I use yui-compressor to minify my files. It’s so easy. In Ubuntu (15.04) it’s available via apt-get.

To install it (on Ubuntu) type the following in your terminal window

apt-get install yui-compressor

The command structure to run it is

yui-compressor SOURCE > TARGET

e.g.
yui-compressor public/css/custom.css > public/css/custom.min.css