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builtin/merge.c contains this important requirement for merge strategies: ...the index must be in sync with the head commit. The strategies are responsible to ensure this. However, Documentation/git-merge.txt says: ...[merge will] abort if there are any changes registered in the index relative to the `HEAD` commit. (One exception is when the changed index entries are in the state that would result from the merge already.) Interestingly, prior to commitc0be8aa06b
("Documentation/git-merge.txt: Partial rewrite of How Merge Works", 2008-07-19), Documentation/git-merge.txt said much more: ...the index file must match the tree of `HEAD` commit... [NOTE] This is a bit of a lie. In certain special cases [explained in detail]... Otherwise, merge will refuse to do any harm to your repository (that is...your working tree...and index are left intact). So, this suggests that the exceptions existed because there were special cases where it would case no harm, and potentially be slightly more convenient for the user. While the current text in git-merge.txt does list a condition under which it would be safe to proceed despite the index not matching HEAD, it does not match what is actually implemented, in three different ways: * The exception is written to describe what unpack-trees allows. Not all merge strategies allow such an exception, though, making this description misleading. 'ours' and 'octopus' merges have strictly enforced index==HEAD for a while, and the commit previous to this one made 'recursive' do so as well. * If someone did a three-way content merge on a specific file using versions from the relevant commits and staged it prior to running merge, then that path would technically satisfy the exception listed in git-merge.txt. unpack-trees.c would still error out on the path, though, because it defers the three-way content merge logic to other parts of the code (resolve, octopus, or recursive) and has no way of checking whether the index entry from before the merge will match the end result of the merge. * The exception as implemented in unpack-trees actually only checked that the index matched the MERGE_HEAD version of the file and that HEAD matched the merge base. Assuming no renames, that would indeed provide cases where the index matches the end result we'd get from a merge. But renames means unpack-trees is checking that it instead matches something other than what the final result will be, risking either erroring out when we shouldn't need to, or not erroring out when we should and overwriting the user's staged changes. In addition to the wording behind this exception being misleading, it is also somewhat surprising to see how many times the code for the special cases were wrong or the check to make sure the index matched head was forgotten altogether: * Prior to commitee6566e8d7
("[PATCH] Rewrite read-tree", 2005-09-05), there were many cases where an unclean index entry was allowed (look for merged_entry_allow_dirty()); it appears that in those cases, the merge would have simply overwritten staged changes with the result of the merge. Thus, the merge result would have been correct, but the user's uncommitted changes could be thrown away without warning. * Prior to commit160252f816
("git-merge-ours: make sure our index matches HEAD", 2005-11-03), the 'ours' merge strategy did not check whether the index matched HEAD. If it didn't, the resulting merge would include all the staged changes, and thus wasn't really an 'ours' strategy. * Prior to commit3ec62ad9ff
("merge-octopus: abort if index does not match HEAD", 2016-04-09), 'octopus' merges did not check whether the index matched HEAD, also resulting in any staged changes from before the commit silently being folded into the resulting merge. commita6ee883b8e
("t6044: new merge testcases for when index doesn't match HEAD", 2016-04-09) was also added at the same time to try to test to make sure all strategies did the necessary checking for the requirement that the index match HEAD. Sadly, it didn't catch all the cases, as evidenced by the remainder of this list... * Prior to commit65170c07d4
("merge-recursive: avoid incorporating uncommitted changes in a merge", 2017-12-21), merge-recursive simply relied on unpack_trees() to do the necessary check, but in one special case it avoided calling unpack_trees() entirely and accidentally ended up silently including any staged changes from before the merge in the resulting merge commit. * The commit immediately before this one in this series noted that the exceptions were written in a way that assumed no renames, making it unsafe for merge-recursive to use. merge-recursive was modified to use its own check to enforce that index==HEAD. This history makes it very tempting to go into builtin/merge.c and replace the comment that strategies must enforce that index matches HEAD with code that just enforces it. At this point, that would only affect the 'resolve' strategy; all other strategies have each been modified to manually enforce it. (However, note that index==HEAD is not strictly enforced for fast-forward merges, as those are not considered a merge strategy and they trigger in builtin/merge.c before the section in the code where the relevant comment is found.) But, even if we don't take the step of just fixing these problems by enforcing index==HEAD for all strategies, we at least need to update this misleading documentation in git-merge.txt. For now, just modify the claim in Documentation/git-merge.txt to fix the error. The precise details around combination of merges strategies and special cases probably is not relevant to most users, so simply state that exceptions may exist but are narrow and vary depending upon which merge strategy is in use. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
356 lines
12 KiB
Plaintext
356 lines
12 KiB
Plaintext
git-merge(1)
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============
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NAME
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----
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git-merge - Join two or more development histories together
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SYNOPSIS
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--------
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[verse]
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'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
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[-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
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[--[no-]allow-unrelated-histories]
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[--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
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'git merge' --abort
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'git merge' --continue
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DESCRIPTION
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-----------
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Incorporates changes from the named commits (since the time their
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histories diverged from the current branch) into the current
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branch. This command is used by 'git pull' to incorporate changes
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from another repository and can be used by hand to merge changes
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from one branch into another.
