git-checkout.txt: improve detached HEAD documentation

[git/dkf.git] / Documentation / git-checkout.txt

git-checkout(1)
===============

NAME
----
git-checkout - Checkout a branch or paths to the working tree

SYNOPSIS
--------
[verse]
'git checkout' [-q] [-f] [-m] [<branch>]
'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] [<start_point>]
'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <paths>...
'git checkout' --patch [<tree-ish>] [--] [<paths>...]

DESCRIPTION
-----------
Updates files in the working tree to match the version in the index
or the specified tree.  If no paths are given, 'git checkout' will
also update `HEAD` to set the specified branch as the current
branch.

'git checkout' [<branch>]::
'git checkout' -b|-B <new_branch> [<start point>]::

        This form switches branches by updating the index, working
        tree, and HEAD to reflect the specified branch.
+
If `-b` is given, a new branch is created as if linkgit:git-branch[1]
were called and then checked out; in this case you can
use the `--track` or `--no-track` options, which will be passed to
'git branch'.  As a convenience, `--track` without `-b` implies branch
creation; see the description of `--track` below.
+
If `-B` is given, <new_branch> is created if it doesn't exist; otherwise, it
is reset. This is the transactional equivalent of
+
------------
$ git branch -f <branch> [<start point>]
$ git checkout <branch>
------------
+
that is to say, the branch is not reset/created unless "git checkout" is
successful.

'git checkout' [--patch] [<tree-ish>] [--] <pathspec>...::

        When <paths> or `--patch` are given, 'git checkout' does *not*
        switch branches.  It updates the named paths in the working tree
        from the index file or from a named <tree-ish> (most often a
        commit).  In this case, the `-b` and `--track` options are
        meaningless and giving either of them results in an error.  The
        <tree-ish> argument can be used to specify a specific tree-ish
        (i.e.  commit, tag or tree) to update the index for the given
        paths before updating the working tree.
+
The index may contain unmerged entries because of a previous failed merge.
By default, if you try to check out such an entry from the index, the
checkout operation will fail and nothing will be checked out.
Using `-f` will ignore these unmerged entries.  The contents from a
specific side of the merge can be checked out of the index by
using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
file can be discarded to re-create the original conflicted merge result.

OPTIONS
-------
-q::
--quiet::
        Quiet, suppress feedback messages.

-f::
--force::
        When switching branches, proceed even if the index or the
        working tree differs from HEAD.  This is used to throw away
        local changes.
+
When checking out paths from the index, do not fail upon unmerged
entries; instead, unmerged entries are ignored.

--ours::
--theirs::
        When checking out paths from the index, check out stage #2
        ('ours') or #3 ('theirs') for unmerged paths.

-b::
        Create a new branch named <new_branch> and start it at
        <start_point>; see linkgit:git-branch[1] for details.

-B::
        Creates the branch <new_branch> and start it at <start_point>;
        if it already exists, then reset it to <start_point>. This is
        equivalent to running "git branch" with "-f"; see
        linkgit:git-branch[1] for details.

-t::
--track::
        When creating a new branch, set up "upstream" configuration. See
        "--track" in linkgit:git-branch[1] for details.
+
If no '-b' option is given, the name of the new branch will be
derived from the remote-tracking branch.  If "remotes/" or "refs/remotes/"
is prefixed it is stripped away, and then the part up to the
next slash (which would be the nickname of the remote) is removed.
This would tell us to use "hack" as the local branch when branching
off of "origin/hack" (or "remotes/origin/hack", or even
"refs/remotes/origin/hack").  If the given name has no slash, or the above
guessing results in an empty name, the guessing is aborted.  You can
explicitly give a name with '-b' in such a case.

--no-track::
        Do not set up "upstream" configuration, even if the
        branch.autosetupmerge configuration variable is true.

-l::
        Create the new branch's reflog; see linkgit:git-branch[1] for
        details.

--orphan::
        Create a new 'orphan' branch, named <new_branch>, started from
        <start_point> and switch to it.  The first commit made on this
        new branch will have no parents and it will be the root of a new
        history totally disconnected from all the other branches and
        commits.
+
The index and the working tree are adjusted as if you had previously run
"git checkout <start_point>".  This allows you to start a new history
that records a set of paths similar to <start_point> by easily running
"git commit -a" to make the root commit.
+
This can be useful when you want to publish the tree from a commit
without exposing its full history. You might want to do this to publish
an open source branch of a project whose current tree is "clean", but
whose full history contains proprietary or otherwise encumbered bits of
code.
+
If you want to start a disconnected history that records a set of paths
that is totally different from the one of <start_point>, then you should
clear the index and the working tree right after creating the orphan
branch by running "git rm -rf ." from the top level of the working tree.
Afterwards you will be ready to prepare your new files, repopulating the
working tree, by copying them from elsewhere, extracting a tarball, etc.

-m::
--merge::
        When switching branches,
        if you have local modifications to one or more files that
        are different between the current branch and the branch to
        which you are switching, the command refuses to switch
        branches in order to preserve your modifications in context.
        However, with this option, a three-way merge between the current
        branch, your working tree contents, and the new branch
        is done, and you will be on the new branch.
+
When a merge conflict happens, the index entries for conflicting
paths are left unmerged, and you need to resolve the conflicts
and mark the resolved paths with `git add` (or `git rm` if the merge
should result in deletion of the path).
+
When checking out paths from the index, this option lets you recreate
the conflicted merge in the specified paths.

--conflict=<style>::
        The same as --merge option above, but changes the way the
        conflicting hunks are presented, overriding the
        merge.conflictstyle configuration variable.  Possible values are
        "merge" (default) and "diff3" (in addition to what is shown by
        "merge" style, shows the original contents).

-p::
--patch::
        Interactively select hunks in the difference between the
        <tree-ish> (or the index, if unspecified) and the working
        tree.  The chosen hunks are then applied in reverse to the
        working tree (and if a <tree-ish> was specified, the index).
+
This means that you can use `git checkout -p` to selectively discard
edits from your current working tree.

<branch>::
        Branch to checkout; if it refers to a branch (i.e., a name that,
        when prepended with "refs/heads/", is a valid ref), then that
        branch is checked out. Otherwise, if it refers to a valid
        commit, your HEAD becomes "detached" and you are no longer on
        any branch (see below for details).
+
As a special case, the `"@\{-N\}"` syntax for the N-th last branch
checks out the branch (instead of detaching).  You may also specify
`-` which is synonymous with `"@\{-1\}"`.
+
As a further special case, you may use `"A\...B"` as a shortcut for the
merge base of `A` and `B` if there is exactly one merge base. You can
leave out at most one of `A` and `B`, in which case it defaults to `HEAD`.

<new_branch>::
        Name for the new branch.

<start_point>::
        The name of a commit at which to start the new branch; see
        linkgit:git-branch[1] for details. Defaults to HEAD.

<tree-ish>::
        Tree to checkout from (when paths are given). If not specified,
        the index will be used.



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:

------------
           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':

------------
$ 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')
------------

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):

------------
$ 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')
------------

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:

------------
$ 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
--------

. The following sequence checks out the `master` branch, reverts
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>
$ rm -f hello.c
$ git checkout hello.c            <3>
------------
+
<1> switch branch
<2> take a file out of another commit
<3> restore hello.c from the index
+
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
------------

. After working in the wrong branch, switching to the correct
branch would be done using:
+
------------
$ git checkout mytopic
------------
+
However, your "wrong" branch and correct "mytopic" branch may
differ in files that you have modified locally, in which case
the above checkout would fail like this:
+
------------
$ git checkout mytopic
error: You have local changes to 'frotz'; not switching branches.
------------
+
You can give the `-m` flag to the command, which would try a
three-way merge:
+
------------
$ git checkout -m mytopic
Auto-merging frotz
------------
+
After this three-way merge, the local modifications are _not_
registered in your index file, so `git diff` would show you what
changes you made since the tip of the new branch.

. When a merge conflict happens during switching branches with
the `-m` option, you would see something like this:
+
------------
$ git checkout -m mytopic
Auto-merging frotz
ERROR: Merge conflict in frotz
fatal: merge program failed
------------
+
At this point, `git diff` shows the changes cleanly merged as in
the previous example, as well as the changes in the conflicted
files.  Edit and resolve the conflict and mark it resolved with
`git add` as usual:
+
------------
$ edit frotz
$ git add frotz
------------


Author
------
Written by Linus Torvalds <torvalds@osdl.org>

Documentation
--------------
Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.

GIT
---
Part of the linkgit:git[1] suite