Re: Misleading text: $(()) is not bash specific

Chris Down wrote:
> Hello,
>
> In the "RETURN VALUES" section of the manual page for smartctl(8), it is
> stated:
>
>         To test within the shell for whether or not the different bits
>         are turned on or off, you can use the following type of
>         construction (this is bash syntax):
>         smartstat=$(($? & 8))

Yes, this should work with any POSIX shell.


>         This bash script prints all status bits:
>         status=$?
>         for ((i=0; i<8; i++)); do
>           echo "Bit $i: $((status & 2**i && 1))"
>         done
>
> All references to bash should be removed and replaced with notes that
> this is instead *POSIX* shell syntax -- this code works in all POSIX
> shells. See section 2.6.4 of POSIX[0].
>
> Perhaps this was confused with `((', which is indeed not POSIX (although
> it is also not bash-specific).

No, the exponentiation operator (2**i) is also not part of POSIX. The 
script is actually bash specific:

$ for sh in bash dash mksh posh zsh; do $sh -c 'echo $((2**4))'; done
16
dash: 1: arithmetic expression: expecting primary: "2**4"
mksh: 2**4: unexpected '*'
posh: 2**4: unexpected `*'
16

On FreeBSD with default shell:
$ echo $((2**4))
arithmetic expression: syntax error: "2**4"

A check of the "for ((.;.;.))" loop gives similar results.

(BTW: The script does also not work with zsh due to different op 
precedence and the R/O status variable)


It would probably make sense to use a more portable example, e.g.:

val=$?; mask=1
for i in 0 1 2 3 4 5 6 7; do
   echo "Bit $i: $(((val & mask) && 1))"
   mask=$((mask << 1))
done


Thanks,
Christian


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