在使用apt-get update 更新时提示：
W: GPG error: http://ftp.debian-unofficial.org etch Release: The following
signatures couldn't be verified because the public key is not available:
NO_PUBKEY D5642BC86823D007
W: You may want to run apt-get update to correct these problems
出现上叙问题是由于没有公钥，http://ftp.debian-unofficial.org 的签名无法进行验证！只要重新导入公钥即可！
gpg --keyserver subkeys.pgp.net --recv-keys D5642BC86823D007
gpg --export D5642BC86823D007 | apt-key add -
编写一个脚本处理此问题：
#!/bin/sh
if [ !# -eq 0 ]
then
echo usage: $0 keys
exit 1
fi
gpg --keyserver subkeys.pgp.net --recv-keys $1
gpg --export $1| apt-key add -