cp -u is illegal on mac. What are the alternatives?

Probably the easiest solution would be to use rsync. It is a backup application that can do what cp -u does (and much, much more).

The basic usage would be something like :

rsync -u source destination

However, you really should check out the man page because there are MANY useful options.

You can use the -nt condition:

[[ source/$file -nt destination/$file ]] && cp source/$file destination/$file

Just run this for all files (via a loop or through find).

I have installed GNU Coreutils with MacPorts by

sudo ports install coreutils findutils 

findutils are not necessary but they suggested to install it too.

I followed the idea from Keith Thompson, (kudos!). It works greatly, the "advanced" copy command is now:

/opt/local/libexec/gnubin/cp

It does not have a man page, but has a long, man-style help with the --help option.