hello@world:~$ telnet
telnet> help
Commands may be abbreviated. Commands are:

close close current connection
logout forcibly logout remote user and close the connection
display display operating parameters
mode try to enter line or character mode ('mode ?' for more)
open connect to a site
quit exit telnet
send transmit special characters ('send ?' for more)
set set operating parameters ('set ?' for more)
unset unset operating parameters ('unset ?' for more)
status print status information
toggle toggle operating parameters ('toggle ?' for more)
slc change state of special charaters ('slc ?' for more)
z suspend telnet
! invoke a subshell
environ change environment variables ('environ ?' for more)
? print help information
telnet>

// 看到 ! 命令嗎？？ 輸入'!', 執行子shell.

Ctrl + ]（中括號的一半，不時j） 將 Telnet 會話模式更改為 Telnet 命令模式；按 Enter 將返回到 Telnet 會話模式