sudo apt-get update
报了一堆错误：
Err http://cn.archive.ubuntu.com trusty InRelease
Err http://cn.archive.ubuntu.com trusty-updates InRelease
Err http://cn.archive.ubuntu.com trusty-backports InRelease
Err http://cn.archive.ubuntu.com trusty Release.gpg
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/trusty/InRelease
错误显示了源有问题，好像是被墙了。
查找解决办法，修改/etc/apt/source.list文件。将源改成163的或者阿里的：
deb http://mirrors.163.com/ubuntu/ vivid main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ vivid-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ vivid-backports main restricted universe multiverse
但是source.list文件是只读的，修改不了。
要使用sudo gedit /etc/apt/source.list修改，但是gedit 指令没有装，使用sudo apt-get install gedit又安装不了。
解决办法：（修改DNS）
修改dns:
sudo vi /etc/resolvconf/resolv.conf.d/base
在里面插入：
nameserver 8.8.8.8
nameserver 8.8.4.4
修改好保存!wq，
然后执行
sudo resolvconf -u
再看/etc/resolv.conf，最下面就多了2行：
cat /etc/resolv.conf
显示
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8
nameserver 8.8.4.4
此时执行
sudo apt-get update
可以运行，然后就可以使用sudo apt-get install **指令了。