Git and libgit2
Mike Slinn

Merge and Pull: Git CLI vs. Libgit2 Wrappers

Published 2023-05-12. Last modified 2023-05-15.
Time to read: 3 minutes.

This page is part of the git collection, categorized under git.

Many people use git-merge for years without realizing how complex this command is. Git-pull utilizes git-merge when merging changes.

Because libgit2 is a low-level API, higher-level functionality such as git’s git-pull is not provided. If you need higher-level functionality you must either write it yourself, or use whatever extra capability provided by the language binding your project uses. This means it is important to understand the low-level complexity when using libgit2 and its language bindings, such as rugged or pygit2 to merge and pull updates.

Shell
$ man git-merge
GIT-MERGE(1)                 Git Manual                 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.

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

       The second syntax ("git merge --abort") can only be run after
       the merge has resulted in conflicts. 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.

       The third syntax ("git merge --continue") can only be run
       after the merge has resulted in conflicts.

OPTIONS
       --commit, --no-commit
           Perform the merge and commit the result. This option can
           be used to override --no-commit.

           With --no-commit perform the merge and stop just before
           creating a merge commit, to give the user a chance to
           inspect and further tweak the merge result before
           committing.

           Note that fast-forward updates do not create a merge
           commit and therefore there is no way to stop those merges
           with --no-commit. Thus, if you want to ensure your branch
           is not changed or updated by the merge command, use
           --no-ff with --no-commit.

       --edit, -e, --no-edit
           Invoke an editor before committing successful mechanical
           merge to further edit the auto-generated merge message,
           so that the user can explain and justify the merge. The
           --no-edit option can be used to accept the auto-generated
           message (this is generally discouraged). The --edit (or
           -e) option is still useful if you are giving a draft
           message with the -m option from the command line and want
           to edit it in the editor.

           Older scripts may depend on the historical behaviour of
           not allowing the user to edit the merge log message. They
           will see an editor opened when they run git merge. To
           make it easier to adjust such scripts to the updated
           behaviour, the environment variable GIT_MERGE_AUTOEDIT
           can be set to no at the beginning of them.

       --cleanup=<mode>
           This option determines how the merge message will be
           cleaned up before committing. See git-commit(1) for more
           details. In addition, if the <mode> is given a value of
           scissors, scissors will be appended to MERGE_MSG before
           being passed on to the commit machinery in the case of a
           merge conflict.

       --ff, --no-ff, --ff-only
           Specifies how a merge is handled when the merged-in
           history is already a descendant of the current history.
           --ff is the default unless merging an annotated (and
           possibly signed) tag that is not stored in its natural
           place in the refs/tags/ hierarchy, in which case --no-ff
           is assumed.

           With --ff, when possible resolve the merge as a
           fast-forward (only update the branch pointer to match the
           merged branch; do not create a merge commit). When not
           possible (when the merged-in history is not a descendant
           of the current history), create a merge commit.

           With --no-ff, create a merge commit in all cases, even
           when the merge could instead be resolved as a
           fast-forward.

           With --ff-only, resolve the merge as a fast-forward when
           possible. When not possible, refuse to merge and exit
           with a non-zero status.

       -S[<keyid>], --gpg-sign[=<keyid>], --no-gpg-sign
           GPG-sign the resulting merge commit. The keyid argument
           is optional and defaults to the committer identity; if
           specified, it must be stuck to the option without a
           space.  --no-gpg-sign is useful to countermand both
           commit.gpgSign configuration variable, and earlier
           --gpg-sign.

       --log[=<n>], --no-log
           In addition to branch names, populate the log message
           with one-line descriptions from at most <n> actual
           commits that are being merged. See also git-fmt-merge-
           msg(1).

           With --no-log do not list one-line descriptions from the
           actual commits being merged.

