UNIX for Advanced & Expert Users

This is for the big guns...
I have been modifying AudioScope.sh to bring it inline with more current practices.
I hit a bug which IS not present in the original code but was after modification.
Shell check first:-

Code:

#!/bin/sh
txt="1234567890"
echo "$(( $txt ))"
echo "$(( ${#txt} - 1 ))"
echo "$(( #txt - 1 ))"

ERROR WINDOW.

$ shellcheck myscript
 
Line 3:
echo "$(( $txt ))"
          ^-- SC2004: $/${} is unnecessary on arithmetic variables.

$

Here is a longhand input to the bash version on 'sh' and yes I did have this inside my code, and what a RPITA it was too. (This also fails inside the version of bash for OSX 10.7.5; I don't know about current versions as i don't have them.)

Code:

Last login: Wed Sep 28 09:36:29 on ttys000
AMIGA:barrywalker~> sh
AMIGA:barrywalker~> txt="1234567890"
AMIGA:barrywalker~> echo "$(( $txt ))"
1234567890
AMIGA:barrywalker~> echo "$(( ${#txt} - 1 ))"
9
AMIGA:barrywalker~> echo "$(( #txt - 1 ))"
sh: bad substitution: no closing `)' in "$(( #txt - 1 ))"
AMIGA:barrywalker~> _

Either "$/${}" is needed or it is not, which is it?
From now on I will be using $ and ${} as I don't want this aggro again...

The
(c.f. man bash) is available in recent shells only, not in sh. As you can see, variable expansion is done implicitly, no $ nor ${} necessary. Your message above is informative, not an error.

It fails on ksh too!

Code:

Last login: Wed Sep 28 10:21:58 on ttys000
AMIGA:barrywalker~> ksh
AMIGA:uw> txt="1234567890"
AMIGA:uw> echo "$(( ${#txt} - 1 ))"
9
AMIGA:uw> echo "$(( #txt - 1 ))"
ksh:  #txt - 1 : arithmetic syntax error
AMIGA:uw> exit 
AMIGA:barrywalker~> _

---------- Post updated at 11:09 AM ---------- Previous update was at 10:30 AM ----------

And the WRONG answer with zsh!

Code:

AMIGA:barrywalker~> zsh
AMIGA:\u\w> zsh --version
zsh 4.3.11 (i386-apple-darwin11.0)
AMIGA:\u\w> txt="1234567890"
AMIGA:\u\w> echo "$(( ${#txt} - 1 ))"
9
AMIGA:\u\w> echo "$(( #txt - 1 ))" 
48
AMIGA:\u\w> _

EDIT:-
This should have been a new post but was added to this one, no idea why!

In an arithmetic evaluation, the value of a variable can be specified by expanding the variable (i.e. $var or ${var}) or by just giving the variable name (i.e. var). But the expression that is the numeric evaluation of the length of the string that is the value of a variable must use the expansion of the variable that produces that number (i.e. ${#var}); you cannot skip the ${ and } when you are using shell variable expansion operators like ${#var} or ${var%%pattern} or ${var:offset:length}.

OK, I'll go with that BUT, and a big but, why does Shell Check echo "$(( #txt - 1 ))" NOT come up with a syntax error knowing from the various scenarios that it does?
Should there not be a warning saying so. Note that in the Shell Check example I was using the worst case sh . Tested on the others gave errors or incorrect results too.
On reading RudiC's post it looks as though it SHOULD work but obviously it doesn't, or I am misinterpreting it?
Anyhow thanks, but I will keep '$/${}' in all cases just to be on the safe side.

There is something in what RudiC quoted in post #2 that says that the string ${#variable_name} inside an arithmetic expansion will undergo parameter expansion resulting in a number that is the number of characters in the string to which $variable_name expands. (I.e., All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal.)

There is nothing in what RudiC quoted in post #2 that says that the string #variable_name inside an arithmetic expansion should have any meaning (and I don't know of any shells where it does).

There is something in what RudiC quoted in post #2 that says that the strings $variable_name and ${variable_name} inside an arithmetic expansion will expand to the string assigned to that variable. (I.e., All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal.)

There is nothing in what RudiC quoted in post #2 that says that the string variable_name inside an arithmetic expansion will expand to the string that has been assigned to that variable. But, in addition to what RudiC quoted, the latest version of the standard also states the following requirement for arithmetic expansions:
and I don't know of any shell that performs arithmetic expansions that does not also provide this shortcut.

I make absolutely no comment as to why shellcheck seems to think that $((#variable_name)) and $(( #variable_name - 1 )) are not errors. You would have to ask the developers or maintainers of that code why they do not flag these constructs as errors in arithmetic expressions in shell scripts.

Thanks Don...

I will attempt to get in touch with the Shell Check developers.
Consider this thread solved.

EDIT:-
As an addendum...
I have left a message on their bug report board...

Thanks for the heads up Don.