Do not use port list, which is likely not what you want

See http://trac.macports.org/wiki/FAQ#portlist. port list prints a port and its most current version number available from MacPorts, not necessarily the one you installed. When getting a list of leaves, you probably want the installed versions and variants, though, which is what port installed (port echo) are for. Plus, it's faster than port list, because it doesn't have to execute Portfiles.