       --signoff, --no-signoff
           Add a Signed-off-by trailer by the committer at the end
           of the commit log message. The meaning of a signoff
           depends on the project to which you’re committing. For
           example, it may certify that the committer has the rights
           to submit the work under the project’s license or agrees
           to some contributor representation, such as a Developer
           Certificate of Origin. (See
           http://developercertificate.org for the one used by the
           Linux kernel and Git projects.) Consult the documentation
           or leadership of the project to which you’re contributing
           to understand how the signoffs are used in that project.

           The --no-signoff option can be used to countermand an
           earlier --signoff option on the command line.

       --stat, -n, --no-stat
           Show a diffstat at the end of the merge. The diffstat is
           also controlled by the configuration option merge.stat.

           With -n or --no-stat do not show a diffstat at the end of
           the merge.

       --squash, --no-squash
           Produce the working tree and index state as if a real
           merge happened (except for the merge information), but do
           not actually make a commit, move the HEAD, or record
           $GIT_DIR/MERGE_HEAD (to cause the next git commit command
           to create a merge commit). This allows you to create a
           single commit on top of the current branch whose effect
           is the same as merging another branch (or more in case of
           an octopus).

           With --no-squash perform the merge and commit the result.
           This option can be used to override --squash.

           With --squash, --commit is not allowed, and will fail.

       --[no-]verify
           By default, the pre-merge and commit-msg hooks are run.
           When --no-verify is given, these are bypassed. See also
           githooks(5).

       -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 (ort when merging a single head, octopus
           otherwise).

       -X <option>, --strategy-option=<option>
           Pass merge strategy specific option through to the merge
           strategy.

       --verify-signatures, --no-verify-signatures
           Verify that the tip commit of the side branch being
           merged is signed with a valid key, i.e. a key that has a
           valid uid: in the default trust model, this means the
           signing key has been signed by a trusted key. If the tip
           commit of the side branch is not signed with a valid key,
           the merge is aborted.

       --summary, --no-summary
           Synonyms to --stat and --no-stat; these are deprecated
           and will be removed in the future.

       -q, --quiet
           Operate quietly. Implies --no-progress.

       -v, --verbose
           Be verbose.

       --progress, --no-progress
           Turn progress on/off explicitly. If neither is specified,
           progress is shown if standard error is connected to a
           terminal. Note that not all merge strategies may support
           progress reporting.

       --autostash, --no-autostash
           Automatically create a temporary stash entry before the
           operation begins, record it in the special ref
           MERGE_AUTOSTASH and apply it after the operation ends.
           This means that you can run the operation on a dirty
           worktree. However, use with care: the final stash
           application after a successful merge might result in
           non-trivial conflicts.

       --allow-unrelated-histories
           By default, git merge command refuses to merge histories
           that do not share a common ancestor. This option can be
           used to override this safety when merging histories of
           two projects that started their lives independently. As
           that is a very rare occasion, no configuration variable
           to enable this by default exists and will not be added.

       -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.

       --rerere-autoupdate, --no-rerere-autoupdate
           After the rerere mechanism reuses a recorded resolution
           on the current conflict to update the files in the
           working tree, allow it to also update the index with the
           result of resolution.  --no-rerere-autoupdate is a good
           way to double-check what rerere did and catch potential
           mismerges, before committing the result to the index with
           a separate git add.

       --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 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"
           program; i.e. 3-way merge results with familiar conflict
           markers <<< === >>>.

        5. 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 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 you through the merge.

       •   Look at the diffs.  git diff will show a three-way diff,
           highlighting changes from both the HEAD and MERGE_HEAD
           versions.

       •   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.

MERGE STRATEGIES
       The merge mechanism (git merge and git pull commands) allows
       the backend merge strategies to be chosen with -s option.
       Some strategies can also take their own options, which can be
       passed by giving -X<option> arguments to git merge and/or git
       pull.

       ort
           This is the default merge strategy when pulling or
           merging one branch. This strategy can only resolve two
           heads using a 3-way merge algorithm. When there is more
           than one common ancestor that can be used for 3-way
           merge, it creates a merged tree of the common ancestors
           and uses that as the reference tree for the 3-way merge.
           This has been reported to result in fewer merge conflicts
           without causing mismerges by tests done on actual merge
           commits taken from Linux 2.6 kernel development history.
           Additionally this strategy can detect and handle merges
           involving renames. It does not make use of detected
           copies. The name for this algorithm is an acronym
           ("Ostensibly Recursive’s Twin") and came from the fact
           that it was written as a replacement for the previous
           default algorithm, recursive.

           The ort strategy can take the following options:

           ours
               This option forces conflicting hunks to be
               auto-resolved cleanly by favoring our version.
               Changes from the other tree that do not conflict with
               our side are reflected in the merge result. For a
               binary file, the entire contents are taken from our
               side.

               This should not be confused with the ours merge
               strategy, which does not even look at what the other
               tree contains at all. It discards everything the
               other tree did, declaring our history contains all
               that happened in it.

           theirs
               This is the opposite of ours; note that, unlike ours,
               there is no theirs merge strategy to confuse this
               merge option with.

           ignore-space-change, ignore-all-space,
           ignore-space-at-eol, ignore-cr-at-eol
               Treats lines with the indicated type of whitespace
               change as unchanged for the sake of a three-way
               merge. Whitespace changes mixed with other changes to
               a line are not ignored. See also git-diff(1) -b, -w,
               --ignore-space-at-eol, and --ignore-cr-at-eol.

               •   If their version only introduces whitespace
                   changes to a line, our version is used;

               •   If our version introduces whitespace changes but
                   their version includes a substantial change,
                   their version is used;

               •   Otherwise, the merge proceeds in the usual way.

           renormalize
               This runs a virtual check-out and check-in of all
               three stages of a file when resolving a three-way
               merge. This option is meant to be used when merging
               branches with different clean filters or end-of-line
               normalization rules. See "Merging branches with
               differing checkin/checkout attributes" in
               gitattributes(5) for details.

           no-renormalize
               Disables the renormalize option. This overrides the
               merge.renormalize configuration variable.

           find-renames[=<n>]
               Turn on rename detection, optionally setting the
               similarity threshold. This is the default. This
               overrides the merge.renames configuration variable.
               See also git-diff(1) --find-renames.

           rename-threshold=<n>
               Deprecated synonym for find-renames=<n>.

           subtree[=<path>]
               This option is a more advanced form of subtree
               strategy, where the strategy makes a guess on how two
               trees must be shifted to match with each other when
               merging. Instead, the specified path is prefixed (or
               stripped from the beginning) to make the shape of two
               trees to match.

       recursive
           This can only resolve two heads using a 3-way merge
           algorithm. When there is more than one common ancestor
           that can be used for 3-way merge, it creates a merged
           tree of the common ancestors and uses that as the
           reference tree for the 3-way merge. This has been
           reported to result in fewer merge conflicts without
           causing mismerges by tests done on actual merge commits
           taken from Linux 2.6 kernel development history.
           Additionally this can detect and handle merges involving
           renames. It does not make use of detected copies. This
           was the default strategy for resolving two heads from Git
           v0.99.9k until v2.33.0.

           The recursive strategy takes the same options as ort.
           However, there are three additional options that ort
           ignores (not documented above) that are potentially
           useful with the recursive strategy:

           patience
               Deprecated synonym for diff-algorithm=patience.

           diff-algorithm=[patience|minimal|histogram|myers]
               Use a different diff algorithm while merging, which
               can help avoid mismerges that occur due to
               unimportant matching lines (such as braces from
               distinct functions). See also git-diff(1)
               --diff-algorithm. Note that ort specifically uses
               diff-algorithm=histogram, while recursive defaults to
               the diff.algorithm config setting.

           no-renames
               Turn off rename detection. This overrides the
               merge.renames configuration variable. See also git-
               diff(1) --no-renames.

       resolve
           This can only resolve two heads (i.e. the current branch
           and another branch you pulled from) using a 3-way merge
           algorithm. It tries to carefully detect criss-cross merge
           ambiguities. It does not handle renames.

       octopus
           This resolves cases with more than two heads, but refuses
           to do a complex merge that needs manual resolution. It is
           primarily meant to be used for bundling topic branch
           heads together. This is the default merge strategy when
           pulling or merging more than one branch.

       ours
           This resolves any number of heads, but the resulting tree
           of the merge is always that of the current branch head,
           effectively ignoring all changes from all other branches.
           It is meant to be used to supersede old development
           history of side branches. Note that this is different
           from the -Xours option to the recursive merge strategy.

       subtree
           This is a modified ort strategy. When merging trees A and
           B, if B corresponds to a subtree of A, B is first
           adjusted to match the tree structure of A, instead of
           reading the trees at the same level. This adjustment is
           also done to the common ancestor tree.

       With the strategies that use 3-way merge (including the
       default, ort), if a change is made on both branches, but
       later reverted on one of the branches, that change will be
       present in the merged result; some people find this behavior
       confusing. It occurs because only the heads and the merge
       base are considered when performing a merge, not the
       individual commits. The merge algorithm therefore considers
       the reverted change as no change at all, and substitutes the
       changed version instead.

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.

       Everything above this line in this section isn’t included
       from the git-config(1) documentation. The content that
       follows is the same as what’s found there:

       merge.conflictStyle
           Specify the style in which conflicted hunks are written
           out to working tree files upon merge. The default is
           "merge", which shows a <<<<<<< conflict marker, changes
           made by one side, a ======= marker, changes made by the
           other side, and then a >>>>>>> marker. An alternate
           style, "diff3", adds a ||||||| marker and the original
           text before the ======= marker. The "merge" style tends
           to produce smaller conflict regions than diff3, both
           because of the exclusion of the original text, and
           because when a subset of lines match on the two sides
           they are just pulled out of the conflict region. Another
           alternate style, "zdiff3", is similar to diff3 but
           removes matching lines on the two sides from the conflict
           region when those matching lines appear near either the
           beginning or end of a conflict region.

       merge.defaultToUpstream
           If merge is called without any commit argument, merge the
           upstream branches configured for the current branch by
           using their last observed values stored in their
           remote-tracking branches. The values of the
           branch.<current branch>.merge that name the branches at
           the remote named by branch.<current branch>.remote are
           consulted, and then they are mapped via
           remote.<remote>.fetch to their corresponding
           remote-tracking branches, and the tips of these tracking
           branches are merged. Defaults to true.

       merge.ff
           By default, Git does not create an extra merge commit
           when merging a commit that is a descendant of the current
           commit. Instead, the tip of the current branch is
           fast-forwarded. When set to false, this variable tells
           Git to create an extra merge commit in such a case
           (equivalent to giving the --no-ff option from the command
           line). When set to only, only such fast-forward merges
           are allowed (equivalent to giving the --ff-only option
           from the command line).

       merge.verifySignatures
           If true, this is equivalent to the --verify-signatures
           command line option. See git-merge(1) for details.

       merge.branchdesc
           In addition to branch names, populate the log message
           with the branch description text associated with them.
           Defaults to false.

       merge.log
           In addition to branch names, populate the log message
           with at most the specified number of one-line
           descriptions from the actual commits that are being
           merged. Defaults to false, and true is a synonym for 20.

       merge.suppressDest
           By adding a glob that matches the names of integration
           branches to this multi-valued configuration variable, the
           default merge message computed for merges into these
           integration branches will omit "into <branch name>" from
           its title.

