ubuntu 安装gcc 或g++ 时提示未发现软件包 gcc或g++

问题：安装gcc 或g++ 时提示未发现软件包？

1.这时，只需更新apt-get即可，那么apt-get是什么呢？
apt-get，是一条linux命令，适用于deb包管理式的操作系统，主要用于自动从互联网的软件仓库中搜索、安装、升级、卸载软件或操作系统。

输入命令：sudo apt-get update

如果更新时出现卡顿或很慢，是由于我们用的是外网，或者我们电脑的网络过慢

对于前者我的建议是换源
2.什么原因造成的呢？
因为我们安装好ubuntu双系统\单系统后，默认的软件更新源是国外的，
我们可以打开源目录看看：
先切换到目录：
cd /etc/apt
查看/编辑目录：
sudo vi sources.list
我们可以发现：里面的源都是关于***cn.archive.ubuntu.com***
这个就是国外的软件更新源

3.怎么解决呢？
国外的软件更新源在国内使用速度很慢，需要更换成国内的源，这样才能正常安装和更新软件。

怎么更换呢？
打开终端命令行界面中，先将sources.list文件备份一个，更新源服务器地址就是存在这个文件里面的，所以主要就是对这个文件进行编辑就可以了，也是很简单的。

1.先切换到目录：cd /etc/apt

2.备份源（防止意外或后续需要）
输入：sudo cp sources.list sources.list.old

3.编辑源:sudo vim sources.list

4.将文件中内容全部删除

5.更换源，有一下几种源可以选择，推荐阿里云
（1）、阿里云

deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse

（2）、清华大学

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse

（3）、中科大

deb http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse

6、复制其中一种粘贴完后，要对照一下，有没有粘贴格式的错误或缺了哪些东西，然后保存退出。最后别忘了更新缓存。
输入：sudo apt-get update
更新缓存需要一点时间，更新完就可以了。

执行sudo apt-get update后续出现的问题，如没出现请忽略：
如输入：sudo apt-get update后出现这些信息

W: Unknown Multi-Arch type 'no' for package 'compiz-core'
W: Unknown Multi-Arch type 'no' for package 'compiz-gnome'
W: Unknown Multi-Arch type 'no' for package 'libxapian-dev'
W: Ignoring Provides line with DepCompareOp for package python-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package python-cffi-backend-api-min
W: Ignoring Provides line with DepCompareOp for package python3-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package python3-cffi-backend-api-min
W: Unknown Multi-Arch type 'no' for package 'kwin'
W: Unknown Multi-Arch type 'no' for package 'kwin-dev'
W: Unknown Multi-Arch type 'no' for package 'kwin-wayland'
W: Unknown Multi-Arch type 'no' for package 'kwin-x11'
W: Unknown Multi-Arch type 'no' for package 'libkf5sysguard-dev'
W: Ignoring Provides line with DepCompareOp for package php-psr-http-message-implementation
W: Ignoring Provides line with DepCompareOp for package php-psr-log-implementation
W: Ignoring Provides line with DepCompareOp for package php-seclib
W: Ignoring Provides line with DepCompareOp for package php-sabre-http
W: Ignoring Provides line with DepCompareOp for package php-math-biginteger
W: Ignoring Provides line with DepCompareOp for package pypy-cffi
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-min
W: Unknown Multi-Arch type 'no' for package 'compiz-core'
W: Unknown Multi-Arch type 'no' for package 'compiz-gnome'
W: Unknown Multi-Arch type 'no' for package 'libxapian-dev'
W: Ignoring Provides line with DepCompareOp for package python-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package python-cffi-backend-api-min
W: Ignoring Provides line with DepCompareOp for package python3-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package python3-cffi-backend-api-min
W: Unknown Multi-Arch type 'no' for package 'kwin-dev'
W: Unknown Multi-Arch type 'no' for package 'kwin-wayland'
W: Unknown Multi-Arch type 'no' for package 'kwin-x11'
W: Unknown Multi-Arch type 'no' for package 'libkf5sysguard-dev'
W: Ignoring Provides line with DepCompareOp for package pypy-cffi
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-min
W: Unknown Multi-Arch type 'no' for package 'compiz-core'
W: Unknown Multi-Arch type 'no' for package 'compiz-gnome'
W: Ignoring Provides line with DepCompareOp for package xserver-xorg
W: Ignoring Provides line with DepCompareOp for package php-math-biginteger
W: Ignoring Provides line with DepCompareOp for package pypy-cffi
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-min
W: Unknown Multi-Arch type 'no' for package 'compiz-core'
W: Unknown Multi-Arch type 'no' for package 'compiz-gnome'
W: Ignoring Provides line with DepCompareOp for package xserver-xorg
W: Ignoring Provides line with DepCompareOp for package pypy-cffi
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-max
W: Ignoring Provides line with DepCompareOp for package pypy-cffi-backend-api-min

W: 您可能需要运行 apt-get update 来解决这些问题
怎么办呢？

执行如下命令：sudo apt-get install -f apt

再次输入：sudo apt-get update
OK,问题解决