Documentation: new upstream rebase recovery section in git-rebase

[git.git] / Documentation / git-rebase.txt

git-rebase(1)
=============

NAME
----
git-rebase - Forward-port local commits to the updated upstream head

SYNOPSIS
--------
[verse]
'git rebase' [-i | --interactive] [-v | --verbose] [-m | --merge]
        [-s <strategy> | --strategy=<strategy>]
        [-C<n>] [ --whitespace=<option>] [-p | --preserve-merges]
        [--onto <newbase>] <upstream> [<branch>]
'git rebase' --continue | --skip | --abort

DESCRIPTION
-----------
If <branch> is specified, 'git-rebase' will perform an automatic
`git checkout <branch>` before doing anything else.  Otherwise
it remains on the current branch.

All changes made by commits in the current branch but that are not
in <upstream> are saved to a temporary area.  This is the same set
of commits that would be shown by `git log <upstream>..HEAD`.

The current branch is reset to <upstream>, or <newbase> if the
--onto option was supplied.  This has the exact same effect as
`git reset --hard <upstream>` (or <newbase>).  ORIG_HEAD is set
to point at the tip of the branch before the reset.

The commits that were previously saved into the temporary area are
then reapplied to the current branch, one by one, in order. Note that
any commits in HEAD which introduce the same textual changes as a commit
in HEAD..<upstream> are omitted (i.e., a patch already accepted upstream
with a different commit message or timestamp will be skipped).

It is possible that a merge failure will prevent this process from being
completely automatic.  You will have to resolve any such merge failure
and run `git rebase --continue`.  Another option is to bypass the commit
that caused the merge failure with `git rebase --skip`.  To restore the
original <branch> and remove the .git/rebase-apply working files, use the
command `git rebase --abort` instead.

Assume the following history exists and the current branch is "topic":

------------
          A---B---C topic
         /
    D---E---F---G master
------------

From this point, the result of either of the following commands:


    git rebase master
    git rebase master topic

would be:

------------
                  A'--B'--C' topic
                 /
    D---E---F---G master
------------

The latter form is just a short-hand of `git checkout topic`
followed by `git rebase master`.

If the upstream branch already contains a change you have made (e.g.,
because you mailed a patch which was applied upstream), then that commit
will be skipped. For example, running `git rebase master` on the
following history (in which A' and A introduce the same set of changes,
but have different committer information):

------------
          A---B---C topic
         /
    D---E---A'---F master
------------

will result in:

------------
                   B'---C' topic
                  /
    D---E---A'---F master
------------

Here is how you would transplant a topic branch based on one
branch to another, to pretend that you forked the topic branch
from the latter branch, using `rebase --onto`.

First let's assume your 'topic' is based on branch 'next'.
For example feature developed in 'topic' depends on some
functionality which is found in 'next'.

------------
    o---o---o---o---o  master
         \
          o---o---o---o---o  next
                           \
                            o---o---o  topic
------------

We would want to make 'topic' forked from branch 'master',
for example because the functionality 'topic' branch depend on
got merged into more stable 'master' branch, like this:

------------
    o---o---o---o---o  master
        |            \
        |             o'--o'--o'  topic
         \
          o---o---o---o---o  next
------------

We can get this using the following command:

    git rebase --onto master next topic


Another example of --onto option is to rebase part of a
branch.  If we have the following situation:

------------
                            H---I---J topicB
                           /
                  E---F---G  topicA
                 /
    A---B---C---D  master
------------

then the command

    git rebase --onto master topicA topicB

would result in:

------------
                 H'--I'--J'  topicB
                /
                | E---F---G  topicA
                |/
    A---B---C---D  master
------------

This is useful when topicB does not depend on topicA.

A range of commits could also be removed with rebase.  If we have
the following situation:

------------
    E---F---G---H---I---J  topicA
------------

then the command

    git rebase --onto topicA~5 topicA~3 topicA

would result in the removal of commits F and G:

------------
    E---H'---I'---J'  topicA
------------

This is useful if F and G were flawed in some way, or should not be
part of topicA.  Note that the argument to --onto and the <upstream>
parameter can be any valid commit-ish.

In case of conflict, 'git-rebase' will stop at the first problematic commit
and leave conflict markers in the tree.  You can use 'git-diff' to locate
the markers (<<<<<<) and make edits to resolve the conflict.  For each
file you edit, you need to tell git that the conflict has been resolved,
typically this would be done with


    git add <filename>


After resolving the conflict manually and updating the index with the
desired resolution, you can continue the rebasing process with


    git rebase --continue


Alternatively, you can undo the 'git-rebase' with


    git rebase --abort

OPTIONS
-------
<newbase>::
        Starting point at which to create the new commits. If the
        --onto option is not specified, the starting point is
        <upstream>.  May be any valid commit, and not just an
        existing branch name.

