GPG error The following signatures couldn't be verified because the public key is not available NO_PUBKEY 解决办法

用ubuntu的同学应该都会遇到过这个问题，在apt-get update时，如果遇到 NO_PUBLIC 错误：

注意

W: GPG error: http://ppa.launchpad.net karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D45DF2E8FC91AE7E

解决办法：

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys D45DF2E8FC91AE7E