factorial () { 
    if (($1 == 1))
    then
        echo 1
        return
    else
        echo $(( $( factorial $(($1 - 1)) ) * $1 ))
    fi
}

function factorial {
  typeset n=$1
  (( n < 2 )) && echo 1 && return
  echo $(( n * $(factorial $((n-1))) ))
}