How do you implement a custom merge strategy in Git?

Git comes with a variety of built-in merge strategies for combining changes from multiple branches, such as the default recursive strategy, as well as more specialized strategies like ours and subtree. However, there may be cases where none of these built-in strategies are suitable for your needs, and you need to implement a custom merge strategy.

A custom merge strategy in Git is implemented as a script that takes three arguments: the merge base commit, the current branch tip commit, and the other branch tip commit. The script must produce a new commit that represents the merged state of the two branches, and exit with a status code of 0 if the merge was successful, or a non-zero status code if the merge failed.

Here’s an example of a simple custom merge strategy that always chooses the changes from the current branch:

    #!/bin/bash
    
    # Get the merge base commit, current branch tip, and other branch tip
    base="$1"
    ours="$2"
    theirs="$3"
    
    # Create a new tree that includes all the changes from the current branch
    tree=$(git merge-tree "$base" "$ours" "$theirs" | sed 's/    /t/g')
    
    # Create a new commit with the merged tree
    commit=$(echo "Merge branch 'ours' into 'theirs'" | git commit-tree "$tree" -p "$ours" -p "$theirs")
    
    # Set the branch tip to the new commit
    git update-ref "HEAD" "$commit"

To use this custom merge strategy, you would save it as a script (let’s say custom-ours.sh) and make it executable. Then you would configure Git to use the script as the merge driver for a specific file pattern in your .gitconfig file:

    [merge "custom-ours"]
    name = always choose changes from current branch
    driver = /path/to/custom-ours.sh %O %A %B
    recursive = binary

Here, we’ve named the merge strategy custom-ours and specified the path to our custom merge script. We’ve also set the recursive option to binary, which tells Git not to try to merge files that have conflicts.

To use the custom merge strategy for a specific file pattern (e.g. all files with the .txt extension), you would add the following to your .gitattributes file:

    *.txt merge=custom-ours

Now, when you run git merge with changes in a .txt file, Git will use our custom merge script instead of the default recursive strategy. Note that you can also specify the merge strategy explicitly on the command line using the -s option:

    git merge -s custom-ours <other-branch>

Custom merge strategies can be a powerful tool for handling complex merge scenarios in Git, but they should be used with care and tested thoroughly to ensure they behave as expected.