           An element with an empty value can be used to clear the
           list of globs accumulated from previous configuration
           entries. When there is no merge.suppressDest variable
           defined, the default value of master is used for backward
           compatibility.

       merge.renameLimit
           The number of files to consider in the exhaustive portion
           of rename detection during a merge. If not specified,
           defaults to the value of diff.renameLimit. If neither
           merge.renameLimit nor diff.renameLimit are specified,
           currently defaults to 7000. This setting has no effect if
           rename detection is turned off.

       merge.renames
           Whether Git detects renames. If set to "false", rename
           detection is disabled. If set to "true", basic rename
           detection is enabled. Defaults to the value of
           diff.renames.

       merge.directoryRenames
           Whether Git detects directory renames, affecting what
           happens at merge time to new files added to a directory
           on one side of history when that directory was renamed on
           the other side of history. If merge.directoryRenames is
           set to "false", directory rename detection is disabled,
           meaning that such new files will be left behind in the
           old directory. If set to "true", directory rename
           detection is enabled, meaning that such new files will be
           moved into the new directory. If set to "conflict", a
           conflict will be reported for such paths. If
           merge.renames is false, merge.directoryRenames is ignored
           and treated as false. Defaults to "conflict".

       merge.renormalize
           Tell Git that canonical representation of files in the
           repository has changed over time (e.g. earlier commits
           record text files with CRLF line endings, but recent ones
           use LF line endings). In such a repository, Git can
           convert the data recorded in commits to a canonical form
           before performing a merge to reduce unnecessary
           conflicts. For more information, see section "Merging
           branches with differing checkin/checkout attributes" in
           gitattributes(5).

       merge.stat
           Whether to print the diffstat between ORIG_HEAD and the
           merge result at the end of the merge. True by default.

       merge.autoStash
           When set to true, automatically create a temporary stash
           entry before the operation begins, and apply it after the
           operation ends. This means that you can run merge on a
           dirty worktree. However, use with care: the final stash
           application after a successful merge might result in
           non-trivial conflicts. This option can be overridden by
           the --no-autostash and --autostash options of git-
           merge(1). Defaults to false.

       merge.tool
           Controls which merge tool is used by git-mergetool(1).
           The list below shows the valid built-in values. Any other
           value is treated as a custom merge tool and requires that
           a corresponding mergetool.<tool>.cmd variable is defined.

       merge.guitool
           Controls which merge tool is used by git-mergetool(1)
           when the -g/--gui flag is specified. The list below shows
           the valid built-in values. Any other value is treated as
           a custom merge tool and requires that a corresponding
           mergetool.<guitool>.cmd variable is defined.

           araxis
               Use Araxis Merge (requires a graphical session)

           bc
               Use Beyond Compare (requires a graphical session)

           bc3
               Use Beyond Compare (requires a graphical session)

           bc4
               Use Beyond Compare (requires a graphical session)

           codecompare
               Use Code Compare (requires a graphical session)

           deltawalker
               Use DeltaWalker (requires a graphical session)

           diffmerge
               Use DiffMerge (requires a graphical session)

           diffuse
               Use Diffuse (requires a graphical session)

           ecmerge
               Use ECMerge (requires a graphical session)

           emerge
               Use Emacs' Emerge

           examdiff
               Use ExamDiff Pro (requires a graphical session)

           guiffy
               Use Guiffy’s Diff Tool (requires a graphical session)

           gvimdiff
               Use gVim (requires a graphical session) with a custom
               layout (see git help mergetool's BACKEND SPECIFIC
               HINTS section)

           gvimdiff1
               Use gVim (requires a graphical session) with a 2
               panes layout (LOCAL and REMOTE)

           gvimdiff2
               Use gVim (requires a graphical session) with a 3
               panes layout (LOCAL, MERGED and REMOTE)

           gvimdiff3
               Use gVim (requires a graphical session) where only
               the MERGED file is shown

           kdiff3
               Use KDiff3 (requires a graphical session)

           meld
               Use Meld (requires a graphical session) with optional
               auto merge (see git help mergetool's CONFIGURATION
               section)

           nvimdiff
               Use Neovim with a custom layout (see git help
               mergetool's BACKEND SPECIFIC HINTS section)

           nvimdiff1
               Use Neovim with a 2 panes layout (LOCAL and REMOTE)

           nvimdiff2
               Use Neovim with a 3 panes layout (LOCAL, MERGED and
               REMOTE)

           nvimdiff3
               Use Neovim where only the MERGED file is shown

           opendiff
               Use FileMerge (requires a graphical session)

           p4merge
               Use HelixCore P4Merge (requires a graphical session)

           smerge
               Use Sublime Merge (requires a graphical session)

           tkdiff
               Use TkDiff (requires a graphical session)

           tortoisemerge
               Use TortoiseMerge (requires a graphical session)

           vimdiff
               Use Vim with a custom layout (see git help
               mergetool's BACKEND SPECIFIC HINTS section)

           vimdiff1
               Use Vim with a 2 panes layout (LOCAL and REMOTE)

           vimdiff2
               Use Vim with a 3 panes layout (LOCAL, MERGED and
               REMOTE)

           vimdiff3
               Use Vim where only the MERGED file is shown

           winmerge
               Use WinMerge (requires a graphical session)

           xxdiff
               Use xxdiff (requires a graphical session)

       merge.verbosity
           Controls the amount of output shown by the recursive
           merge strategy. Level 0 outputs nothing except a final
           error message if conflicts were detected. Level 1 outputs
           only conflicts, 2 outputs conflicts and file changes.
           Level 5 and above outputs debugging information. The
           default is level 2. Can be overridden by the
           GIT_MERGE_VERBOSITY environment variable.

       merge.<driver>.name
           Defines a human-readable name for a custom low-level
           merge driver. See gitattributes(5) for details.

       merge.<driver>.driver
           Defines the command that implements a custom low-level
           merge driver. See gitattributes(5) for details.

       merge.<driver>.recursive
           Names a low-level merge driver to be used when performing
           an internal merge between common ancestors. See
           gitattributes(5) for details.

SEE ALSO
       git-fmt-merge-msg(1), git-pull(1), gitattributes(5), git-
       reset(1), git-diff(1), git-ls-files(1), git-add(1), git-
       rm(1), git-mergetool(1)

GIT
       Part of the git(1) suite

Git 2.39.2                   04/24/2023                 GIT-MERGE(1)

Git-Merge Scenarios

Merging two commit sequences is generally performed when the sequences are on different branches. The commits on the branch to be merged are incorporated into the commits of the current branch, also known as the target branch. This article has several references to the target branch; remember that this is always the current branch. This is so the merge commit can be created, and the working tree and index can be updated.

When discussing the mechanics of a merge, and the HEAD of the target branch is called the target HEAD, the branches to be merged are called merge branch(es). and the HEAD(s) of the merge branches are called merge HEAD(s).

If the git repository is not corrupt, the target branch and the branch to be merged share a common ancestor commit. This commit is called the merge base. The git-merge-base command shows this commit.

Shell
$ man git-merge-base
GIT-MERGE-BASE(1)            Git Manual            GIT-MERGE-BASE(1)

NAME
       git-merge-base - Find as good common ancestors as possible
       for a merge

SYNOPSIS
       git merge-base [-a | --all] <commit> <commit>...
       git merge-base [-a | --all] --octopus <commit>...
       git merge-base --is-ancestor <commit> <commit>
       git merge-base --independent <commit>...
       git merge-base --fork-point <ref> [<commit>]

DESCRIPTION
       git merge-base finds best common ancestor(s) between two
       commits to use in a three-way merge. One common ancestor is
       better than another common ancestor if the latter is an
       ancestor of the former. A common ancestor that does not have
       any better common ancestor is a best common ancestor, i.e. a
       merge base. Note that there can be more than one merge base
       for a pair of commits.

OPERATION MODES
       As the most common special case, specifying only two commits
       on the command line means computing the merge base between
       the given two commits.

       More generally, among the two commits to compute the merge
       base from, one is specified by the first commit argument on
       the command line; the other commit is a (possibly
       hypothetical) commit that is a merge across all the remaining
       commits on the command line.

       As a consequence, the merge base is not necessarily contained
       in each of the commit arguments if more than two commits are
       specified. This is different from git-show-branch(1) when
       used with the --merge-base option.

