High leveling common tools

Tools like bash worths a "man bash" reading. You can even make recursive scripts! Lets play...

#!/bin/bash

function generalista ()
{
    if [ $1 == "0" ] || [ $1 == "1" ]
    then
            echo 1
    else
            VARF=$(($1-1))
            echo $VARF
            generalista $VARF
    fi
}

if [ "$#" == "1" ]
then
    if [ $1 -ge 0 ]
    then
            time LISTA=$(generalista $1)
            echo El factorial es el producto de los factores de esta lista : $LISTA
            FACTORIAL=1;
            for FACTOR in $LISTA
            do
                    TEMP=$(($FACTORIAL*$FACTOR))
                    FACTORIAL=$TEMP
            done
            echo Entonces, el factorial de $1 es $FACTORIAL, aunque eso no es correcto...
    else
            echo Error: integer MUST be positive
    fi
else
    echo Usage: $0 positive_integer
fi