ubuntu server18.04 更换默认源为阿里源-加速
Ubuntu安装完毕后，默认使用的是官方的源，在国内访问速度很慢，这里介绍更换为阿里云的源方法。
安装server时建议替换镜像源，可以提速
原镜像：
http://cn.archive.ubuntu.com/ubuntu/
更换为：
http://mirrors.aliyun.com/ubuntu/
http://mirrors.cn99.com/ubuntu/
步骤如下：
1.备份源配置文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
2.用编辑器打开源配置文件
sudo vim /etc/apt/sources.list
3.删除文件内容，更换为以下内容（也可使用其他源）
deb http://mirrors.aliyun.com/ubuntu trusty main restricted
deb-src http://mirrors.aliyun.com/ubuntu trusty main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.aliyun.com/ubuntu trusty-updates main restricted
deb-src http://mirrors.aliyun.com/ubuntu trusty-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://mirrors.aliyun.com/ubuntu trusty universe
deb-src http://mirrors.aliyun.com/ubuntu trusty universe
deb http://mirrors.aliyun.com/ubuntu trusty-updates universe
deb-src http://mirrors.aliyun.com/ubuntu trusty-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.aliyun.com/ubuntu trusty multiverse
deb-src http://mirrors.aliyun.com/ubuntu trusty multiverse
deb http://mirrors.aliyun.com/ubuntu trusty-updates multiverse
deb-src http://mirrors.aliyun.com/ubuntu trusty-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.aliyun.com/ubuntu trusty-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu trusty-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse
4.执行以下命令，刷新
sudo apt-get clean
sudo apt-get update
