login
login功能说明：登入系统。
语 法：login
补充说明：login指令让用户登入系统，您亦可通过它的功能随时更换登入身份。在Slackware发行版中 ，您可在指令后面附加欲登入的用户名称，它会直接询问密码，等待用户输入。当/etc目录里含名称为nologin的文件时，系统只root帐号登入系统，其他用户一律不准登入。
login
login [name | option]
Log into the system. login asks for a username (name can be supplied on the command line) and password (if appropriate).
If successful, login updates accounting files, sets various environment variables, notifies users if they have mail, and executes startup shell files.
Only the root user can log in when /etc/nologin exists. That file is displayed before the connection is terminated. Furthermore, root may connect only on a tty that is listed in /etc/securetty. If ~/.hushlogin exists, execute a quiet login. If /var/adm/lastlog exists, print the time of the last login.
Options
Suppress second login authentication.
Specify name of remote host. Normally used by servers, not humans; may be used only by root.
Preserve previous environment.