Merge, update, and pull Git branches without using checkouts

Question

I work on a project that has 2 branches, A and B. I typically work on branch A, and merge stuff from branch B. For the merging, I would typically do:

git mer         


        

Answer 1

For many cases (such as merging), you can just use the remote branch without having to update the local tracking branch. Adding a message in the reflog sounds like overkill and will stop it being quicker. To make it easier to recover, add the following into your git config

[core]
    logallrefupdates=true

Then type

git reflog show mybranch

to see the recent history for your branch

Answer 2

You can simply git pull origin branchB into your branchA and git will do the trick for you.

Answer 3

It is absolutely possible to do any merge, even non-fast forward merges, without git checkout. The worktree answer by @grego is a good hint. To expand on that:

cd local_repo
git worktree add _master_wt master
cd _master_wt
git pull origin master:master
git merge --no-ff -m "merging workbranch" my_work_branch
cd ..
git worktree remove _master_wt

You have now merged the local work branch to the local master branch without switching your checkout.

Answer 4

As Amber said, fast-forward merges are the only case in which you could conceivably do this. Any other merge conceivably needs to go through the whole three-way merge, applying patches, resolving conflicts deal - and that means there need to be files around.

I happen to have a script around I use for exactly this: doing fast-forward merges without touching the work tree (unless you're merging into HEAD). It's a little long, because it's at least a bit robust - it checks to make sure that the merge would be a fast-forward, then performs it without checking out the branch, but producing the same results as if you had - you see the diff --stat summary of changes, and the entry in the reflog is exactly like a fast forward merge, instead of the "reset" one you get if you use branch -f. If you name it git-merge-ff and drop it in your bin directory, you can call it as a git command: git merge-ff.

#!/bin/bash

_usage() {
    echo "Usage: git merge-ff <branch> <committish-to-merge>" 1>&2
    exit 1
}

_merge_ff() {
    branch="$1"
    commit="$2"

    branch_orig_hash="$(git show-ref -s --verify refs/heads/$branch 2> /dev/null)"
    if [ $? -ne 0 ]; then
        echo "Error: unknown branch $branch" 1>&2
        _usage
    fi

    commit_orig_hash="$(git rev-parse --verify $commit 2> /dev/null)"
    if [ $? -ne 0 ]; then
        echo "Error: unknown revision $commit" 1>&2
        _usage
    fi

    if [ "$(git symbolic-ref HEAD)" = "refs/heads/$branch" ]; then
        git merge $quiet --ff-only "$commit"
    else
        if [ "$(git merge-base $branch_orig_hash $commit_orig_hash)" != "$branch_orig_hash" ]; then
            echo "Error: merging $commit into $branch would not be a fast-forward" 1>&2
            exit 1
        fi
        echo "Updating ${branch_orig_hash:0:7}..${commit_orig_hash:0:7}"
        if git update-ref -m "merge $commit: Fast forward" "refs/heads/$branch" "$commit_orig_hash" "$branch_orig_hash"; then
            if [ -z $quiet ]; then
                echo "Fast forward"
                git diff --stat "$branch@{1}" "$branch"
            fi
        else
            echo "Error: fast forward using update-ref failed" 1>&2
        fi
    fi
}

while getopts "q" opt; do
    case $opt in
        q ) quiet="-q";;
        * ) ;;
    esac
done
shift $((OPTIND-1))

case $# in
    2 ) _merge_ff "$1" "$2";;
    * ) _usage
esac

P.S. If anyone sees any issues with that script, please comment! It was a write-and-forget job, but I'd be happy to improve it.

Answer 5

You can only do this if the merge is a fast-forward. If it's not, then git needs to have the files checked out so it can merge them!

To do it for a fast-forward only:

git fetch <branch that would be pulled for branchB>
git update-ref -m "merge <commit>: Fast forward" refs/heads/<branch> <commit>

where <commit> is the fetched commit, the one you want to fast-forward to. This is basically like using git branch -f to move the branch, except it also records it in the reflog as if you actually did the merge.

Please, please, please don't do this for something that's not a fast-forward, or you'll just be resetting your branch to the other commit. (To check, see if git merge-base <branch> <commit> gives the branch's SHA1.)

Answer 6

Enter git-forward-merge:

Without needing to checkout destination, git-forward-merge <source> <destination> merges source into destination branch.

https://github.com/schuyler1d/git-forward-merge

Only works for automatic merges, if there are conflicts you need to use the regular merge.