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Assume the following history exists and the current branch is
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"`master`":
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------------
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A---B---C topic
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/
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D---E---F---G master
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------------
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Then "`git merge topic`" will replay the changes made on the
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`topic` branch since it diverged from `master` (i.e., `E`) until
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its current commit (`C`) on top of `master`, and record the result
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in a new commit along with the names of the two parent commits and
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a log message from the user describing the changes.
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------------
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A---B---C topic
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/ \
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D---E---F---G---H master
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------------
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The second syntax ("`git merge --abort`") can only be run after the
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merge has resulted in conflicts. 'git merge --abort' will abort the
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merge process and try to reconstruct the pre-merge state. However,
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if there were uncommitted changes when the merge started (and
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especially if those changes were further modified after the merge
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was started), 'git merge --abort' will in some cases be unable to
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reconstruct the original (pre-merge) changes. Therefore:
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*Warning*: Running 'git merge' with non-trivial uncommitted changes is
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discouraged: while possible, it may leave you in a state that is hard to
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back out of in the case of a conflict.
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The fourth syntax ("`git merge --continue`") can only be run after the
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merge has resulted in conflicts.
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OPTIONS
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-------
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include::merge-options.txt[]
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-m <msg>::
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Set the commit message to be used for the merge commit (in
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case one is created).
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+
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If `--log` is specified, a shortlog of the commits being merged
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will be appended to the specified message.
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+
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The 'git fmt-merge-msg' command can be
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used to give a good default for automated 'git merge'
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invocations. The automated message can include the branch description.
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--[no-]rerere-autoupdate::
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Allow the rerere mechanism to update the index with the
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result of auto-conflict resolution if possible.
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--abort::
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Abort the current conflict resolution process, and
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try to reconstruct the pre-merge state.
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+
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If there were uncommitted worktree changes present when the merge
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started, 'git merge --abort' will in some cases be unable to
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reconstruct these changes. It is therefore recommended to always
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commit or stash your changes before running 'git merge'.
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+
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'git merge --abort' is equivalent to 'git reset --merge' when
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`MERGE_HEAD` is present.
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--continue::
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After a 'git merge' stops due to conflicts you can conclude the
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merge by running 'git merge --continue' (see "HOW TO RESOLVE
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CONFLICTS" section below).
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<commit>...::
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Commits, usually other branch heads, to merge into our branch.
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Specifying more than one commit will create a merge with
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more than two parents (affectionately called an Octopus merge).
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+
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If no commit is given from the command line, merge the remote-tracking
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branches that the current branch is configured to use as its upstream.
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See also the configuration section of this manual page.
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+
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When `FETCH_HEAD` (and no other commit) is specified, the branches
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recorded in the `.git/FETCH_HEAD` file by the previous invocation
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of `git fetch` for merging are merged to the current branch.
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PRE-MERGE CHECKS
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----------------
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Before applying outside changes, you should get your own work in
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good shape and committed locally, so it will not be clobbered if
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there are conflicts. See also linkgit:git-stash[1].
