Diferença entre dois tipos de aritmética de shell

Question

Qual é a diferença entre os dois comandos aritméticos de shell a seguir:

echo $[ $var1 - 1 ]
echo $(( $var1 - 1 ))

Assumindo var1 = 5 por exemplo.

    

Answer 1

Do homem bash:

Arithmetic Expansion
   Arithmetic expansion allows the evaluation of an arithmetic 
   expression and the substitution of the result.  The format 
   for arithmetic expansion is:

          $((expression))

   The old format $[expression] is deprecated and will be 
   removed in upcoming versions of bash.