Question

Using the following shell code for remote telnet request:

{
sleep 5
echo admin
sleep 3
echo pass
sleep 3
echo ls
sleep 5
echo exit
} | telnet 172.16.1.1

I want to check if telnet connection was successful or not. Trying to use $?:

echo $?

But it always returns “1”, even if telnet connection was OK.

Answer 1

Telnet is exceptionally difficult to script in this way, there is a high degree of asynchronicity with the time it takes to establish a connection and for your intended actions to complete. expect was created for exactly this kind of purpose. You launch a program, like telnet, then declare a series of expectations – eg, when ‘username: ‘ is emitted from the program, and an action to trigger (eg: typing in the username).

There are also libraries or wrappers for expect in many languages:

• python expect
• ruby expect
• perl expect

Here is an example that drives telnet to make an HTTP HEAD request:

set timeout 20

spawn telnet localhost 80

expect "Connected to "
send "HEAD / HTTP/1.0\r\n\r\n"

expect "HTTP 200 OK"

Given all of this, I feel I must point out that telnet is considered insecure. Ssh is a much better choice and supports better choices for authentication (eg: public/private key auth), restrictions for commands that can be run (via .ssh/authorized_keys). With ssh, and ssh-keys set up, your script reduces to a single shell command:

ssh user@hostname ls

ssh has great support for safe, secure remote command execution.