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git-merge.adoc

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git-merge(1)

NAME

git-merge - Join two or more development histories together

SYNOPSIS

git merge [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
	[--no-verify] [-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
	[--[no-]allow-unrelated-histories]
	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>]
	[--into-name <branch>] [<commit>…​]
git merge (--continue | --abort | --quit)

DESCRIPTION

Incorporates changes from the named commits (since the time their histories diverged from the current branch) into the current branch. This command is used by git pull to incorporate changes from another repository and can be used by hand to merge changes from one branch into another.

Assume the following history exists and the current branch is master:

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

Then git merge topic will replay the changes made on the topic branch since it diverged from master (i.e., E) until its current commit (C) on top of master, and record the result in a new commit along with the names of the two parent commits and a log message from the user describing the changes. Before the operation, ORIG_HEAD is set to the tip of the current branch (C).

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

A merge stops if there’s a conflict that cannot be resolved automatically or if --no-commit was provided when initiating the merge. At that point you can run git merge --abort or git merge --continue.

git merge --abort will abort the merge process and try to reconstruct the pre-merge state. However, if there were uncommitted changes when the merge started (and especially if those changes were further modified after the merge was started), git merge --abort will in some cases be unable to reconstruct the original (pre-merge) changes. Therefore:

Warning: Running git merge with non-trivial uncommitted changes is discouraged: while possible, it may leave you in a state that is hard to back out of in the case of a conflict.

OPTIONS

-m <msg>

Set the commit message to be used for the merge commit (in case one is created).

If --log is specified, a shortlog of the commits being merged will be appended to the specified message.

The git fmt-merge-msg command can be used to give a good default for automated git merge invocations. The automated message can include the branch description.

--into-name <branch>

Prepare the default merge message as if merging to the branch <branch>, instead of the name of the real branch to which the merge is made.

-F <file>
--file=<file>

Read the commit message to be used for the merge commit (in case one is created).

If --log is specified, a shortlog of the commits being merged will be appended to the specified message.

--overwrite-ignore
--no-overwrite-ignore

Silently overwrite ignored files from the merge result. This is the default behavior. Use --no-overwrite-ignore to abort.

--abort

Abort the current conflict resolution process, and try to reconstruct the pre-merge state. If an autostash entry is present, apply it to the worktree.

If there were uncommitted worktree changes present when the merge started, git merge --abort will in some cases be unable to reconstruct these changes. It is therefore recommended to always commit or stash your changes before running git merge.

git merge --abort is equivalent to git reset --merge when MERGE_HEAD is present unless MERGE_AUTOSTASH is also present in which case git merge --abort applies the stash entry to the worktree whereas git reset --merge will save the stashed changes in the stash list.

--quit

Forget about the current merge in progress. Leave the index and the working tree as-is. If MERGE_AUTOSTASH is present, the stash entry will be saved to the stash list.

--continue

After a git merge stops due to conflicts you can conclude the merge by running git merge --continue (see "HOW TO RESOLVE CONFLICTS" section below).

<commit>…​

Commits, usually other branch heads, to merge into our branch. Specifying more than one commit will create a merge with more than two parents (affectionately called an Octopus merge).

If no commit is given from the command line, merge the remote-tracking branches that the current branch is configured to use as its upstream. See also the configuration section of this manual page.

When FETCH_HEAD (and no other commit) is specified, the branches recorded in the .git/FETCH_HEAD file by the previous invocation of git fetch for merging are merged to the current branch.

PRE-MERGE CHECKS

Before applying outside changes, you should get your own work in good shape and committed locally, so it will not be clobbered if there are conflicts. See also linkgit:git-stash[1]. git pull and git merge will stop without doing anything when local uncommitted changes overlap with files that git pull/git merge may need to update.

To avoid recording unrelated changes in the merge commit, git pull and git merge will also abort if there are any changes registered in the index relative to the HEAD commit. (Special narrow exceptions to this rule may exist depending on which merge strategy is in use, but generally, the index must match HEAD.)

If all named commits are already ancestors of HEAD, git merge will exit early with the message "Already up to date."

FAST-FORWARD MERGE

Often the current branch head is an ancestor of the named commit. This is the most common case especially when invoked from git pull: you are tracking an upstream repository, you have committed no local changes, and now you want to update to a newer upstream revision. In this case, a new commit is not needed to store the combined history; instead, the HEAD (along with the index) is updated to point at the named commit, without creating an extra merge commit.

This behavior can be suppressed with the --no-ff option.

TRUE MERGE

Except in a fast-forward merge (see above), the branches to be merged must be tied together by a merge commit that has both of them as its parents.

A merged version reconciling the changes from all branches to be merged is committed, and your HEAD, index, and working tree are updated to it. It is possible to have modifications in the working tree as long as they do not overlap; the update will preserve them.

When it is not obvious how to reconcile the changes, the following happens:

  1. The HEAD pointer stays the same.

  2. The MERGE_HEAD ref is set to point to the other branch head.

  3. Paths that merged cleanly are updated both in the index file and in your working tree.

  4. For conflicting paths, the index file records up to three versions: stage 1 stores the version from the common ancestor, stage 2 from HEAD, and stage 3 from MERGE_HEAD (you can inspect the stages with git ls-files -u). The working tree files contain the result of the merge operation; i.e. 3-way merge results with familiar conflict markers <<< === >>>.

  5. A ref named AUTO_MERGE is written, pointing to a tree corresponding to the current content of the working tree (including conflict markers for textual conflicts). Note that this ref is only written when the ort merge strategy is used (the default).

  6. No other changes are made. In particular, the local modifications you had before you started merge will stay the same and the index entries for them stay as they were, i.e. matching HEAD.

