By default, jekyll will copy everything in the source directory into _site, but there’s an undocumented feature that lets you exclude or ignore files. A configuration variable named exclude exists. Any files or directories listed will be ignored. For example, the following is in my _config.yml file:

exclude: makelive.sh, ideas.txt, newpost.rb

so that some scripts and a text file of ideas aren’t published on the live site.