1. 首先用help理解su， sudo su， sudo

2. 其次理解"login shell" and "interactive shell". 

"When Bash starts executes the commands in a variety of different scripts. 
(1) When started as an interactive login shell: Bash reads and executes the /etc/profile (if it exists). After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile in that order, and reads and executes the first one (that exists and is readable).  When a login shell exits: Bash reads and executes ~/.bash_logout (if it exists).
(2) When started as an interactive shell (but not a login shell): Bash reads and executes ~/.bashrc (if it exists)."
显然login shell和interactive shell在环境的设置上是存在差别的，详细可以参考： http://groups.google.com/group/Linux.debian.user/browse_thread/thread/2b71ecfc45789958/7bff24e3bae74b36?lnk=raot

3.  然后su命令的横杠就非常容易理解了，参数横杠-， 是为了让切换获取的shell是一个login shell

 -, -l, --login               make the shell a login shell

Linux-命令-su