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From: William Entriken <fulldecent@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: Double substitution issue
Date: Wed, 28 Feb 2018 11:13:58 -0500
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References: <CAFwrLX4FoKEm8pAGgrT+SkeUOk=zKxA5-h2KuoVsfwJSMz+kew@mail.gmail.com> <5cc476fa-8293-ba55-1325-8d672982ad60@case.edu>
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Xref: csiph.com gnu.bash.bug:13771

Thank you for explaining this fine point.

I have found a way to use double expansion, and of course this is not safe
in all contexts:

    from=1;to=3;eval echo {$from..$to}

Regards,
Will


William Entriken
+1 267-738-4201


On Wed, Feb 28, 2018 at 10:03 AM, Chet Ramey <chet.ramey@case.edu> wrote:

> 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/
>