test.sh: 4: test.sh: let: not found
解决办法：
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.
use:
sudo dpkg-reconfigure dash
<password>
and when you get the option select "no" to actually use bash instead of dash
