• Write a shell script to calculate $n!=\prod\limits_{k=1}^n k = 1\times 2\times 3\times 4\times... \times (k-1) \times k$ for any positive integer $n$.
  In addition, for the case that given $n\lt0$, print an error message "Input number must be a positive integer!!" to the user;
  for $n=0$, print $1$ because by definition, $0!=1$.
  
  Send the filename of these shell scripts including their absolute path to me by email.
  
  Hint: For the judgement of condition in bash, refer here. Using if...then would be the simplest but you could also try other syntax.
  
  

• 一個長整數各自表述 ^{local cache}
  
• 大數問題 ^{local cache}
  
• Test codes
  

jsyu@Ukko:~$ grep "[ugd][0-9a][0-9]\{6\}" /etc/passwd | awk -F: '{ print something wrong here }'
>
>
>
> ^C
jsyu@Ukko:~$