X-Git-Url: https://git.tokkee.org/?a=blobdiff_plain;f=Documentation%2Fgit-checkout.txt;h=c44a4a8004a49e42c01bcd238ae22b8d32b33cc3;hb=e0d10e1c63bc52b37bbec99b07deee794058d9b4;hp=985bb2f8272254f0e52816981171a2df8ea6e83a;hpb=ac5a85181ac18b0119705f46cfa825389643df33;p=git.git

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 985bb2f82..c44a4a800 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -8,8 +8,8 @@ git-checkout - Checkout and switch to a branch
 SYNOPSIS
 --------
 [verse]
-'git-checkout' [-f] [-b <new_branch>] [-m] [<branch>]
-'git-checkout' [-m] [<branch>] <paths>...
+'git-checkout' [-f] [-b <new_branch> [-l]] [-m] [<branch>]
+'git-checkout' [<branch>] <paths>...
 
 DESCRIPTION
 -----------
@@ -35,7 +35,15 @@ OPTIONS
 	Force a re-read of everything.
 
 -b::
-	Create a new branch and start it at <branch>.
+	Create a new branch named <new_branch> and start it at
+	<branch>.  The new branch name must pass all checks defined
+	by gitlink:git-check-ref-format[1].  Some of these checks
+	may restrict the characters allowed in a branch name.
+
+-l::
+	Create the new branch's ref log.  This activates recording of
+	all changes to made the branch ref, enabling use of date
+	based sha1 expressions such as "<branchname>@{yesterday}".
 
 -m::
 	If you have local modifications to one or more files that
@@ -55,7 +63,57 @@ and mark the resolved paths with `git update-index`.
 
 <branch>::
 	Branch to checkout; may be any object ID that resolves to a
-	commit. Defaults to HEAD.
+	commit.  Defaults to HEAD.
++
+When this parameter names a non-branch (but still a valid commit object),
+your HEAD becomes 'detached'.
+
+
+Detached HEAD
+-------------
+
+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:
+
+------------
+$ git checkout v2.6.18
+------------
+
+Earlier versions of git did not allow this and asked you to
+create a temporary branch using `-b` option, but starting from
+version 1.5.0, the above command 'detaches' your HEAD from the
+current branch and directly point at the commit named by the tag
+(`v2.6.18` in the above example).
+
+You can use usual 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`.
+
+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.
+
+The command would refuse to switch back to make sure that you do
+not discard your temporary state by mistake when your detached
+HEAD is not pointed at by any existing ref.  If you did want to
+save your state (e.g. "I was interested in the fifth commit from
+the top of 'master' branch", or "I made two commits to fix minor
+bugs while on a detached HEAD" -- and if you do not want to lose
+these facts), you can create a new branch and switch to it with
+`git checkout -b newbranch` so that you can keep building on
+that state, or tag it first so that you can come back to it
+later and switch to the branch you wanted to switch to with `git
+tag that_state; git checkout master`.  On the other hand, if you
+did want to discard the temporary state, you can give `-f`
+option (e.g. `git checkout -f master`) to override this
+behaviour.
 
 
 EXAMPLES
@@ -66,19 +124,19 @@ the `Makefile` to two revisions back, deletes hello.c by
 mistake, and gets it back from the index.
 +
 ------------
-$ git checkout master <1>
-$ git checkout master~2 Makefile <2>
+$ git checkout master             <1>
+$ git checkout master~2 Makefile  <2>
 $ rm -f hello.c
-$ git checkout hello.c <3>
-
+$ git checkout hello.c            <3>
+------------
++
 <1> switch branch
 <2> take out a file out of other commit
-<3> or "git checkout -- hello.c", as in the next example.
-------------
+<3> restore hello.c from HEAD of current branch
 +
-If you have an unfortunate branch that is named `hello.c`, the
-last step above would be confused as an instruction to switch to
-that branch.  You should instead write:
+If you have an unfortunate branch that is named `hello.c`, this
+step would be confused as an instruction to switch to that branch.
+You should instead write:
 +
 ------------
 $ git checkout -- hello.c