The difference between "-" and "no hyphen" 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).
带横杠的会创建一个全新的环境，使用的是真实用户的设置。 不带横杠的还保留了之前用户的设置，例如当前目录，环境变量等等。
https://wiki.archlinux.org/index.php/Su
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).
