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From: Robert Elz <kre@munnari.OZ.AU>
Newsgroups: gnu.bash.bug
Subject: Re: Assign Default Value expansion inserts ASCII DEL for empty 'word'
Date: Tue, 10 Mar 2020 12:16:33 +0700
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To: Martin Castillo <castilma@uni-bremen.de>
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Xref: csiph.com gnu.bash.bug:16005

    Date:        Mon, 9 Mar 2020 20:28:30 +0059.55
    From:        Martin Castillo <castilma@uni-bremen.de>
    Message-ID:  <5ad25080-7c13-9b9c-cff6-12ffee7afeb8@uni-bremen.de>

  | Repeat-By:
  | $ unset foo bar
  | $ echo "${foo:=}/baz:${foo}/bat"|xxd
  | 00000000: 7f2f 6261 7a3a 2f62 6174 0a              ./baz:/bat.
  | $ echo "${foo:=$bar}/baz:${foo}/bat"|xxd
  | 00000000: 7f2f 6261 7a3a 2f62 6174 0a              ./baz:/bat.

Your tests are overly complex, and arguably stretch into unspecified
behaviour (not that that should have any bearing on this).

The problem occurs to happen when (and only when) a empty "default"
value is assigned to a variable which is (within the same quoted string)
concatenated with some non-empty text (before or after).

So
	"${foo:=}y"
or
	"x${foo:=}"
or
	"x${foo:=}y"

but not for any of

	"${foo:=}"
	x"${foo:=}"
	"${foo:=}"y
	x${foo:=}

(etc).   It is irrelevant whether the x & y here are literals, or the
results of expansions, provided that there is actual data, that is

	x= ; echo "${x}${foo:=}"
works
	x=x ; echo ${x}${foo:=}"
inserts the DEL as the value of ${foo:=}


kre