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The following signatures couldn‘t be verified because the public key is not available: NO_PUBKEY 8D5A09DC9B929006
解决方法:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8D5A09DC9B929006
sudo apt upgrade
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原文地址：http://www.cnblogs.com/IT-lss/p/5577003.html
