执行apt-get update报错

W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A1715D88E1DF1F24

具体的解决方法步骤是(注意下面的KEY 是根据每次报错而不同的，可能需要用实际报错的KEY替换命令中的KEY值)：

$ sudo gpg --keyserver keyserver.ubuntu.com --recv A1715D88E1DF1F24
$ sudo gpg --export --armor A1715D88E1DF1F24 | sudo apt-key add -

注：如果执行gpg报错如下

Executing: /tmp/apt-key-gpghome.1uzliCJjyg/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv 4B7C549A058F8B6B
gpg: failed to start the dirmngr '/usr/bin/dirmngr': No such file or directory
gpg: connecting dirmngr at '/run/user/0/gnupg/d.9awoasrggmw7i87e3cdkmbgp/S.dirmngr' failed: No such file or directory
gpg: keyserver receive failed: No dirmngr

安装dirmngr

$ sudo apt install dirmngr --install-recommends