su(super user)
su(super user)功能说明：变更用户身份。
语 法：su [-flmp][--help][--version][-][-c <指令>][-s <shell>][用户帐号]
补充说明：su可让用户暂时变更登入的身份。变更时须输入所要变更的用户帐号与密码。
参 数：
-c<指令>或--command=<指令> 执行完指定的指令后，即恢复原来的身份。
-f或--fast 适用于csh与tsch，使shell不用去读取启动文件。
-.-l或--login 改变身份时，也同时变更工作目录，以及HOME,SHELL,USER,LOGNAME。此外，也会变更PATH变量。
-m,-p或--preserve-environment 变更身份时，不要变更环境变量。
-s<shell>或--shell=<shell> 指定要执行的shell。
--help 显示帮助。
--version 显示版本信息。
[用户帐号] 指定要变更的用户。若不指定此参数，则预设变更为root
su
su [option] [user] [shell_args]
Create a shell with the effective user ID user. If no user is specified, create a shell for a privileged user (i.e., become a superuser). Enter EOF to terminate. You can run the shell with particular options by passing them as shell_args (e.g., if the shell runs bash, you can specify -c command to execute command via bash, or -r to create a restricted shell).
Options
Go through the entire login sequence (i.e., change to user's environment).
Execute command in the new shell and then exit immediately. If command is more than one word, it should be enclosed in quotes. For example:
su -c 'find / -name *.c -print' nobody
Start the shell with the -f option, which suppresses the reading of the .cshrc or .tcshrc file. Applies to csh and tcsh.
Do not reset environment variables.
Execute shell, not the shell specified in /etc/passwd, unless shell is restricted.
Print a help message and then exit.
Print version information and then exit.
Examples
Become root and obtain all of root's user environment:
$ su -
Become root long enough to restart the Apache httpd web server, then revert to the current user:
$ su -c /etc/rc.d/init.d/httpd restart