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How to use `git mergetool` to resolve conflicts

Table Of Content

Skip to the relevant sections if needed.

Concepts for resolving Git conflicts

For using mergetool in git, we need to understand the following terminology to understand what is being merged:

  • LOCAL - the head for the file(s) from the current branch on the machine that you are using.
  • REMOTE - the head for files(s) from a remote location that you are trying to merge into your LOCAL branch.
  • BASE - the common ancestor(s) of LOCAL and BASE.
  • MERGED - the tag / HEAD object after the merge - this is saved as a new commit.

Common mergetool from editors will display both LOCAL and REMOTE so you can decide which changes to keep. Please read this tutorial explaining the HEAD objects if you do not know what it is. It will help your understanding of Git tremendously.

Setting up different editors / tool for using git mergetool

We have to change the git config to set a default mergetool. In this example, we will use vimdiff:

$ git config merge.tool vimdiff      

We can also set the editor to display the common ancestor BASE while we examine what changes are in LOCAL and REMOTE with the following setting:

$ git config merge.conflictstyle diff3  

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Finding out what mergetool editors are supported

$ git mergetool --tool-help

And we list a few of them:

Command line mergetool editors

  • Emacs based diff tools: emerge, or Ediff
  • Vim based diff tool: vimdiff

GUI mergetool editors

  • gvimdiff - almost identical to vimdiff but uses the Linux GUI for Vim, please refer to vimdiff if you still use the keyboard commands for GVim.
  • kdiff3
  • meld
  • tortoisemerge

Or consult the community of your favorite editor to see how to do the equivalent operations for your editor.

Other useful mergetool settings

Do not prompt before launching the merge resolution tool

$ git config mergetool.prompt false

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mergetool simple code example

Ref1 for the example

creating the git repo

$ mkdir galaxyZoo
$ cd galaxyZoo
$ git init
$ vim astrophy_obj.txt

Add some galaxy types into astrophy_obj.txt then save the file.

# content of astrophy_obj.txt

save then commit the file.

$ git add astrophy_obj.txt
$ git commit -m 'Initial commit'
$ git branch astrophy_objects   # create a new branch
$ git checkout astrophy_objects # change to new branch
$ vim astrophy_obj.txt          # make changes to file 

Change bar to barred in the file.

$ git commit -am 'changed bar to barred'
$ git checkout master   # change back to master branch
$ vim astrophy_obj.txt  
# add the word `galaxy` to the end of each line using Vim REGEX 
# type `:%s/$/ galaxy/g` in Vim then press enter and save `:wq`

$ git commit -am 'added galaxy to each line'
# merge from the astrophy_objects branch to current branch, i.e. master
$ git merge astrophy_objects  

Then you will see some error messages:

Auto-merging astrophy_obj.txt
CONFLICT (content): Merge conflict in astrophy_obj.txt
Automatic merge failed; fix conflicts and then commit the result.

We can bring up the mergetool:

$ git mergetool

Then it will bring up the different versions of the file in different Vim splits panels.

| LOCAL  |     BASE     | REMOTE |
|             MERGED             |

The top left split panel is the LOCAL, top middle split is BASE and top right split is REMOTE. The bottom split refers to the MERGED version. You can find this info in the bottom bar of each split (I have put 3 yellow rectangles to highlight that info).

As you can see form the below image, my Vim has highlighted the differences in red for me. Vim mergetool image

Now if your terminal has any GUI capability and you have compiled Vim correctly with GUI support, you can use your mouse to click on the bottom split to edit it. Or if you are a Vim ninja, you can use the keyboard shortcut to move to different splits.

Ctrl w + h   # move to the split on the left 
Ctrl w + j   # move to the split below
Ctrl w + k   # move to the split on top
Ctrl w + l   # move to the split on the right

You can either incorporate the changes by manually editing the MERGED split, or use Vim shortcuts pull from one of the LOCAL, BASE ad REMOTE versions.

:diffg RE  # get from REMOTE
:diffg BA  # get from BASE
:diffg LO  # get from LOCAL

save the changes then quit with :wqa to close all the splits. Remember to commit the merge.

$ git commit -am 'merged from several branches'

Resolving conflict from a git pull

If you were trying to do a git pull when you ran into merge conflicts, follow all steps in the previous section for using the mergetool, then do:

$ git rebase –continue

This command will

Forward-port local commits to the updated upstream HEAD.

according to the documentation, meaning your local commits will be pushed to the upstream remote branch as a new forward commit that doesn't interfere with previous commits. Hooray now you can claim that you can collaborate with others with Git without messing up with your collaborators' commits.

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Other vimdiff keyboard shortcuts

]c - Jump to the next change.
[c - Jump to the previous change.


Other great references and tutorials

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