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From: Denys Vlasenko <dvlasenk@redhat.com>
Newsgroups: gnu.bash.bug
Subject: Single quotes in ARG in "${v:+ARG}" are erroneously required to be paired
Date: Wed, 18 Jul 2018 14:25:03 +0200
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Xref: csiph.com gnu.bash.bug:14358

Unquoted ${v:+ARG} operator allows single-quoted strings
in ARG:

$ x=a; echo ${x:+'b c'}
b c

In this case, obviously single quotes must be paired.

If ${v:+ARG} is in double-quoted string, single quotes lose their special status:

$ x=a; echo "${x:+'b c'}"
'b c'

IOW: they work similarly to ordinary dquoted strings: echo "It's sunny"
treats single quote as ordinary char.

However, bash errors out if they are not paired:

$ x=a; echo "${x:+'b c}"
>          <------------- bash thinks 'str' did not end yet

Most other shells - dash, mksh, busybox ash, posh, bosh -
do not require paired single quotes in this case:

$ dash
$ x=a; echo "${x:+'b c}"
'b c

 From all tested shells, only yash behaves like bash.

I think it's a bug.