https://guides.rubygems.org/command-reference/
https://jekyllrb.com/docs/installation/ubuntu/

gem uninstall           # uninstall gems from the local directory
gem uninstall -aIx      # uninstall all gems without prompt
gem update              # update installed gems to the latest version

# install jekyll (https://jekyllrb.com/docs/installation/ubuntu/)
sudo apt-get install ruby-dev libffi-dev
gem install bundler jekyll

bundle update --bundler

If Gemfile.lock is present in the directory remove them:
$ bundle install
$ bundle exec jekyll serve

1. gem install rails
2. gem install bundler jekyll
3. cd ~/Sites
4. rm Gemfile.lock
5. bundle install
6. bundle exec jekyll serve
7. open: http://127.0.0.1:4000/
Ref: https://stackoverflow.com/questions/10012181/bundle-install-returns-could-not-locate-gemfile
