Is there a Terminal shell command to compare the contents of folders?
Yes. The diff command will do exactly what you want. To use it just type diff -rq and either drag the folders you want to compare into the terminal window or type the folders' path yourself. 

The -r is to recursively compare any subdirectories found, and the -q toggles diff into brief mode. Without it, diff, will not only tell you which files are different, but also the actual line-by-line differences for any text files that exist in both locations but are not identical.

On the other hand, Kaleidoscope is a great Application to compare text, images and folders in a very neat and clean way. You can even merge changes with it.