Git Post-Merge Hook: The Ultimate Guide (with Examples)

Introduction

Welcome to the ultimate guide on using Git post-merge hooks. If you’re familiar with Git, you’ll know it supports hooks, which are scripts that run automatically whenever certain important actions occur in the repository, such as committing, merging, or receiving pushes. A post-merge hook is a client-side script that’s invoked by Git after a successful merge command.

In this comprehensive tutorial, we’ll explore how to set up and use Git post-merge hooks for various purposes, from simple notifications to automating tasks to maintain your codebase’s cleanliness and stability. We’ll delve into a series of practical examples, progressing from basic to more advanced use cases, to help you fully leverage post-merge hooks in your workflow.

A Quick Overview of Git Hooks

Git hooks are scripts that Git executes before or after events such as: commit, push, and receive. They’re stored in the ‘hooks’ subdirectory of the .git directory in a Git project. By default, Git hooks are disabled and must be made executable before they will run.

There are two types of hooks:

• Client-side: These hooks are triggered by operations such as committing and merging.
• Server-side: These hooks run on network operations such as receiving pushed commits.

The post-merge hook is a client-side hook, which means it runs on your local machine after a successful git merge.

Creating a Simple Post-Merge Hook

Let’s start with a basic example: creating a post-merge hook that outputs a simple message after a successful merge.

Create a file named ‘post-merge’ in the .git/hooks/ directory of your repository and add the following:

#!/bin/sh

echo "Merge was successful, thank you for merging!"

Make sure to make it executable:

$ chmod +x .git/hooks/post-merge

Output after a merge:

Merge was successful, thank you for merging!

This is the most basic type of post-merge hook, useful for ensuring your script is working as expected.

Automating Task Runs with Post-Merge Hook

One common use of post-merge hooks is to run automated tasks such as dependency installations or builds. Below is an example of a post-merge hook that checks if package.json has changed and runs npm install if it has:

#!/bin/sh

# Check for changes in the 'package.json'.
if git diff --name-only HEAD@{1} HEAD -- | grep -q 'package.json'; then
  echo "package.json changed. Running 'npm install'..."
  npm install
else
  echo "No changes detected in package.json"
fi

If there are changes to the package.json file after the merge, the hook will automatically execute npm install. This ensures that the dependencies are updated immediately after the relevant updates are brought into the local codebase through the merge operation.

Validating Code Post-Merge

To keep your codebase stable and clean, you can run tests automatically after a merge to catch any problems as early as possible. While this can also be done with a CI/CD pipeline, doing a quick local check can prevent breaking changes from impacting your work prematurely. Here’s an example post-merge hook that runs automated tests:

#!/bin/sh

# Run automated tests.
echo "Running automated tests..."
npm test

When you merge code that introduces errors, the tests will fail and alert you right away, helping to maintain a stable main branch.

Advanced Post-Merge Hook: Custom Actions per Branch

In this advanced example, the post-merge hook will perform different actions based on the branch that has been merged:

#!/bin/sh

BRANCH=$(git rev-parse --abbrev-ref HEAD)

echo "Post-merge actions for branch $BRANCH..."

case "$BRANCH" in
    development)
        echo 'Merged into development, running tests...'
        npm test
        ;;
    staging)
        echo 'Merged into staging, running linting...'
        npm run lint
        ;;
    production)
        echo 'Merged into production, performing deployment...'
        ./deploy_production.sh
        ;;
    *)
        echo 'No special actions for this branch.'
        ;;
esac

This script uses a case statement to match the branch name and run corresponding tasks that are appropriate for that branch. It’s an example of how you can tailor your post-merge hook to fit the specific workflow of your project.

Conditional Execution Based on Merge Source

A more complex hook may require knowing whether the merge is local (from another branch in the same repository) or remote (from a fetched branch). To implement conditional execution based on the origin of the merged content, you could modify your hook to look like this:

#!/bin/sh

MERGED_FROM=$(git name-rev --name-only MERGE_HEAD)

if [[ $MERGED_FROM == origin/* ]]; then
    echo "Merge made from remote branch $MERGED_FROM. Executing specific tasks..."
    # Specific tasks here
else
    echo "Merge made from the local branch $MERGED_FROM."
fi

This technique can be quite helpful in multi-developer environments where merges are frequent and from various sources.

Conclusion

In this guide, we’ve covered from the basics to more complex uses of the Git post-merge hook. Employing these hooks can streamline your workflow considerably by automating routine tasks, ensuring code stability, and reducing errors introduced by manual processes. Start small with simple notification scripts, and as you become more comfortable, implement more tailored, conditional-based hooks to suit your project’s needs.