolic ref).

In the symbolic ref case, operations like "commit" or "reset" work on the branch you have checked out (found through the symbolic ref), in the non-symbolic ref case (detached HEAD), those operations work on HEAD itself.

Björn