Ttyjfyk

I'm trying to automatically query a status via telnet (this isn't something I'm able to work around in this case). The idea is to grep for something and assign the result to a variable, to pass to a conditional statement later. The complication is the link to the target device may not be up, and therefore the script could hang indefinitely if not killed via a timeout.

This sets output to what I'd expect, but does not handle the link being down:

output=$(telnet 1.2.3.4 1234 | grep "something")

This will spit out the expected output while handling the link being down:

timeout --signal=9 3 telnet 1.2.3.4 1234 | grep "something"

I can even direct the output to a file and the file will contain the output:

timeout --signal=9 3 telnet 1.2.3.4 1234 | grep "something" > /tmp/tmpfile.txt

Unfortunately rapid write/read of a file like this isn't an option because of how much it'll fill up log files.

But, when I try to combine everything, the variable doesn't set:

output=$(timeout --signal=9 3 telnet 1.2.3.4 1234 | grep "something")

Or, rather, it sets it to a blank value, because if I set it before running the above, the variable is blank afterwards.

telnet expects a tty on its stdin, but timeout takes that away.

If you really insist on using telnet you may do so by adding --foreground option to timeout, as in:

output=$(timeout --foreground --signal=9 3 telnet 1.2.3.4 1234 2>&1 | grep "something")

Besides, if you can have nc on your system you should really rather use that for your purpose:

output=$(timeout 3 nc 1.2.3.4 1234 | grep "something")

If neither nc nor timeout --foreground are an option for you then you really need an alternative to telnet that won’t need a tty.

I see you tagged your question bash, so you could use Bash’s own networking facilities, and thus your line might become like:

output=$(timeout 3 cat < /dev/tcp/1.2.3.4/1234 | grep "something")

If not even cat is an option then you could replace it with a one-liner script in Bash, like in:

output=$(timeout 3 stdbuf -oL bash -c 'while read line ; do echo "$line" ; done < /dev/tcp/1.2.3.4/1234 | grep "something"')

Hoping that at least stdbuf (which is part of standard coreutils package) is available in your system.

In this last alternative however pay attention to your grep regex: if you have single-quotes in there then you need to escape them by first quitting the main single-quote pair.

That is needed also if you need to pass variables (eg hostname and/or port number) from your shell to the one-liner script. For instance:

hostname=1.2.3.4
portnumber=1234

output=$(timeout 3 stdbuf -oL bash -c 'while read line ; do echo "$line" ; done < /dev/tcp/'"$hostname"'/'"$portnumber"' | grep "something"')

Here I'm assuming that $hostname and $portnumber values can be trusted, ie provided by you or by other trusted sources that won't give illegitimate, invalid, or dangerous values.

It is the standard error that is not being redirected (when your link is down)!
This will solve your problem, because it redirects the standard error to the standard output.

output=$(timeout --signal=9 3 telnet 1.2.3.4 1234 2>&1 | grep "something")