Ubuntu 18.04 apt-get update出现NO_PUBKEY错误的解决方法
最近在VPS中使用apt-get update命令时，出现了如下的错误，形式如“W: GPG error: http://security.ubuntu.com trusty-security Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY XXXXX”
该问题也会出现在Ubuntu其他的版本中，实际上在商家上线的系统模版中或多或少都有一些类似的细节错误，值得我们在使用中注意并修正。当然这一类的问题实际上并不算大，只是解决起来可能需要一些搜索工作。
apt-get update返回NO_PUBKEY错误的解决方法
报错代码如下：
Err:5 https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu bionic InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F Fetched 23.3 kB in 9s (2,673 B/s) Reading package lists... Done W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F W: Failed to fetch https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu/dists/bionic/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F W: Some index files failed to download. They have been ignored, or old ones used instead.
解决办法：
将公钥添加至服务器，运行如下命令解决：
apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32
添加成功后终端输出为：
Executing: /tmp/apt-key-gpghome.4JQ9tOjCB7/gpg.1.sh --recv-keys --keyserver keyserver.ubuntu.com 3F01618A51312F3F gpg: key 3F01618A51312F3F: public key "GitLab B.V. (package repository signing key) <packages@gitlab.com>" imported gpg: Total number processed: 1 gpg: imported: 1
这样再去sudo apt-get update就不会有问题了。