2>&1

sudo           mkdir            /var/run/wt       2>&1
root 用户    创建文件夹   /var/run/wt       标准错误输出（2）并入（>）标准输出（1）

& 领取文件描述符；

来自： http://tldp.org/LDP/abs/html/io-redirection.html

   M>N
     # "M" is a file descriptor, which defaults to 1, if not explicitly set.
     # "N" is a filename.
     # File descriptor "M" is redirect to file "N."
   M>&N
     # "M" is a file descriptor, which defaults to 1, if not set.
     # "N" is another file descriptor.

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