Linux系统命令su和su - 区别验证

Linux系统命令su和su - 区别验证

以下内容为reed个人见解，不保证内容的准确性，仅供参考讨论，如有错误之处，万望提出！
由于工作需要，Linux下经常在各个用户之间切换，所以经常用到这个命令:su -，因为一直用这个命令，所以也没留意su后面为什么要加一个横杠，不加的话有什么区别。今天脑海里一直浮现su和su - 有什么区别，既然有疑问，就要解决之，哪怕是最基本最简单的问题。

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A.用info su命令查看帮助如下（不用翻译啦，大概看的明白吧）:
1）默认的su，也就是不加参数
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'参数
`-'
`-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个例子：
1.例子1，现有reed普通用户，用su切换root用户。

[reed@bkjia ~]$ pwd
/home/reed
[reed@bkjia ~]$ id
uid=510(reed) gid=510(reed) groups=510(reed)
[reed@bkjia ~]$ su
口令：
[root@bkjia reed]# pwd
/home/reed
[root@bkjia reed]# id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
[root@bkjia 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@bkjia reed]# useradd test
bash: useradd: command not found
[root@bkjia reed]#

例子1用[su]命令切换root用户，从例子看出的确是切换到root用户了，但是工作目录和环境变量竟然还是reed的，用useradd命令竟然提示找不到。