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Documentation: detached HEAD
Add discussion section to git-checkout documentation and mention detached HEAD in repository-layout document. Signed-off-by: Junio C Hamano <junkio@cox.net>
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@ -9,7 +9,7 @@ SYNOPSIS
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--------
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[verse]
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'git-checkout' [-f] [-b <new_branch> [-l]] [-m] [<branch>]
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'git-checkout' [-m] [<branch>] <paths>...
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'git-checkout' [<branch>] <paths>...
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DESCRIPTION
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-----------
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@ -63,7 +63,57 @@ and mark the resolved paths with `git update-index`.
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<branch>::
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Branch to checkout; may be any object ID that resolves to a
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commit. Defaults to HEAD.
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commit. Defaults to HEAD.
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+
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When this parameter names a non-branch (but still a valid commit object),
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your HEAD becomes 'detached'.
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Detached HEAD
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-------------
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It is sometimes useful to be able to 'checkout' a commit that is
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not at the tip of one of your branches. The most obvious
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example is to check out the commit at a tagged official release
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point, like this:
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------------
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$ git checkout v2.6.18
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------------
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Earlier versions of git did not allow this and asked you to
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create a temporary branch using `-b` option, but starting from
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version 1.5.0, the above command 'detaches' your HEAD from the
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current branch and directly point at the commit named by the tag
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(`v2.6.18` in the above example).
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You can use usual git commands while in this state. You can use
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`git-reset --hard $othercommit` to further move around, for
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example. You can make changes and create a new commit on top of
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a detached HEAD. You can even create a merge by using `git
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merge $othercommit`.
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The state you are in while your HEAD is detached is not recorded
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by any branch (which is natural --- you are not on any branch).
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What this means is that you can discard your temporary commits
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and merges by switching back to an existing branch (e.g. `git
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checkout master`), and a later `git prune` or `git gc` would
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garbage-collect them.
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The command would refuse to switch back to make sure that you do
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not discard your temporary state by mistake when your detached
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HEAD is not pointed at by any existing ref. If you did want to
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save your state (e.g. "I was interested in the fifth commit from
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the top of 'master' branch", or "I made two commits to fix minor
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bugs while on a detached HEAD" -- and if you do not want to lose
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these facts), you can create a new branch and switch to it with
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`git checkout -b newbranch` so that you can keep building on
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that state, or tag it first so that you can come back to it
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later and switch to the branch you wanted to switch to with `git
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tag that_state; git checkout master`. On the other hand, if you
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did want to discard the temporary state, you can give `-f`
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option (e.g. `git checkout -f master`) to override this
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behaviour.
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EXAMPLES
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@ -91,6 +91,12 @@ HEAD::
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'name' does not (yet) exist. In some legacy setups, it is
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a symbolic link instead of a symref that points at the current
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branch.
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+
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HEAD can also record a specific commit directly, instead of
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being a symref to point at the current branch. Such a state
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is often called 'detached HEAD', and almost all commands work
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identically as normal. See gitlink:git-checkout[1] for
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details.
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branches::
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A slightly deprecated way to store shorthands to be used
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