Create a global git commit hook

1. Enable git templates:

git config --global init.templatedir '~/.git-templates'

This tells git to copy everything in ~/.git-templates to your per-project .git/ directory when you run git init

2. Create a directory to hold the global hooks:

mkdir -p ~/.git-templates/hooks

3. Write your hooks in ~/.git-templates/hooks.

For example, here's a post-commit hook (located in ~/.git-templates/hooks/post-commit):

#!/bin/sh

# Copy last commit hash to clipboard on commit
git log -1 --format=format:%h | pbcopy

# Add other post-commit hooks 

4. Make sure the hook is executable.

chmod a+x ~/.git-templates/hooks/post-commit

5. Re-initialize git in each existing repo you'd like to use this in:

git init

NOTE if you already have a hook defined in your local git repo, this will not overwrite it.