Detailed explanation of scenario linux examples

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.

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