Janis Papanagnou <janis_papanagnou@hotmail.com>:
>On 09.08.2022 11:32, Martijn Dekker wrote:
>> Op 06-08-22 om 19:13 schreef Janis Papanagnou:
>>> For readability I prefer the [non-standard] $(( RANDOM % 3 ))
>>
>> FYI, referring to variables without a leading $ in arithmetic
>> expressions is perfectly standard:
>>
>> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
>>
>> | If the shell variable x contains a value that forms a valid integer
>> | constant, optionally including a leading <plus-sign> or <hyphen-
>> | minus>, then the arithmetic expansions "$((x))" and "$(($x))" shall
>> | return the same value.
>
>Oh, thanks! - Has that changed? (I seem to recall that last time I
>looked at that issue it didn't support $-less variables in arithmetic
>expressions.) - Anyway, good to know.

There might be a difference, though, if the value of the variable is
not a literal integer, but a string resembling an integer
expression.  Try, using different shells, the following commands:

v='6*7'
printf '%s\n' "$((v))"
printf '%s\n' "$((${v}))"

On 09.08.2022 19:09, Helmut Waitzmann wrote:
> Janis Papanagnou <janis_papanagnou@hotmail.com>:
>> On 09.08.2022 11:32, Martijn Dekker wrote:
>>> Op 06-08-22 om 19:13 schreef Janis Papanagnou:
>>>> For readability I prefer the [non-standard] $(( RANDOM % 3 ))
>>>
>>> FYI, referring to variables without a leading $ in arithmetic
>>> expressions is perfectly standard:
>>>
>>> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
>>>
>>>
>>> | If the shell variable x contains a value that forms a valid integer
>>> | constant, optionally including a leading <plus-sign> or <hyphen-
>>> | minus>, then the arithmetic expansions "$((x))" and "$(($x))" shall
>>> | return the same value.
>>
>> Oh, thanks! - Has that changed? (I seem to recall that last time I
>> looked at that issue it didn't support $-less variables in arithmetic
>> expressions.) - Anyway, good to know.
>
> There might be a difference, though, if the value of the variable is not
> a literal integer, but a string resembling an integer expression. Try,
> using different shells, the following commands:
>
> v='6*7'
> printf '%s\n' "$((v))"
> printf '%s\n' "$((${v}))"

I anyway wouldn't have expected that formulas in string variables
would have been handled (like ksh/bash/zsh do) in the first place.

A probably more interesting case (where these three shells differ)
is
printf '%s\n' $(("${v}"))

Janis

Janis Papanagnou <janis_papanagnou@hotmail.com>:
>On 09.08.2022 19:09, Helmut Waitzmann wrote:
>> v='6*7'
>> printf '%s\n' "$((v))"
>> printf '%s\n' "$((${v}))"
>
>I anyway wouldn't have expected that formulas in string variables
>would have been handled (like ksh/bash/zsh do) in the first place.
>
>A probably more interesting case (where these three shells differ)
>is
> printf '%s\n' $(("${v}"))

POSIX
(<https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04>)
says:

$((expression))

The expression shall be treated as if it were in
double-quotes, except that a double-quote inside the
expression is not treated specially. The shell shall
expand all tokens in the expression for parameter
expansion, command substitution, and quote removal.

Does "a double-quote inside the expression is not treated specially"
say that double-quotes are treated literally, i.e. excluded from
quote removal, thus left as (a syntactic invalid) part of the
arithmetic expression?

On 10.08.2022 16:46, Helmut Waitzmann wrote:
> Janis Papanagnou <janis_papanagnou@hotmail.com>:
>> On 09.08.2022 19:09, Helmut Waitzmann wrote:
>>> v='6*7'
>>> printf '%s\n' "$((v))"
>>> printf '%s\n' "$((${v}))"
>>
>> I anyway wouldn't have expected that formulas in string variables
>> would have been handled (like ksh/bash/zsh do) in the first place.
>>
>> A probably more interesting case (where these three shells differ) is
>> printf '%s\n' $(("${v}"))
>
> POSIX
> (<https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04>)
> says:
>
> $((expression))
>
> The expression shall be treated as if it were in
> double-quotes, except that a double-quote inside the
> expression is not treated specially. The shell shall
> expand all tokens in the expression for parameter
> expansion, command substitution, and quote removal.
>
> Does "a double-quote inside the expression is not treated specially" say
> that double-quotes are treated literally, i.e. excluded from quote
> removal, thus left as (a syntactic invalid) part of the arithmetic
> expression?

My interpretation would be that it's effectively just ignored because
the expression is anyway treated like being double-quoted. It would,
IMO, also make no sense to assume differently. Throw an error for a
quoted $-expression?!

But shells behave differently, so maybe it's not that clear...

$ ksh -c 'v="6*7" ; printf "%s\n" "$((v))" ; printf "%s\n" $(("${v}"))'
42
42

$ zsh -c 'v="6*7" ; printf "%s\n" "$((v))" ; printf "%s\n" $(("${v}"))'
42
zsh:1: bad math expression: illegal character: "

$ bash -c 'v="6*7" ; printf "%s\n" "$((v))" ; printf "%s\n" $(("${v}"))'
42
bash: "6*7": syntax error: operand expected (error token is ""6*7"")

At least bash and zsh seem to interpret it the way you suspected. (And
as so often, I'm happy to use ksh here which serves my interpretation.)

And dash complains completely...

$ dash -c 'v="6*7" ; printf "%s\n" "$((v))" ; printf "%s\n" $(("${v}"))'
dash: 1: Illegal number: 6*7

Janis