PORTDIR
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The information in this article has been deprecated. It may or may not be relevant for contemporary usage. Handle with care!
The PORTDIR variable was[1] 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 /var/db/repos/gentoo) 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