Create a github-pages branch from scratch on an existing repo

Check out branch directly

git clone -b gh-pages <remote_repo> <folder-name>

script.sh

# Full instructions at
# https://help.github.com/articles/creating-project-pages-using-the-command-line/
# Go to the full instructions for other examples like pushing gh-pages to master or to a docs folder in master 

cd my-project
git checkout --orphan master
git rm -rf .

# <add content here>

git commit -a -m "First pages commit"
git push -u origin gh-pages