用户登录时，实际进入了shell，它遵循一定的语法将输入的命令加以解释并传给系统。命令行中输入的第一个字必须是一个命令的名字，第二个字是命令的选项或参数，命令行中的每个字必须由空格或TAB隔开，格式如下：

　　

　　$ Command Option Arguments -User is logged on, the actual into the shell, it follows a certain syntax of the command input to be interpreted and passed to the system. The command line, type the first word must be a command name, the second word is the command options or parameters, the command line every word must be separated by a space or TAB format is as follows: