原 su user 与 su - user 之间的区别
The difference between "-" and "no hyphen(连字符)" is that the latter keeps your existing environment (variables, etc); the former creates a new environment (with the settings of the actual user, not your own).
区别：su - 会带上用户本身所执行的环境。如果不加上，还是使用本身的参数。
The hyphen has two effects:
1) switches from the current directory to the home directory of the new user (e.g., to /root in the case of the root user) by logging in as that user
2) changes the environmental variables to those of the new user as dictated by their ~/.bashrc. That is, if the first argument to su is a hyphen, the current directory and environment will be changed to what would be expected if the new user had actually logged on to a new session (rather than just taking over an existing session).