<upstream>::
        Upstream branch to compare against.  May be any valid commit,
        not just an existing branch name.

<branch>::
        Working branch; defaults to HEAD.

--continue::
        Restart the rebasing process after having resolved a merge conflict.

--abort::
        Restore the original branch and abort the rebase operation.

--skip::
        Restart the rebasing process by skipping the current patch.

-m::
--merge::
        Use merging strategies to rebase.  When the recursive (default) merge
        strategy is used, this allows rebase to be aware of renames on the
        upstream side.

-s <strategy>::
--strategy=<strategy>::
        Use the given merge strategy; can be supplied more than
        once to specify them in the order they should be tried.
        If there is no `-s` option, a built-in list of strategies
        is used instead ('git-merge-recursive' when merging a single
        head, 'git-merge-octopus' otherwise).  This implies --merge.

-v::
--verbose::
        Display a diffstat of what changed upstream since the last rebase.

-C<n>::
        Ensure at least <n> lines of surrounding context match before
        and after each change.  When fewer lines of surrounding
        context exist they all must match.  By default no context is
        ever ignored.

--whitespace=<nowarn|warn|error|error-all|strip>::
        This flag is passed to the 'git-apply' program
        (see linkgit:git-apply[1]) that applies the patch.

-i::
--interactive::
        Make a list of the commits which are about to be rebased.  Let the
        user edit that list before rebasing.  This mode can also be used to
        split commits (see SPLITTING COMMITS below).

-p::
--preserve-merges::
        Instead of ignoring merges, try to recreate them.  This option
        only works in interactive mode.

include::merge-strategies.txt[]

NOTES
-----

You should understand the implications of using 'git-rebase' on a
repository that you share.  See also RECOVERING FROM UPSTREAM REBASE
below.

When the git-rebase command is run, it will first execute a "pre-rebase"
hook if one exists.  You can use this hook to do sanity checks and
reject the rebase if it isn't appropriate.  Please see the template
pre-rebase hook script for an example.

Upon completion, <branch> will be the current branch.

INTERACTIVE MODE
----------------

Rebasing interactively means that you have a chance to edit the commits
which are rebased.  You can reorder the commits, and you can
remove them (weeding out bad or otherwise unwanted patches).

The interactive mode is meant for this type of workflow:

1. have a wonderful idea
2. hack on the code
3. prepare a series for submission
4. submit

where point 2. consists of several instances of

a. regular use
 1. finish something worthy of a commit
 2. commit
b. independent fixup
 1. realize that something does not work
 2. fix that
 3. commit it

Sometimes the thing fixed in b.2. cannot be amended to the not-quite
perfect commit it fixes, because that commit is buried deeply in a
patch series.  That is exactly what interactive rebase is for: use it
after plenty of "a"s and "b"s, by rearranging and editing
commits, and squashing multiple commits into one.

Start it with the last commit you want to retain as-is:

        git rebase -i <after-this-commit>

An editor will be fired up with all the commits in your current branch
(ignoring merge commits), which come after the given commit.  You can
reorder the commits in this list to your heart's content, and you can
remove them.  The list looks more or less like this:

-------------------------------------------
pick deadbee The oneline of this commit
pick fa1afe1 The oneline of the next commit
...
-------------------------------------------

The oneline descriptions are purely for your pleasure; 'git-rebase' will
not look at them but at the commit names ("deadbee" and "fa1afe1" in this
example), so do not delete or edit the names.

By replacing the command "pick" with the command "edit", you can tell
'git-rebase' to stop after applying that commit, so that you can edit
the files and/or the commit message, amend the commit, and continue
rebasing.

If you want to fold two or more commits into one, replace the command
"pick" with "squash" for the second and subsequent commit.  If the
commits had different authors, it will attribute the squashed commit to
the author of the first commit.

In both cases, or when a "pick" does not succeed (because of merge
errors), the loop will stop to let you fix things, and you can continue
the loop with `git rebase --continue`.

For example, if you want to reorder the last 5 commits, such that what
was HEAD~4 becomes the new HEAD. To achieve that, you would call
'git-rebase' like this:

----------------------
$ git rebase -i HEAD~5
----------------------

And move the first patch to the end of the list.

You might want to preserve merges, if you have a history like this:

------------------
           X
            \
         A---M---B
        /
---o---O---P---Q
------------------

Suppose you want to rebase the side branch starting at "A" to "Q". Make
sure that the current HEAD is "B", and call

-----------------------------
$ git rebase -i -p --onto Q O
-----------------------------


SPLITTING COMMITS
-----------------

In interactive mode, you can mark commits with the action "edit".  However,
this does not necessarily mean that 'git-rebase' expects the result of this
edit to be exactly one commit.  Indeed, you can undo the commit, or you can
add other commits.  This can be used to split a commit into two:

- Start an interactive rebase with `git rebase -i <commit>^`, where
  <commit> is the commit you want to split.  In fact, any commit range
  will do, as long as it contains that commit.

