mjhjmtu

I have numerous linux boxes with a very limited set of commands and disk space. But it has the telnet command on it.

I remotely connect to each of these probes (programmatically) and issue one line linux command through SSH.

I need to run a single command to connect to a specific machine, using telnet, and then disconnect right away.

I can do all that, but the disconnection right away part. Telnet opens some sort of a console, or terminal and I can't figure out a one-line command to run the telnet command and then disconnect right away.

If I do that, I can easily parse the textual output for error messages for not being able to connect to the machine on the specified port and that's exactly what I am looking for.

So how can I run a one-line command to connect to a machine using telnet and disconnect afterwards ?

You ought to be able to pipe the exit command into STDIN in telnet.
Try:

echo exit | telnet site port

and see if that works. (it seems to work on my web server, but YMMV).

The simplest and easiest method is given below.

 sleep <n> | telnet <server> <port>

n - The wait time in seconds before auto exit. It could be fractional like 0.5. Note that some required output may not be returned in the specified wait time. So we may need to increase accordingly.

server - The target server IP or hostname.

port - Target service port number.

You can also redirect the output to file like this,

sleep 1 | telnet <server> <port> > output.log

I think better tool for sending commands directly and just getting output would be netcat. It just simple, but powerful tool for putting commands through ports.
You could see usage example in this superuser question: https://superuser.com/questions/261900/how-can-i-pipe-commands-to-a-netcat-that-will-stay-alive - asker gives working example in which connection closes after few seconds.

And if you want just to test connectivity use this:
http://terminalinflection.com/use-netcat-not-telnet-to-test-network-connectivity/

In my case this works. (CentOs7):

while read host port; do
r=$(bash -c 'exec 3<> /dev/tcp/'$host'/'$port';echo $?' 2>/dev/null)
if [ "$r" = "0" ]; then
 echo "$host $port is open"
else
 echo "$host $port is closed"
 exit 1 # To force fail result in ShellScript
fi
done

:) Regards

This is another version of the answer above that makes it act a little more like "normal" telnet syntax. If you like my answer, please give an upvote not to this, but to the original.

#!/bin/bash
if [ "$2" == "" ]; then
 echo "Syntax: $0 <host> <port>"
 exit;
fi

host=$1
port=$2

r=$(bash -c 'exec 3<> /dev/tcp/'$host'/'$port';echo $?' 2>/dev/null)
if [ "$r" = "0" ]; then
 echo "$host $port is open"
else
 echo "$host $port is closed"
 exit 1 # To force fail result in ShellScript
fi

Here is solution I found on Internet:

( echo open 127.0.0.1 23
sleep 5
echo your_login
sleep 5
echo your_password
sleep 5
echo hostname
sleep 5
echo exit ) | telnet

It works for me on SunOS & HP-UX