當出現GPG的錯誤的時候，來這裏

獲取：56 http://security.ubuntu.com precise-security/main Translation-en [279 kB]                                                                                                                                  
獲取：57 http://security.ubuntu.com precise-security/multiverse Translation-en [1,993 B]                                                                                                                           
獲取：58 http://security.ubuntu.com precise-security/restricted Translation-en [2,805 B]                                                                                                                           
獲取：59 http://security.ubuntu.com precise-security/universe Translation-en [91.9 kB]                                                                                                                             
下載 5,917 kB，耗時 1分 36秒 (61.3 kB/s)                                                                                                                                                                           
正在讀取軟件包列表... 完成
W: GPG 錯誤：http://ppa.launchpad.net precise Release: 因爲沒有公鑰，沒法驗證下列簽名： NO_PUBKEY EAA903E3A2F4C039
nice@lap2:~/xxx$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EAA903E3A2F4C039
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /tmp/tmp.XCAi9m9kOD --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv-keys EAA903E3A2F4C039
gpg: 下載密鑰‘A2F4C039’，從 hkp 服務器 keyserver.ubuntu.com
gpg: 密鑰 A2F4C039：公鑰「Launchpad Subversion」已導入
gpg: 合計被處理的數量：1
gpg:               已導入：1  (RSA: 1)

sudo apt-get update