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Question:

I'm using git on a new project that has two parallel -- but currently experimental -- development branches:

  • master: import of existing codebase plus a few mods that I'm generally sure of
  • exp1: experimental branch #1
  • exp2: experimental branch #2

exp1 and exp2 represent two very different architectural approaches. Until I get further along I have no way of knowing which one (if either) will work. As I make progress in one branch I sometimes have edits that would be useful in the other branch and would like to merge just those.

What is the best way to merge selective files from one development branch to another while leaving behind everything else?

Approaches I've considered:

  1. git merge --no-commit followed by manual unstaging of a large number of edits that I don't want to make common between the branches.

  2. Manual copying of common files into a temp directory followed by git checkout to move to the other branch and then more manual copying out of the temp directory into the working tree.

  3. A variation on the above. Abandon the exp branches for now and use two additional local repositories for experimentation. This makes the manual copying of files much more straightforward.

All three of these approaches seem tedious and error-prone. I'm hoping there is a better approach; something akin to a filter path parameter that would make git-merge more selective.


Best Answer:


To selectively merge files from one branch into another branch, run

git merge --no-ff --no-commit branchX

where: branchX is the branch you want to merge from into the current branch

The --no-commit option will stage the files that have been merged by Git without actually committing them. This will give you the opportunity to modify the merged files however you want to and then commit them yourself.

Depending on how you want to merge files, there are four cases:

1) You want a true merge. In this case, you accept the merged files the way Git merged them automatically and then commit them.

2) There are some files you don't want to merge. For example, you want to retain the version in the current branch and ignore the version in the branch you are merging from.

To select the version in the current branch, run:

git checkout HEAD file1

This will retrieve the version of file1 in the current branch and overwrite the file automerged by Git.

3) If you want the version in branchX (and not a true merge), run:

git checkout branchX file1

This will retrieve the version of file1 in branchX and overwrite the file auto-merged by Git.

4) The last case is if you want to select only specific merges in file1. In this case, you can edit the modified file1 directly, update it to whatever you'd want the version of file1 to become, and then commit.

If Git cannot merge a file automatically, it will report it as "unmerged" file and produce a copy where you will need to resolve the conflicts manually.

To explain further with an example, let's say you want to merge branchX into the current branch:

git merge --no-ff --no-commit branchX

You then run the git status command to view the status of modified files.

For example:

git status
# On branch master
# Changes to be committed:
#
#       modified:   file1
#       modified:   file2
#       modified:   file3
# Unmerged paths:
#   (use "git add/rm <file>..." as appropriate to mark resolution)
#
#       both modified:      file4
#

Where file1, file2, and file3 are the files git have successfully auto-merged.

What this means is that changes in the master and branchX for all those three files have been combined together without any conflicts.

You can inspect how the merge was done by running the git diff --cached file. For example:

git diff --cached file1
git diff --cached file2
git diff --cached file3

If you find some merge undesirable, you can edit the file directly, save, and then commit.

If you don't want to merge file1 and want to retain the version in the current branch, run:

git checkout HEAD file1

If you don't want to merge file2 and only want the version in branchX, run

git checkout branchX file2

If you want file3 to be merged automatically, don't do anything. Git has already merged it at this point.

file4 above is a failed merge by Git. This means there are changes in both branches that occur on the same line. This is where you will need to resolve the conflicts manually. You can discard the merged done by editing the file directly or running the checkout command for the version in the branch you want file4 to become.

Finally, don't forget to commit.

git commit



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