If you tried a merge which resulted in complex conflicts and want to start over, you can recover with git merge --abort.

MERGING TAG

When merging an annotated (and possibly signed) tag, Git always creates a merge commit even if a fast-forward merge is possible, and the commit message template is prepared with the tag message. Additionally, if the tag is signed, the signature check is reported as a comment in the message template. See also linkgit:git-tag[1].

When you want to just integrate with the work leading to the commit that happens to be tagged, e.g. synchronizing with an upstream release point, you may not want to make an unnecessary merge commit.

In such a case, you can "unwrap" the tag yourself before feeding it to git merge, or pass --ff-only when you do not have any work on your own. e.g.

git fetch origin
git merge v1.2.3^0
git merge --ff-only v1.2.3

HOW CONFLICTS ARE PRESENTED

During a merge, the working tree files are updated to reflect the result of the merge. Among the changes made to the common ancestor’s version, non-overlapping ones (that is, you changed an area of the file while the other side left that area intact, or vice versa) are incorporated in the final result verbatim. When both sides made changes to the same area, however, Git cannot randomly pick one side over the other, and asks you to resolve it by leaving what both sides did to that area.

By default, Git uses the same style as the one used by the "merge" program from the RCS suite to present such a conflicted hunk, like this:

Here are lines that are either unchanged from the common
ancestor, or cleanly resolved because only one side changed,
or cleanly resolved because both sides changed the same way.
<<<<<<< yours:sample.txt
Conflict resolution is hard;
let's go shopping.
=======
Git makes conflict resolution easy.
>>>>>>> theirs:sample.txt
And here is another line that is cleanly resolved or unmodified.

The area where a pair of conflicting changes happened is marked with markers <<<<<<<, =======, and >>>>>>>. The part before the ======= is typically your side, and the part afterwards is typically their side.

The default format does not show what the original said in the conflicting area. You cannot tell how many lines are deleted and replaced with Barbie’s remark on your side. The only thing you can tell is that your side wants to say it is hard and you’d prefer to go shopping, while the other side wants to claim it is easy.

An alternative style can be used by setting the merge.conflictStyle configuration variable to either "diff3" or "zdiff3". In "diff3" style, the above conflict may look like this:

Here are lines that are either unchanged from the common
ancestor, or cleanly resolved because only one side changed,
<<<<<<< yours:sample.txt
or cleanly resolved because both sides changed the same way.
Conflict resolution is hard;
let's go shopping.
||||||| base:sample.txt
or cleanly resolved because both sides changed identically.
Conflict resolution is hard.
=======
or cleanly resolved because both sides changed the same way.
Git makes conflict resolution easy.
>>>>>>> theirs:sample.txt
And here is another line that is cleanly resolved or unmodified.

while in "zdiff3" style, it may look like this:

Here are lines that are either unchanged from the common
ancestor, or cleanly resolved because only one side changed,
or cleanly resolved because both sides changed the same way.
<<<<<<< yours:sample.txt
Conflict resolution is hard;
let's go shopping.
||||||| base:sample.txt
or cleanly resolved because both sides changed identically.
Conflict resolution is hard.
=======
Git makes conflict resolution easy.
>>>>>>> theirs:sample.txt
And here is another line that is cleanly resolved or unmodified.

In addition to the <<<<<<<, =======, and >>>>>>> markers, it uses another ||||||| marker that is followed by the original text. You can tell that the original just stated a fact, and your side simply gave in to that statement and gave up, while the other side tried to have a more positive attitude. You can sometimes come up with a better resolution by viewing the original.

HOW TO RESOLVE CONFLICTS

After seeing a conflict, you can do two things:

  • Decide not to merge. The only clean-ups you need are to reset the index file to the HEAD commit to reverse 2. and to clean up working tree changes made by 2. and 3.; git merge --abort can be used for this.

  • Resolve the conflicts. Git will mark the conflicts in the working tree. Edit the files into shape and git add them to the index. Use git commit or git merge --continue to seal the deal. The latter command checks whether there is a (interrupted) merge in progress before calling git commit.

You can work through the conflict with a number of tools:

  • Use a mergetool. git mergetool to launch a graphical mergetool which will work through the merge with you.

  • Look at the diffs. git diff will show a three-way diff, highlighting changes from both the HEAD and MERGE_HEAD versions. git diff AUTO_MERGE will show what changes you’ve made so far to resolve textual conflicts.

  • Look at the diffs from each branch. git log --merge -p <path> will show diffs first for the HEAD version and then the MERGE_HEAD version.

  • Look at the originals. git show :1:filename shows the common ancestor, git show :2:filename shows the HEAD version, and git show :3:filename shows the MERGE_HEAD version.

EXAMPLES

  • Merge branches fixes and enhancements on top of the current branch, making an octopus merge:

    $ git merge fixes enhancements

  • Merge branch obsolete into the current branch, using ours merge strategy:

    $ git merge -s ours obsolete

  • Merge branch maint into the current branch, but do not make a new commit automatically:

    $ git merge --no-commit maint

    This can be used when you want to include further changes to the merge, or want to write your own merge commit message.

    You should refrain from abusing this option to sneak substantial changes into a merge commit. Small fixups like bumping release/version name would be acceptable.

CONFIGURATION

branch.<name>.mergeOptions

Sets default options for merging into branch <name>. The syntax and supported options are the same as those of git merge, but option values containing whitespace characters are currently not supported.

SEE ALSO

linkgit:git-fmt-merge-msg[1], linkgit:git-pull[1], linkgit:gitattributes[5], linkgit:git-reset[1], linkgit:git-diff[1], linkgit:git-ls-files[1], linkgit:git-add[1], linkgit:git-rm[1], linkgit:git-mergetool[1]

GIT

Part of the linkgit:git[1] suite