执行apt-get update的时候报错如下：
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5BB92C09DB82666C
详情：
Err:4 http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5BB92C09DB82666C sh: 1: /usr/lib/cnf-update-db: not found Reading package lists... Done W: GPG error: http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5BB92C09DB82666C E: The repository 'http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease' 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. E: Problem executing scripts APT::Update::Post-Invoke-Success 'if /usr/bin/test -w /var/lib/command-not-found/ -a -e /usr/lib/cnf-update-db; then /usr/lib/cnf-update-db > /dev/null; fi' E: Sub-process returned an error code解决方式：
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 5BB92C09DB82666C
到这里就不报这个错了
