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From: Stephane Chazelas <stephane.chazelas@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: T/F var expansion?
Date: Mon, 29 Jul 2019 10:28:15 +0100
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Xref: csiph.com gnu.bash.bug:15257

2019-07-28 21:17:43 -0700, L A Walsh:
> Is there a T/F var expansion that does:
>  
>     var=${tst:+$yes}${tst:-$no}
> 
> but with yes/no in 1 expansion?
[...]

You can also do:

no_yes=(no yes)
echo "${no_yes[${var+1}]}"

For the reverse:

echo "${no_yes[!0${var+1}]}"

See also:

map=(unset empty non-empty)
echo "${map[${var+1}+0${var:+1}]}"

-- 
Stephane