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'git pull' and 'git merge' will stop without doing anything when
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local uncommitted changes overlap with files that 'git pull'/'git
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merge' may need to update.
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To avoid recording unrelated changes in the merge commit,
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'git pull' and 'git merge' will also abort if there are any changes
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registered in the index relative to the `HEAD` commit. (Special
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narrow exceptions to this rule may exist depending on which merge
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strategy is in use, but generally, the index must match HEAD.)
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If all named commits are already ancestors of `HEAD`, 'git merge'
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will exit early with the message "Already up to date."
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FAST-FORWARD MERGE
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------------------
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Often the current branch head is an ancestor of the named commit.
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This is the most common case especially when invoked from 'git
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pull': you are tracking an upstream repository, you have committed
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no local changes, and now you want to update to a newer upstream
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revision. In this case, a new commit is not needed to store the
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combined history; instead, the `HEAD` (along with the index) is
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updated to point at the named commit, without creating an extra
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merge commit.
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This behavior can be suppressed with the `--no-ff` option.
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TRUE MERGE
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----------
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Except in a fast-forward merge (see above), the branches to be
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merged must be tied together by a merge commit that has both of them
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as its parents.
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A merged version reconciling the changes from all branches to be
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merged is committed, and your `HEAD`, index, and working tree are
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updated to it. It is possible to have modifications in the working
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tree as long as they do not overlap; the update will preserve them.
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When it is not obvious how to reconcile the changes, the following
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happens:
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1. The `HEAD` pointer stays the same.
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2. The `MERGE_HEAD` ref is set to point to the other branch head.
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3. Paths that merged cleanly are updated both in the index file and
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in your working tree.
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4. For conflicting paths, the index file records up to three
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versions: stage 1 stores the version from the common ancestor,
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stage 2 from `HEAD`, and stage 3 from `MERGE_HEAD` (you
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can inspect the stages with `git ls-files -u`). The working
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tree files contain the result of the "merge" program; i.e. 3-way
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merge results with familiar conflict markers `<<<` `===` `>>>`.
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5. No other changes are made. In particular, the local
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modifications you had before you started merge will stay the
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same and the index entries for them stay as they were,
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i.e. matching `HEAD`.
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If you tried a merge which resulted in complex conflicts and
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want to start over, you can recover with `git merge --abort`.
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MERGING TAG
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-----------
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When merging an annotated (and possibly signed) tag, Git always
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creates a merge commit even if a fast-forward merge is possible, and
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the commit message template is prepared with the tag message.
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Additionally, if the tag is signed, the signature check is reported
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as a comment in the message template. See also linkgit:git-tag[1].
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When you want to just integrate with the work leading to the commit
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that happens to be tagged, e.g. synchronizing with an upstream
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release point, you may not want to make an unnecessary merge commit.
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In such a case, you can "unwrap" the tag yourself before feeding it
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to `git merge`, or pass `--ff-only` when you do not have any work on
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your own. e.g.
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----
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git fetch origin
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git merge v1.2.3^0
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git merge --ff-only v1.2.3
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----
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HOW CONFLICTS ARE PRESENTED
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---------------------------
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During a merge, the working tree files are updated to reflect the result
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of the merge. Among the changes made to the common ancestor's version,
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non-overlapping ones (that is, you changed an area of the file while the
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other side left that area intact, or vice versa) are incorporated in the
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final result verbatim. When both sides made changes to the same area,
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however, Git cannot randomly pick one side over the other, and asks you to
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resolve it by leaving what both sides did to that area.
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By default, Git uses the same style as the one used by the "merge" program
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from the RCS suite to present such a conflicted hunk, like this:
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------------
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Here are lines that are either unchanged from the common
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ancestor, or cleanly resolved because only one side changed.
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<<<<<<< yours:sample.txt
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Conflict resolution is hard;
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let's go shopping.
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=======
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Git makes conflict resolution easy.
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>>>>>>> theirs:sample.txt
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And here is another line that is cleanly resolved or unmodified.
