以下內容為reed個人見解，不保證內容的准確性，僅供參考討論，如有錯誤之處，萬望提出！
由於工作需要，Linux下經常在各個用戶之間切換，所以經常用到這個命令:su -，因為一直用這個命令，所以也沒留意su後面為什麼要加一個橫槓，不加的話有什麼區別。今天腦海裡一直浮現su和su - 有什麼區別，既然有疑問，就要解決之，哪怕是最基本最簡單的問題。
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A.用info su命令查看幫助如下（不用翻譯啦，大概看的明白吧）:
1）默認的su，也就是不加參數
By default, `su' does not change the current directory. It sets the
environment variables `HOME' and `SHELL' from the password entry for
USER, and if USER is not the super-user, sets `USER' and `LOGNAME' to
USER. By default, the shell is not a login shell.
2）su加'-'/'-l'/'--login'參數
`-'
`-l'
`--login'
Make the shell a login shell. This means the following. Unset all
environment variables except `TERM', `HOME', and `SHELL' (which
are set as described above), and `USER' and `LOGNAME' (which are
set, even for the super-user, as described above), and set `PATH'
to a compiled-in default value. Change to USER's home directory.
Prepend `-' to the shell's name, intended to make it read its
login startup file(s). Additionaly `DISPLAY' and `XAUTHORITY'
environment variables are preserved as well for PAM functionality.
B.再看看2個例子：
1.例子1，現有reed普通用戶，用su切換root用戶。
[reed@linuxidc ~]$ pwd
/home/reed
[reed@linuxidc ~]$ id
uid=510(reed) gid=510(reed) groups=510(reed)
[reed@linuxidc ~]$ su
口令：
[root@linuxidc reed]# pwd
/home/reed
[root@linuxidc reed]# id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
[root@linuxidc reed]# echo $PATH
/usr/kerberos/sbin:/usr/lib/qt-3.3/bin:/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/usr/java/jdk1.5.0_11/bin:/usr/local/mysql/bin:/usr/apache/bin:/home/reed/bin
[root@linuxidc reed]# useradd test
bash: useradd: command not found
[root@linuxidc reed]#
例子1用[su]命令切換root用戶，從例子看出的確是切換到root用戶了，但是工作目錄和環境變量竟然還是reed的，用useradd命令竟然提示找不到。
