Re: Fwd: Strange results

[Kerin Millar]

On Fri, 27 Oct 2023 02:00:01 +0700
Victor Pasko <victor.pasko@gmail.com> wrote:

> ---------- Forwarded message ---------
> From: Victor Pasko <victor.pasko@gmail.com>
> Date: Fri, Oct 27, 2023 at 1:57 AM
> Subject: Re: Strange results
> To: Dennis Williamson <dennistwilliamson@gmail.com>
> 
> 
> 
> Also
> 
> echo10 ${ASCII_SET:$((-10)):1}

This is the "Substring Expansion" kind of parameter expansion.
> 
> and
> 
> echo11 ${ASCII_SET:-10:1}

This is the "Use Default Values" kind of parameter expansion.

> 
> have different behaviour:(

Substring expansions already imply a numeric context. A single pair of 
enclosing brackets is enough to avoid this pitfall.

${ASCII_SET:(-10):1}

Another method is to have a leading space.

${ASCII_SET: -10:1}

> 
> Both of these say "output the character that's 10th from the end" which is
> > "u". What did you expect it to output?
> >
> > echo "echo11 ${ASCII_SET:-10:1}"
> >
> 
> Firstly, expected the only one symbol from  ASCII_SET string
> 
> This says, according to the man page:
> >
> >        ${parameter:-word}
> >               Use Default Values.  If parameter is unset or null, the
> > expansion of word is substituted.  Otherwise, the value of parameter is
> > substituted
> >
> > which means output "10:1" if ASCII_SET is unset or null. Since it isn't,
> > the contents of that variable are output giving you a long sequence of
> > ASCII characters.
> >
> 
> But ASCII_SET is not unset so -word must not be used

It behaves precisely as the manual states. The parameter, ASCII_SET, is neither 
unset nor null (empty). Therefore, the value of the parameter is substituted, 
rather than the given word of "10:1".

-- 
Kerin Millar