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PORTDIR
From Gentoo Wiki (test)
The PORTDIR variable was used to point to the main package repository hosted on the system. It has since been deprecated in favor of the location attribute (whose default value is /usr/portage) of the Gentoo repository configuration inside /etc/portage/repos.conf.
The current value of the location attribute can be obtained by using portageq:
user $portageq get_repo_path / gentoo/usr/portage