Return / exit values

[Chris Elvidge]

I've used 'return $((!$#))' and/or 'return $[!$#]' to return an error if 
no parameters given to function.

Tested in a bash script 'exit $((!$#)) / $[!$#]' - both work.

'echo  $((!$#)) / $[!$#]' - both echo 1 when no params, 0 when any 
number of params.

I'm told ( https://wiki.bash-hackers.org/scripting/obsolete ) that 
$[...] is obsolete and that $((...)) should be used instead. OK so far.

However 'N=0; echo $((!$N))' gives an error at the bash prompt. 'echo 
$[!$N]' echo's 1 as expected.

My question - is $[...] actually obsolete? If so, what should I use at 
the bash prompt to get the same effect?

Cheers