Linux 命令 let: not found

ubuntu sh ???.sh 时出现 let: not found
（我的sh文件里有mv命令，结果文件全丢失了）
出现问题的原因：It's because Ubuntu uses the dash shell as default and doesn't always recognize when you try to set the shell in a script. Even if you enter "echo $SHELL" into the console it will tell you you're using /bin/bash but for some reason it's actually using dash instead.
#就是bash环境指向了/bin/sh，而在/bin/bash里才有let命令。
解决办法:
sudo dpkg-reconfigure dash
<password>
and when you get the option select"no" to actually use bash instead of dash

其实也可改用 bash ???.sh指令