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From: Chet Ramey <chet.ramey@case.edu>
Newsgroups: gnu.bash.bug
Subject: Re: Double substitution issue
Date: Wed, 28 Feb 2018 10:03:11 -0500
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On 2/28/18 3:25 AM, William Entriken wrote:
> This behavior is different in zsh and bash, and maybe bash behavior is a
> bug.
> 
> # Test case
> 
> touch 1 2 3
> cat > script.sh <<EOL
> from=1
> to=3
> ls {$from..$to}
> EOL
> chmod a+x script.sh
> 
> bash ./script.sh
> 
> zsh ./script.sh
> 
> # Expected
> 
> Both list files 1, 2, 3
> 
> # Actual
> 
> zsh passes.
> 
> Bash fails the chained substitution with:
> 
> ls: {1..3}: No such file or directory

This is how bash works, how it's always worked, and how it's documented to
work:

"Brace expansion is performed before any other expansions, and any char-
 acters special to other expansions are preserved in the result.  It  is
 strictly  textual.  Bash does not apply any syntactic interpretation to
 the context of the expansion or the text between the braces."

So you have {$from..$to}, which is not a valid sequence expression because
$from and $to are not integers. Invalid brace expansions are left
unchanged. When the word undergoes parameter expansion, you get {1..3}.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/