apt-get update后，可能会出现以下问题：
Get:5 http://xxxxxx
Err:5 http://xxxxxx
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY xxxxxxxx（’公钥编码’）
并且伴随出现
E: The repository ‘http://xxxxxx‘ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
解决办法：
将公钥添加至服务器，即终端中输入
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys '公钥编码'
添加成功后终端输出为
gpg: key ‘公钥编码’: public key “xxxxxx” imported
gpg: Total number processed: 1
gpg: imported: 1
