Re: [PATCH] unset/null variables should expand to 0 during arithmetic expansion

[Michael Greenberg <Michael.Greenberg@pomona.edu>]

Hi Martijn,

> On Jan 29, 2017, at 4:13 PM, Martijn Dekker <martijn@inlv.org> wrote:
> 
> Op 19-01-17 om 21:06 schreef Michael Greenberg:
>> unset x ; echo $((x+=2))
>> 
>> Running bash on this program echoes the number 2 to standard out and sets x to 2. Running dash (git HEAD/release 0.5.9.1) yields an error:
>> 
>> src/dash: 1: Illegal number: 
> 
> Yes, looks like a dash-specific bug. The related shells Busybox ash and
> FreeBSD /bin/sh act like POSIX says they should.

Great! Please pardon my naivete, but is there anything else I can or should do to see that this bug patch gets accepted and applied?

>> The standard is a little bit unclear about what to do for variables that are set to null (or non-numeric values). I’d interpret the standard as defaulting for unset variables, having null and other non-numeric strings cause some kind of error. bash, as usual, goes well beyond what the standard indicates: any non-numerical value is defaulted to 0. So:
>> 
>> x="" ; echo $((x+=2))
>> 
>> x=“yo” ; echo $((x+=2))
>> 
>> both yield 2! If you ask me, that’s a bridge too far—and asking for bugs.
> 
> That's recursive arithmetic expression parsing in bash, ksh and zsh; if
> the value of a variable is an arithmetic expression (including simply
> another variable name, such as "yo") this is evaluated, which in this
> case means the value of "x" is taken as the value of "yo" which is taken
> as zero. That behaviour is not specified by POSIX, of course.

Right. I only mentioned it because I—as I think we both agree—this behavior is underspecified and probably good as-is. :)

Cheers,
Michael