UNIX for Dummies Questions & Answers

Hi,
Hope all your are doing well. Need a suggestion from you.
I am writing an automated shell script that will effectively check the telnet connectivity with different backends from present deployment server.

Ideally, this script reads each backend hostname from a configuration file and fires telnet command on that. like

Code:

telnet $hostname $port

but the catch here is, if the connection is established shell holds the prompts there, it does not automatically exits. I get something like

Code:

telnet  hostname 443
Trying **.***.***.*...
Connected to **.**.**.**
Escape character is '^]'.

Sometime if the remote host does not respond then it holds for long time,

Code:

telnet  hostname 443
Trying **.***.***.*...

This symbolises we need manual intervention clearly to come out. I want script should wait for certain interval if it does not get response come out and try next host.

Any ideas how to implement?
I can not use expect technogloy as it is not permitted in our environment.
I tried google and search in this forum as well, but did find a suitable approach.

Please let me know your ideas, any help would be very much appreciated.

Thanks in anticipation.
Regards,
Bhaskar

Making it quit once it connects at least might be more doable. Redirect standard input from /dev/null. It will try to read from stdin once it connects, get EOF, and quit normally.

telnet doesn't seem to have any timeout options. It's a pain in general to automate. You'll have to kill it externally.

Code:

telnet hostname port < /dev/null > /dev/null &
PID=$!

sleep 3
# Can't kill something that's dead, so if telnet's finished already
# killing it won't change its exit status from zero to nonzero.
kill "$PID"
if ! wait
then
        echo "No telnet"
fi

Is netcat (nc) available? It has timeouts and such, and doesn't depend on terminals, so is much less torturous to automate.

Hi Corona,

It was indeed a great experience to run your snippet.

I varied the waiting time and launched the process in background was indeed a good idea. May be I tried reading from config file rather than /dev/null it also worked.

Thanks a lot for this idea!!

I think nc is available in our env and it can be used as a substitute of telnet command. I will suggest these to team members.

Once again Thanks a lot.
Have great time.

Cheers!!
Bhaskar

??? That's stuff that'd be fed raw into telnet! Make it an empty file

Slightly off topic. Port 443 is not normally used for telnet.
If you are trying to write some sort of port scanner be very careful. It is possible to crash a system by firing whatever ports are live in "inetd". I have also seen systems block all incoming TCP/IP connections when under attack thus causing a "denial of service" situation.