新部署的云服务器出现如下错误：

root@iZj6cbjalvhsw0fhndmm5xZ:~# apt-get install git
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  git-man liberror-perl
Suggested packages:
  git-daemon-run | git-daemon-sysvinit git-doc git-el git-email git-gui gitk gitweb git-arch git-cvs git-mediawiki git-svn
The following NEW packages will be installed:
  git git-man liberror-perl
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 3,898 kB/3,918 kB of archives.
After this operation, 25.6 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Ign:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-updates/main i386 git-man all 1:2.7.4-0ubuntu1.2
Ign:2 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 git amd64 1:2.7.4-0ubuntu1.2
Ign:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 git-man all 1:2.7.4-0ubuntu1.2
Err:2 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main amd64 git amd64 1:2.7.4-0ubuntu1.2
  404  Not Found
Err:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main i386 git-man all 1:2.7.4-0ubuntu1.2
  404  Not Found
Err:1 http://mirrors.cloud.aliyuncs.com/ubuntu xenial-security/main i386 git-man all 1:2.7.4-0ubuntu1.2
  404  Not Found
E: Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/g/git/git-man_2.7.4-0ubuntu1.2_all.deb  404  Not Found

E: Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/g/git/git_2.7.4-0ubuntu1.2_amd64.deb  404  Not Found

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

（这不很明显是一些安装源失效，甚至还有命令提示）为了避免像我一样看到错误就百度一波，结果没找到一个靠谱的方法。

超简单，更新一下源就可以了。

sudo apt-get update