Usually grunt is supposed to perform the compilation but there are instances where you would like to compress SASS files recursively from a shell manually. In order to do that, change to the top-level under which all .scss files are contained and issue:
find . -name \*.scss -exec sh -c 'sass -t compressed {} `i={}; echo ${i%.scss}.css`' \;
This will compile all .scss files to .css files recursively and place them on the same level as the .scss files in the filesystem tree.