Re: Code Execution in Mathematical Context

[Ilkka Virta]

On 6.6. 15:53, Greg Wooledge wrote:
> > wooledg:~$ echo $(( a[$i] ))
> > Tue 04 Jun 2019 09:23:28 AM EDT
> > 0
>
> > wooledg:~$ echo $(( 'a[$i]' ))
> > bash: 'a[$(date >&2)]' : syntax error: operand expected (error token is "'a[$(date 
> > >&2)]' ")
>
> I definitely got different results when I added single quotes.

Well, yes... The point I was trying to make before (and which Chet 
seemed to confirm) is that the quotes break the $((..)) expression 
completely, even if 'i' there is just a number, and not any fancier than 
that.

  $ a=(123 456 789)
  $ i=2
  $ echo $(( a[$i] ))
  789
  $ echo $(( 'a[$i]' ))
  bash5.0.3: 'a[2]' : syntax error: operand expected (error token is 
"'a[2]' ")
  $ echo $(( 'a[2]' ))
  bash5.0.3: 'a[2]' : syntax error: operand expected (error token is 
"'a[2]' ")

And with ((..)), the command substitution runs with a slightly different 
i, quotes or not and with $i or i:

  $ i='a[$(date >&2)]'
  $ (( a[$i]++ ))
  Thu Jun  6 16:46:06 EEST 2019
  bash5.0.3: a[]: bad array subscript
  bash5.0.3: a[]: bad array subscript
  $ (( a[i]++ ))
  Thu Jun  6 16:46:12 EEST 2019
  $ (( 'a[$i]++' ))
  Thu Jun  6 16:46:18 EEST 2019
  $ (( 'a[i]++' ))
  Thu Jun  6 16:46:31 EEST 2019


So if we want "valid" values in the index to work, and invalid ones to 
not do anything nasty, I can't seem to find a case where quotes would 
help with that.

--
Ilkka Virta / itvirta@iki.fi