Southy is a command line app that will check you in to your Southwest flights exactly 24 hours in advance, and email you your boarding passes.
Important: if you want emails to be sent with boarding passes attached as a PDF, you must first install wkhtmltopdf. Good instructions for OS X and various Linux distros are here:
https://github.com/jdpace/PDFKit/wiki/Installing-WKHTMLTOPDF
I strongly suggest you download the pre-compiled binaries for both OS X and Ubuntu.
After that is installed, you just install Southy as a gem:
% gem install southy
To see how to use Southy, just run it with no arguments:
% southy