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From: Bob Proulx <bob@proulx.com>
Newsgroups: gnu.bash.bug
Subject: Re: Empty ""s in ARG in ${x:+ARG} expand to no words instead of the empty word if prepended/appended with space
Date: Fri, 20 Jul 2018 22:44:10 -0600
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Denys Vlasenko wrote:
> $ f() { for i; do echo "|$i|"; done; }
> $ x=x
> $ e=
> $ f ${x:+ ""}
> ^^^^^^^^^^^ prints nothing, bug?
> 
> $  ${x:+"" }
> ^^^^^^^^^^^ prints nothing, bug?

Insufficient quoting.  That argument should be quoted to avoid the
whitespace getting stripped.  (Is that during word splitting phase
using the IFS?  I think so.)

Try this:

  f "${x:+ ""}"
  f "${x:+"" }"

Also in the future please say what version of bash you are using.
There have been a lot of bugs fixed in various versions.

Bob