The Jekyll gem makes a jekyll executable available to you in your Terminal window. You can use this command in a number of ways:
$ jekyll build
# => The current folder will be generated into ./_site
$ jekyll build --destination <destination>
# => The current folder will be generated into <destination>
$ jekyll build --source <source> --destination <destination>
# => The <source> folder will be generated into <destination>
$ jekyll build --watch
# => The current folder will be generated into ./_site,
# watched for changes, and regenerated automatically.
Jekyll also comes with a built-in development server that will allow you to preview what the generated site will look like in your browser locally.
$ jekyll serve
# => A development server will run at http://localhost:4000/
# Auto-regeneration: enabled. Use `--no-watch` to disable.