Re: whats wrong with (( a = 8 , a > 4 && a -= 2 || a-- )) , bash: ((: a = 8 , a > 4 && a -= 2 || a-- : attempted assignment to non-variable (error token is "-= 2 || a-- ")

[Kerin Millar]

On Thu, 23 Mar 2023 11:32:33 +0100
alex xmb ratchev <fxmbsw7@gmail.com> wrote:

> i remember doing && (( code
> maybe i didnt '=' in action there
> 
> (( a = 8 , a > 4 && a -= 2 || a-- ))
> 
> bash: ((: a = 8 , a > 4 && a -= 2 || a-- : attempted assignment to
> non-variable (error token is "-= 2 || a-- ")
> 
> 
> (( a = 8 , a > 4 && a-- && a-- || a-- ))
> 
> works
> a=6
> 
> ..
> i suppose this is a (( lex bug where u didnt include || && in op for =

I agree that it looks like a possible bug in the parser. It works if 
parentheses are used to disambiguate one of the subtractions.

$ (( a = 8 , a > 4 && (a -= 2) || a-- )); declare -p a
declare -- a="6"

Even when made to work, I would consider it to be poor. Consider what happens 
below. It is a fine example of what can happen in the course of writing 
unconventional code without understanding the consequences.

$ (( a = 2 , a > 1 && (a -= 2) || a-- )); declare -p a
declare -- a="-1"

If you must do this sort of thing, use the conditional operator instead e.g. (( 
a = 8, a = (a > 4) ? a - 2 : a - 1 )). Alternatively, just avoid doing 
assignments in arithmetic expressions except where there's a clear advantage (I 
cannot see one in this case).

-- 
Kerin Millar