#!/bin/sh
    # XXX命令不存在
    set -o nounset

-o option-name
 Set the option corresponding to option-name:
   nounset
   Same as -u.

-u
 Treat unset variables and parameters other than the special 
 parameters ‘@’ or ‘*’ as an error when performing parameter expansion. 
 An error message will be written to the standard error, 
 and a non-interactive shell will exit.