Ubuntu中apt-get更新源

1. cd /etc/apt进入配置目录
2. cp ./sources.list ./sources.list.backup备份默认源列表文件
3. rm ./sources.list删除默认源列表文件
4. vim ./sources.list新建源列表文件，拷贝以下内容并保存（注：以北理工的源为例）

deb http://mirror.bit.edu.cn/ubuntu/ precise main restricted 
deb http://mirror.bit.edu.cn/ubuntu/ precise-updates main restricted 
deb http://mirror.bit.edu.cn/ubuntu/ precise universe 
deb http://mirror.bit.edu.cn/ubuntu/ precise-updates universe 
deb http://mirror.bit.edu.cn/ubuntu/ precise multiverse 
deb http://mirror.bit.edu.cn/ubuntu/ precise-updates multiverse 
deb http://mirror.bit.edu.cn/ubuntu/ precise-backports main restricted universe multiverse 
deb http://mirror.bit.edu.cn/ubuntu/ precise-security main restricted 
deb http://mirror.bit.edu.cn/ubuntu/ precise-security universe 
deb http://mirror.bit.edu.cn/ubuntu/ precise-security multiverse

5. sudo apt-get update更新包依赖关系