Portage keywords used in ebuilds

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There does not seem to be any index of all the keywords used by portage, in particular within ebuilds. As we wonder about the meaning of such or such keyword, we can build up the index below, with links to the most appropriate documentation.

See: https://devmanual.gentoo.org/
Another partial list can be found here:
Install Functions Reference https://devmanual.gentoo.org/function-reference/install-functions/index....

In alphabetical order:

CHECKREQS_DISK_BUILD: Ensure we have enough disk space to compile. Typical use in .ebuild file:

pkg_pretend() {
    # Ensure we have enough disk space to compile

In order to test the space required by a compile, you can use the build environment /etc/portage/env/hack.conf with has FEATURES="keepwork noclean". Note that the space required to build a package may depend on the USE flags set for the package. Note that emerge indicates the required space at the end of the build process, e.g.:
* Final size of build directory:  312164 KiB (304.8 MiB)
* Final size of installed tree:  1194532 KiB (  1.1 GiB)

DEPEND: build dependencies. See: https://devmanual.gentoo.org/general-concepts/dependencies/index.html

DISTDIR: see: http://linux.overshoot.tv/usr/portage/distfiles

inherit: see: https://devmanual.gentoo.org/ebuild-writing/using-eclasses/index.html and https://devmanual.gentoo.org/eclass-reference/index.html

PDEPEND: post-merge dependencies. See: https://devmanual.gentoo.org/general-concepts/dependencies/index.html

RDEPEND: runtime dependencies. See: https://devmanual.gentoo.org/general-concepts/dependencies/index.html