githook to convert iPython README to Markdown

post-commit

READPY=$(git log --name-only HEAD^.. | grep "^README.ipynb$")
READMD=$(git log --name-only HEAD^.. | grep "^README.md$")

if [ -n "$READPY" ] && [ -z "$READMD" ]; then
    echo "It looks like a new README was committed, appending a Markdown version"
    ipython nbconvert --to markdown README.ipynb

    # Adding this file doesn't work in pre-commit hooks, which is
    # why we're appending post-commit
    git add README.md

    # We don't want to run this hook again after appending
    git commit --amend -C HEAD --no-verify
fi

Why not simply generate the file pre-commit and stage? Because git add does not actually stage in pre-commit scripts. This is why the script checks post-commit and steps back, amending the commit.