:~$ echo'deb https://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ stretch main contrib non-free rpi' >> /etc/apt/sources.list :~$ echo'deb https://mirrors.tuna.tsinghua.edu.cn/raspberrypi/ stretch main ui' >> /etc/apt/souces.list.d/raspi.list
### `NO_PUBKEY` Error
```sh :~$ sudo apt-get update .... .... W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://raspbian.raspberrypi.org/raspbian/ stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E ... # fix the problem ## Method 1 :~$ wget http://raspbian.raspberrypi.org/raspbina.public.key -O | sudo apt-key add - ## Method 2 :~$ wget https://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian.public.key -O | sudo apt-key add -
