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原文：Here’s a quick-and-dirty method that helps you in using su, even on Ubuntu. It will enable you to execute privileged commands from now on. First, use the command sudo su When asked for a password, enter the password assigned to your user account. Next, use the command passwd to give the user root a password. From this moment on, you can also log in as root on Ubuntu.
方式一：普通用户登录系统后，运行

$ sudo su 输入当前用户密码,进入root用户
# passwd 修改root密码

方式二：新装系统时，进行root密码修改

$ sudo passwd root

之后就可以使用su命令切换到root用户来执行某些更高权限的操作。