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------------
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The area where a pair of conflicting changes happened is marked with markers
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`<<<<<<<`, `=======`, and `>>>>>>>`. The part before the `=======`
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is typically your side, and the part afterwards is typically their side.
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The default format does not show what the original said in the conflicting
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area. You cannot tell how many lines are deleted and replaced with
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Barbie's remark on your side. The only thing you can tell is that your
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side wants to say it is hard and you'd prefer to go shopping, while the
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other side wants to claim it is easy.
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An alternative style can be used by setting the "merge.conflictStyle"
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configuration variable to "diff3". In "diff3" style, the above conflict
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may look like this:
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------------
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Here are lines that are either unchanged from the common
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ancestor, or cleanly resolved because only one side changed.
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<<<<<<< yours:sample.txt
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Conflict resolution is hard;
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let's go shopping.
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|||||||
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Conflict resolution is hard.
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=======
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Git makes conflict resolution easy.
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>>>>>>> theirs:sample.txt
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And here is another line that is cleanly resolved or unmodified.
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------------
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In addition to the `<<<<<<<`, `=======`, and `>>>>>>>` markers, it uses
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another `|||||||` marker that is followed by the original text. You can
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tell that the original just stated a fact, and your side simply gave in to
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that statement and gave up, while the other side tried to have a more
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positive attitude. You can sometimes come up with a better resolution by
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viewing the original.
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HOW TO RESOLVE CONFLICTS
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------------------------
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After seeing a conflict, you can do two things:
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* Decide not to merge. The only clean-ups you need are to reset
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the index file to the `HEAD` commit to reverse 2. and to clean
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up working tree changes made by 2. and 3.; `git merge --abort`
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can be used for this.
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* Resolve the conflicts. Git will mark the conflicts in
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the working tree. Edit the files into shape and
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'git add' them to the index. Use 'git commit' or
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'git merge --continue' to seal the deal. The latter command
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checks whether there is a (interrupted) merge in progress
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before calling 'git commit'.
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You can work through the conflict with a number of tools:
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* Use a mergetool. `git mergetool` to launch a graphical
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mergetool which will work you through the merge.
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* Look at the diffs. `git diff` will show a three-way diff,
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highlighting changes from both the `HEAD` and `MERGE_HEAD`
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versions.
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* Look at the diffs from each branch. `git log --merge -p <path>`
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will show diffs first for the `HEAD` version and then the
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`MERGE_HEAD` version.
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* Look at the originals. `git show :1:filename` shows the
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common ancestor, `git show :2:filename` shows the `HEAD`
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version, and `git show :3:filename` shows the `MERGE_HEAD`
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version.
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EXAMPLES
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--------
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* Merge branches `fixes` and `enhancements` on top of
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the current branch, making an octopus merge:
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------------------------------------------------
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$ git merge fixes enhancements
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------------------------------------------------
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* Merge branch `obsolete` into the current branch, using `ours`
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merge strategy:
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------------------------------------------------
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$ git merge -s ours obsolete
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------------------------------------------------
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* Merge branch `maint` into the current branch, but do not make
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a new commit automatically:
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------------------------------------------------
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$ git merge --no-commit maint
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------------------------------------------------
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This can be used when you want to include further changes to the
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merge, or want to write your own merge commit message.
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You should refrain from abusing this option to sneak substantial
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changes into a merge commit. Small fixups like bumping
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release/version name would be acceptable.
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include::merge-strategies.txt[]
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CONFIGURATION
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-------------
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include::merge-config.txt[]
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branch.<name>.mergeOptions::
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Sets default options for merging into branch <name>. The syntax and
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supported options are the same as those of 'git merge', but option
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values containing whitespace characters are currently not supported.
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SEE ALSO
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--------
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linkgit:git-fmt-merge-msg[1], linkgit:git-pull[1],
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linkgit:gitattributes[5],
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linkgit:git-reset[1],
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linkgit:git-diff[1], linkgit:git-ls-files[1],
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linkgit:git-add[1], linkgit:git-rm[1],
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linkgit:git-mergetool[1]
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GIT
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---
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Part of the linkgit:git[1] suite
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