//Ubuntu20.04.1安装完成之后默认是没有root用户登录权限：
//1、使用普通用户登录，创建root用户的密码
yanqi@ubuntu:~$ sudo passwd
[sudo] password for yanqi:
New password:
Retype new password:
passwd: password updated successfully
//2、修改/usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf配置文件
yanqi@ubuntu:~$ ll /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
-rw-r--r-- 1 root root 29 3月 26 2020 /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
yanqi@ubuntu:~$ sudo chmod 666 /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
yanqi@ubuntu:~$ ll /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
-rw-rw-rw- 1 root root 29 3月 26 2020 /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
yanqi@ubuntu:~$ sudo vim /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
greeter-show-manual-login=true
all-guest=false
//3、修改/etc/pam.d/gdm-autologin配置文件：
yanqi@ubuntu:~$ ll /etc/pam.d/gdm-autologin
-rw-r--r-- 1 root root 1192 10月 8 2019 /etc/pam.d/gdm-autologin
yanqi@ubuntu:~$ sudo chmod 666 /etc/pam.d/gdm-autologin
yanqi@ubuntu:~$ ll /etc/pam.d/gdm-autologin
-rw-rw-rw- 1 root root 1192 10月 8 2019 /etc/pam.d/gdm-autologin
yanqi@ubuntu:~$ sudo vim /etc/pam.d/gdm-autologin
#auth required pam_succeed_if.so user != root quiet_success //注释掉第三行
//4、修改/etc/pam.d/gdm-password配置文件：
yanqi@ubuntu:~$ ll /etc/pam.d/gdm-password
-rw-r--r-- 1 root root 1320 10月 8 2019 /etc/pam.d/gdm-password
yanqi@ubuntu:~$ sudo chmod 666 /etc/pam.d/gdm-password
yanqi@ubuntu:~$ ll /etc/pam.d/gdm-password
-rw-rw-rw- 1 root root 1320 10月 8 2019 /etc/pam.d/gdm-password
yanqi@ubuntu:~$ sudo vim /etc/pam.d/gdm-password
#auth required pam_succeed_if.so user != root quiet_success //注释掉第三行的
//5、修改/root/.profile配置文件
yanqi@ubuntu:~$ su root
root@ubuntu:~# ll /root/.profile
-rw-r--r-- 1 root root 161 12月 5 2019 /root/.profile
root@ubuntu:~# chmod 666 /root/.profile
root@ubuntu:~# ll /root/.profile
-rw-rw-rw- 1 root root 161 12月 5 2019 /root/.profile
root@ubuntu:~# vim /root/.profile
# mesg n 2> /dev/null || true //注释掉这一行
tty -s&&mesg n || true //添加tty开启状态二、Ubuntu配置虚拟机静态固定ip地址：
root@ubuntu:~# vim /etc/netplan/01-network-manager-all.yaml
ethernets:
ens33:
dhcp4: false
addresses: [192.168.1.92/24]
gateway4: 192.168.1.1
nameservers:
addresses: [114.114.114.114,8.8.8.8]
root@ubuntu:~# netplan apply三、Ubuntu开启ssh服务，并通过xshell远程连接：
root@ubuntu:~# apt-get install openssh-server openssh-client
root@ubuntu:~# chmod 666 /etc/ssh/sshd_config
root@ubuntu:~# vim /etc/ssh/sshd_config
# Authentication:
#LoginGraceTime 2m
#PermitRootLogin prohibit-password
PermitRootLogin yes
StrictModes yes
root@ubuntu:~# systemctl restart ssh四、更改Ubuntu下载原地址为国内源
root@ubuntu:~# chmod 666 /etc/apt/sources.list
root@ubuntu:~# vim /etc/apt/sources.list //编辑配置文件，更换为国内源地址 //使用清华源地址
root@ubuntu:~# apt-get update && apt-get upgrade //可以保证系统的完整性（upgrade更新，若依赖关系无法解决可能会报错或停止）
root@ubuntu:~# apt-get dist-upgrade //该命令也是更新所有软件包（慎用：自动解决依赖关系，可能更新一些不需要更新的包，导致一些需要依赖就报的程序无法运行）。
root@ubuntu:~# apt-get clean 国内常用源地址：
# 阿里云源地址
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
# 清华源地址
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-updates main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-backports main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-security main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-proposed main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ eoan-proposed main restricted universe multiverse