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From: Martin Vaeth <martin@mvath.de>
Subject: [gentoo-dev] Re: [PATCH] linux-mod.eclass: IUSE default support for
 MODULES_OPTIONAL_USE
Date: Tue, 16 Jan 2018 17:01:09 +0000 (UTC)
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References: <robbat2-20180114T234941-135666777Z@orbis-terrarum.net>
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 <20180115202619.GA25005@linux1.home>
 <robbat2-20180115T234605-078394673Z@orbis-terrarum.net>
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Robin H. Johnson <robbat2@gentoo.org> wrote:
> That is counter-intuitive for somebody that puts
> MODULES_OPTIONAL_USE_IUSE_DEFAULT=0
> Or tries to otherwise have it unset.

What I usually do is:

case ${MODULES_OPTIONAL_USE_IUSE_DEFAULT:-n} in
  [nNfF]*|[oO][fF]*|0|-)  # false case
    ;;
  *) # true case
esac

This covers most of the "intuitive" attempts (0 - Off FALSE no ...)
to switch it off, and everything else (nonempty) means "on".