Scenario linux--How to exit telnet gracefully in a script

Scenario

The telnet command is the user interface of the TELNET protocol. It supports two modes: Command mode and Session Mode. Although telnet supports many commands, in most cases, we just use it to check whether the target host has opened a certain port (default is 23).

There are two execution results:

• The port is not open

$ telnet 101.199.97.65 62715

Trying 101.199.97.65...
telnet: connect to address 101.199.97.65: Connection refused

At this point, the command has exited.

• The port is open

$ telnet 101.199.97.65 62715

Trying 101.199.97.65...
Connected to 101.199.97.65.
Escape character is '^]'.

The command has not exited at this time.

According to the prompt
Escape character is '^]'.It can be seen that the exit character is '^]' (CTRL+]). Entering other characters at this time will not cause it to exit, nor will CTRL+C. After entering CTRL+], it will be automatically executed and enter the command mode:

^]

telnet>

Run

quit at this time to truly exit.

telnet> quit

Connection closed.

Among them, the Escape character can be customized, using the parameter

-e:

$ telnet -e p 101.199.97.65 62715
Telnet escape character is 'p'.
Trying 101.199.97.65...
Connected to 101.199.97.65.
Escape character is 'p'.
p
telnet> quit
Connection closed.

Even so, exiting telnet is still troublesome. So, going one step further, how should I (gracefully) exit telnet if it appears in a script?

Plan

In fact, it can be like this:

• Exit immediately after outputting the results

  $ echo "" | telnet 101.199.97.65 62715

Trying 101.199.97.65...
Connected to 101.199.97.65.
Escape character is '^]'.
Connection closed by foreign host.

$ echo "" | telnet 101.199.97.65 62715

Trying 101.199.97.65...
telnet: connect to address 101.199.97.65: Connection refused

• Delayed exit after outputting results

  sleep 2 causes telnet to output results and stay for 2 seconds before exiting command mode.
  

  $ sleep 2 | telnet 101.199.97.65 62715

Trying 101.199.97.65...
Connected to 101.199.97.65.
Escape character is '^]'.
Connection closed by foreign host.

Using this method, the standard output and standard error can be redirected to a file, and the port open status can be determined by analyzing the contents of the file.

The above is the detailed content of Detailed explanation of scenario linux examples. For more information, please follow other related articles on the PHP Chinese website!