       --octopus
           Compute the best common ancestors of all supplied
           commits, in preparation for an n-way merge. This mimics
           the behavior of git show-branch --merge-base.

       --independent
           Instead of printing merge bases, print a minimal subset
           of the supplied commits with the same ancestors. In other
           words, among the commits given, list those which cannot
           be reached from any other. This mimics the behavior of
           git show-branch --independent.

       --is-ancestor
           Check if the first <commit> is an ancestor of the second
           <commit>, and exit with status 0 if true, or with status
           1 if not. Errors are signaled by a non-zero status that
           is not 1.

       --fork-point
           Find the point at which a branch (or any history that
           leads to <commit>) forked from another branch (or any
           reference) <ref>. This does not just look for the common
           ancestor of the two commits, but also takes into account
           the reflog of <ref> to see if the history leading to
           <commit> forked from an earlier incarnation of the branch
           <ref> (see discussion on this mode below).

OPTIONS
       -a, --all
           Output all merge bases for the commits, instead of just
           one.

DISCUSSION
       Given two commits A and B, git merge-base A B will output a
       commit which is reachable from both A and B through the
       parent relationship.

       For example, with this topology:

                    o---o---o---B
                   /
           ---o---1---o---o---o---A

       the merge base between A and B is 1.

       Given three commits A, B and C, git merge-base A B C will
       compute the merge base between A and a hypothetical commit M,
       which is a merge between B and C. For example, with this
       topology:

                  o---o---o---o---C
                 /
                /   o---o---o---B
               /   /
           ---2---1---o---o---o---A

       the result of git merge-base A B C is 1. This is because the
       equivalent topology with a merge commit M between B and C is:

                  o---o---o---o---o
                 /                 \
                /   o---o---o---o---M
               /   /
           ---2---1---o---o---o---A

       and the result of git merge-base A M is 1. Commit 2 is also a
       common ancestor between A and M, but 1 is a better common
       ancestor, because 2 is an ancestor of 1. Hence, 2 is not a
       merge base.

       The result of git merge-base --octopus A B C is 2, because 2
       is the best common ancestor of all commits.

       When the history involves criss-cross merges, there can be
       more than one best common ancestor for two commits. For
       example, with this topology:

           ---1---o---A
               \ /
                X
               / \
           ---2---o---o---B

       both 1 and 2 are merge-bases of A and B. Neither one is
       better than the other (both are best merge bases). When the
       --all option is not given, it is unspecified which best one
       is output.

       A common idiom to check "fast-forward-ness" between two
       commits A and B is (or at least used to be) to compute the
       merge base between A and B, and check if it is the same as A,
       in which case, A is an ancestor of B. You will see this idiom
       used often in older scripts.

           A=$(git rev-parse --verify A)
           if test "$A" = "$(git merge-base A B)"
           then
                   ... A is an ancestor of B ...
           fi

       In modern git, you can say this in a more direct way:

           if git merge-base --is-ancestor A B
           then
                   ... A is an ancestor of B ...
           fi

       instead.

DISCUSSION ON FORK-POINT MODE
       After working on the topic branch created with git switch -c
       topic origin/master, the history of remote-tracking branch
       origin/master may have been rewound and rebuilt, leading to a
       history of this shape:

                            o---B2
                           /
           ---o---o---B1--o---o---o---B (origin/master)
                   \
                    B0
                     \
                      D0---D1---D (topic)

       where origin/master used to point at commits B0, B1, B2 and
       now it points at B, and your topic branch was started on top
       of it back when origin/master was at B0, and you built three
       commits, D0, D1, and D, on top of it. Imagine that you now
       want to rebase the work you did on the topic on top of the
       updated origin/master.

       In such a case, git merge-base origin/master topic would
       return the parent of B0 in the above picture, but B0^..D is
       not the range of commits you would want to replay on top of B
       (it includes B0, which is not what you wrote; it is a commit
       the other side discarded when it moved its tip from B0 to
       B1).

       git merge-base --fork-point origin/master topic is designed
       to help in such a case. It takes not only B but also B0, B1,
       and B2 (i.e. old tips of the remote-tracking branches your
       repository’s reflog knows about) into account to see on which
       commit your topic branch was built and finds B0, allowing you
       to replay only the commits on your topic, excluding the
       commits the other side later discarded.

       Hence

           $ fork_point=$(git merge-base --fork-point origin/master topic)

       will find B0, and

           $ git rebase --onto origin/master $fork_point topic

       will replay D0, D1 and D on top of B to create a new history
       of this shape:

                            o---B2
                           /
           ---o---o---B1--o---o---o---B (origin/master)
                   \                   \
                    B0                  D0'--D1'--D' (topic - updated)
                     \
                      D0---D1---D (topic - old)

       A caveat is that older reflog entries in your repository may
       be expired by git gc. If B0 no longer appears in the reflog
       of the remote-tracking branch origin/master, the --fork-point
       mode obviously cannot find it and fails, avoiding to give a
       random and useless result (such as the parent of B0, like the
       same command without the --fork-point option gives).

       Also, the remote-tracking branch you use the --fork-point
       mode with must be the one your topic forked from its tip. If
       you forked from an older commit than the tip, this mode would
       not find the fork point (imagine in the above sample history
       B0 did not exist, origin/master started at B1, moved to B2
       and then B, and you forked your topic at origin/master^ when
       origin/master was B1; the shape of the history would be the
       same as above, without B0, and the parent of B1 is what git
       merge-base origin/master topic correctly finds, but the
       --fork-point mode will not, because it is not one of the
       commits that used to be at the tip of origin/master).

SEE ALSO
       git-rev-list(1), git-show-branch(1), git-merge(1)

GIT
       Part of the git(1) suite

Git 2.39.2                   04/24/2023            GIT-MERGE-BASE(1)

Merge Analysis

The git-merge command first analyzes the commit sequences to merge. It categorizes the situation into one of four possible scenarios:

up_to_date
The merge HEADs are reachable from the target HEAD. This means the target HEAD is up-to-date, and no action needs to be taken.
fastforward
There can be only one merge head when fast-forwarding, so if merge analysis returns this value we know:
  • There is only one merge HEAD.
  • The target HEAD can be reached by fast-forwarding from the merge HEAD
  • Fast-forwarding was explicitly requested on the command line or implicitly permitted via configuration setting.
For this scenario, a true/normal merge is not required; instead, the target HEAD can simply be set to the merge HEAD.
normal

In this scenario, both the target branch and the commit sequence to merge have diverged from their common ancestor. A normal merge (sometimes called a true merge) is required to reconcile divergent branches.

A normal merge will also be performed even though fast-forwarding might be possible when:

  • The ‑‑no‑ff option to the git-merge command is specified.
  • Git was configured to default to disabling fast-forward prior to running the git-merge command.

    Shell
    $ git config --add merge.ff false

The divergent commit sequences are merged by creating a new commit that joins them; this is called a merge commit. As you should know, commits are always made on the current branch. The reason why target branch must be the current branch for a merge is so the merge commit is created on the proper branch.

The merge commit has a reference to the first commit in the sequence to merge, and a reference to the common ancestor commit in the target branch; this is why we say the merge commit has two parents.

Octupus commits have more than two parents. Because this article does not discuss merge strategies, no further mention will be made of Cthulhu.

At this point the newly created merge commit is HEAD. After the new commit is created, the working tree and the index are updated.

unborn
The HEAD of the target branch is said to be unborn when it does not point to anything yet. Since there is no common ancestor for HEAD and the commit sequences to merge, a normal merge is not possible. If there is only one commit sequence to merge, the logical action would be to set HEAD of the current branch to the HEAD of the commit sequence to merge.

Examining the libgit2 C source code for merge.c and merge.h should help understand the mechanics better.

Libgit2 merge_analysis

Libgit2 has a method called git_merge_analysis that performs the same analysis as just described. It is used in the merge.c example.