Automating telnet prompts in shell scripts

Telnet is a common tool we use to confirm if connectivity on a particular port is working or not.
However, since it's an interactive program it's difficult to use it in a script.
Since you have to press ctrl+] followed by quit every time you exit the telnet prompt.

I found a neat little trick to get around this using echo command.
If we pipe the word quit to our telnet test then the telnet prompt exits automatically once the test concludes.

[ssuri@:~] $ echo "quit" | telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Connection to localhost closed by foreign host.


This can prove to be very helpful while writing scripts.
For example, if want a particular action to be performed if connectivity exists & the code should exit otherwise.
One thing I couldn't get around is that "Connection to localhost closed by foreign host." printed no matter what text filter I tried.
So, I finally did a workaround by redirecting the telnet test output to a file & then running an if condition on the file.

Given below is the example:


#!/usr/bin/bash

VAR1=$(echo "quit" | telnet localhost 25 | awk 'NR==2 {print $1}')

echo  $VAR1 > /tmp/sa

VAR2=$(cat /tmp/sa)

if [ $VAR2 == "Connected" ] ; then
        echo "connectivity exists"
fi

rm /tmp/sa


This script provides the following output when executed.

[ssuri@:~] $ bash def.sh
Connection to localhost closed by foreign host.
connectivity exists