- Mark the commit you want to split with the action "edit".

- When it comes to editing that commit, execute `git reset HEAD^`.  The
  effect is that the HEAD is rewound by one, and the index follows suit.
  However, the working tree stays the same.

- Now add the changes to the index that you want to have in the first
  commit.  You can use `git add` (possibly interactively) or
  'git-gui' (or both) to do that.

- Commit the now-current index with whatever commit message is appropriate
  now.

- Repeat the last two steps until your working tree is clean.

- Continue the rebase with `git rebase --continue`.

If you are not absolutely sure that the intermediate revisions are
consistent (they compile, pass the testsuite, etc.) you should use
'git-stash' to stash away the not-yet-committed changes
after each commit, test, and amend the commit if fixes are necessary.


RECOVERING FROM UPSTREAM REBASE
-------------------------------

Rebasing (or any other form of rewriting) a branch that others have
based work on is a bad idea: anyone downstream of it is forced to
manually fix their history.  This section explains how to do the fix
from the downstream's point of view.  The real fix, however, would be
to avoid rebasing the upstream in the first place.

To illustrate, suppose you are in a situation where someone develops a
'subsystem' branch, and you are working on a 'topic' that is dependent
on this 'subsystem'.  You might end up with a history like the
following:

------------
    o---o---o---o---o---o---o---o---o  master
         \
          o---o---o---o---o  subsystem
                           \
                            *---*---*  topic
------------

If 'subsystem' is rebased against 'master', the following happens:

------------
    o---o---o---o---o---o---o---o  master
         \                       \
          o---o---o---o---o       o'--o'--o'--o'--o'  subsystem
                           \
                            *---*---*  topic
------------

If you now continue development as usual, and eventually merge 'topic'
to 'subsystem', the commits from 'subsystem' will remain duplicated forever:

------------
    o---o---o---o---o---o---o---o  master
         \                       \
          o---o---o---o---o       o'--o'--o'--o'--o'--M  subsystem
                           \                         /
                            *---*---*-..........-*--*  topic
------------

Such duplicates are generally frowned upon because they clutter up
history, making it harder to follow.  To clean things up, you need to
transplant the commits on 'topic' to the new 'subsystem' tip, i.e.,
rebase 'topic'.  This becomes a ripple effect: anyone downstream from
'topic' is forced to rebase too, and so on!

There are two kinds of fixes, discussed in the following subsections:

Easy case: The changes are literally the same.::

        This happens if the 'subsystem' rebase was a simple rebase and
        had no conflicts.

Hard case: The changes are not the same.::

        This happens if the 'subsystem' rebase had conflicts, or used
        `\--interactive` to omit, edit, or squash commits; or if the
        upstream used one of `commit \--amend`, `reset`, or
        `filter-branch`.


The easy case
~~~~~~~~~~~~~

Only works if the changes (patch IDs based on the diff contents) on
'subsystem' are literally the same before and after the rebase
'subsystem' did.

In that case, the fix is easy because 'git-rebase' knows to skip
changes that are already present in the new upstream.  So if you say
(assuming you're on 'topic')
------------
    $ git rebase subsystem
------------
you will end up with the fixed history
------------
    o---o---o---o---o---o---o---o  master
                                 \
                                  o'--o'--o'--o'--o'  subsystem
                                                   \
                                                    *---*---*  topic
------------


The hard case
~~~~~~~~~~~~~

Things get more complicated if the 'subsystem' changes do not exactly
correspond to the ones before the rebase.

NOTE: While an "easy case recovery" sometimes appears to be successful
      even in the hard case, it may have unintended consequences.  For
      example, a commit that was removed via `git rebase
      \--interactive` will be **resurrected**!

The idea is to manually tell 'git-rebase' "where the old 'subsystem'
ended and your 'topic' began", that is, what the old merge-base
between them was.  You will have to find a way to name the last commit
of the old 'subsystem', for example:

* With the 'subsystem' reflog: after 'git-fetch', the old tip of
  'subsystem' is at `subsystem@\{1}`.  Subsequent fetches will
  increase the number.  (See linkgit:git-reflog[1].)

* Relative to the tip of 'topic': knowing that your 'topic' has three
  commits, the old tip of 'subsystem' must be `topic~3`.

You can then transplant the old `subsystem..topic` to the new tip by
saying (for the reflog case, and assuming you are on 'topic' already):
------------
    $ git rebase --onto subsystem subsystem@{1}
------------

The ripple effect of a "hard case" recovery is especially bad:
'everyone' downstream from 'topic' will now have to perform a "hard
case" recovery too!


Authors
------
Written by Junio C Hamano <gitster@pobox.com> and
Johannes E. Schindelin <johannes.schindelin@gmx.de>

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

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