Detached HEAD
-------------
+HEAD normally refers to a named branch (e.g. 'master'). Meanwhile, each
+branch refers to a specific commit. Let's look at a repo with three
+commits, one of them tagged, and with branch 'master' checked out:
-It is sometimes useful to be able to 'checkout' a commit that is
-not at the tip of one of your branches. The most obvious
-example is to check out the commit at a tagged official release
-point, like this:
+------------
+ HEAD (refers to branch 'master')
+ |
+ v
+a---b---c branch 'master' (refers to commit 'c')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
+------------
+
+When a commit is created in this state, the branch is updated to refer to
+the new commit. Specifically, 'git commit' creates a new commit 'd', whose
+parent is commit 'c', and then updates branch 'master' to refer to new
+commit 'd'. HEAD still refers to branch 'master' and so indirectly now refers
+to commit 'd':
------------
-$ git checkout v2.6.18
+$ edit; git add; git commit
+
+ HEAD (refers to branch 'master')
+ |
+ v
+a---b---c---d branch 'master' (refers to commit 'd')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
------------
-Earlier versions of git did not allow this and asked you to
-create a temporary branch using the `-b` option, but starting from
-version 1.5.0, the above command 'detaches' your HEAD from the
-current branch and directly points at the commit named by the tag
-(`v2.6.18` in the example above).
+It is sometimes useful to be able to checkout a commit that is not at
+the tip of any named branch, or even to create a new commit that is not
+referenced by a named branch. Let's look at what happens when we
+checkout commit 'b' (here we show two ways this may be done):
-You can use all git commands while in this state. You can use
-`git reset --hard $othercommit` to further move around, for
-example. You can make changes and create a new commit on top of
-a detached HEAD. You can even create a merge by using `git
-merge $othercommit`.
+------------
+$ git checkout v2.0 # or
+$ git checkout master^^
+
+ HEAD (refers to commit 'b')
+ |
+ v
+a---b---c---d branch 'master' (refers to commit 'd')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
+------------
-The state you are in while your HEAD is detached is not recorded
-by any branch (which is natural --- you are not on any branch).
-What this means is that you can discard your temporary commits
-and merges by switching back to an existing branch (e.g. `git
-checkout master`), and a later `git prune` or `git gc` would
-garbage-collect them. If you did this by mistake, you can ask
-the reflog for HEAD where you were, e.g.
+Notice that regardless of which checkout command we use, HEAD now refers
+directly to commit 'b'. This is known as being in detached HEAD state.
+It means simply that HEAD refers to a specific commit, as opposed to
+referring to a named branch. Let's see what happens when we create a commit:
------------
-$ git log -g -2 HEAD
+$ edit; git add; git commit
+
+ HEAD (refers to commit 'e')
+ |
+ v
+ e
+ /
+a---b---c---d branch 'master' (refers to commit 'd')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
------------
+There is now a new commit 'e', but it is referenced only by HEAD. We can
+of course add yet another commit in this state:
+
+------------
+$ edit; git add; git commit
+
+ HEAD (refers to commit 'f')
+ |
+ v
+ e---f
+ /
+a---b---c---d branch 'master' (refers to commit 'd')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
+------------
+
+In fact, we can perform all the normal git operations. But, let's look
+at what happens when we then checkout master:
+
+------------
+$ git checkout master
+
+ HEAD (refers to branch 'master')
+ e---f |
+ / v
+a---b---c---d branch 'master' (refers to commit 'd')
+ ^
+ |
+ tag 'v2.0' (refers to commit 'b')
+------------
+
+It is important to realize that at this point nothing refers to commit
+'f'. Eventually commit 'f' (and by extension commit 'e') will be deleted
+by the routine git garbage collection process, unless we create a reference
+before that happens. If we have not yet moved away from commit 'f',
+any of these will create a reference to it:
+
+------------
+$ git checkout -b foo <1>
+$ git branch foo <2>
+$ git tag foo <3>
+------------
+
+<1> creates a new branch 'foo', which refers to commit 'f', and then
+updates HEAD to refer to branch 'foo'. In other words, we'll no longer
+be in detached HEAD state after this command.
+
+<2> similarly creates a new branch 'foo', which refers to commit 'f',
+but leaves HEAD detached.
+
+<3> creates a new tag 'foo', which refers to commit 'f',
+leaving HEAD detached.
+
+If we have moved away from commit 'f', then we must first recover its object
+name (typically by using git reflog), and then we can create a reference to
+it. For example, to see the last two commits to which HEAD referred, we
+can use either of these commands:
+
+------------
+$ git reflog -2 HEAD # or
+$ git log -g -2 HEAD
+------------
EXAMPLES
--------