Prevent commits in master branch

Yes, it is possible. You must create a pre-commit hook which rejects commits to the master branch. Git doesn't call a pre-commit hook when you call the merge command, so this hook will be rejecting only regular commits.

1. Go to your repository.

2. Create a file, .git/hooks/pre-commit, with the following content:

   #!/bin/sh
   
   branch="$(git rev-parse --abbrev-ref HEAD)"
   
   if [ "$branch" = "master" ]; then
     echo "You can't commit directly to master branch"
     exit 1
   fi
   

3. Make it executable (not required on Windows):

   chmod +x .git/hooks/pre-commit
   

To disable fast-forward merges, you must also add the following option to your .git/config file:

[branch "master"]
    mergeoptions = --no-ff

If you want also protect the master branch on your remote, check this answer: How to restrict access to master branch in Git

You can use the pre-commit utility to do this. It has a built-in no-commit-to-branch hook that can be used to prevent commits to one or more branches.

Setup

The basic setup process is:

• Install using pip or Homebrew (instructions at https://pre-commit.com/#install)
• Create a .pre-commit-config.yaml file in the root of your project (see below for a first draft)
• Install the hooks into your Git configuration by running pre-commit install.

Basic configuration for protecting branches

Here is a basic configuration that includes just the no-commit-to-branch hook:

repos:
- repo: https://github.com/pre-commit/pre-commit-hooks
  rev: v3.3.0
  hooks:
    - id: no-commit-to-branch
      args: ['--branch', 'master']

If you want to protect multiple branches, you can use include multiple --branch arguments in the argument list:

repos:
- repo: https://github.com/pre-commit/pre-commit-hooks
  rev: v3.3.0
  hooks:
    - id: no-commit-to-branch
      args: ['--branch', 'master', '--branch', 'staging']

Isn't this all overkill?

Pre-commit has many other built-in hooks, and a large collection of community-built hooks that will transform the way you clean up and validate your commits. The reason I mention this is because, while this tool may be overkill for just preventing commits to a protected branch, it has many other features that make it a compelling and simple addition to any Git project.

It may make sense to install it globally via

git config --global core.hooksPath ~/githooks

and moving that pre-commit file into that directory