sudo apt-get update
報了一堆錯誤：
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
錯誤顯示了源有問題，好像是被牆了。
查找解決辦法，修改/etc/apt/source.list文件。將源改成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文件是只讀的，修改不了。
要使用sudo gedit /etc/apt/source.list修改，但是gedit 指令沒有裝，使用sudo apt-get install gedit又安裝不了。
解決辦法：（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，
然后執行
sudo resolvconf -u
再看/etc/resolv.conf，最下面就多了2行：
cat /etc/resolv.conf
顯示
# 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
此時執行
sudo apt-get update
可以運行，然后就可以使用sudo apt-get install **指令了。