how to make SSH command execution to timeout

Question

I have a program like this:

ssh -q [email protected] exit
echo "output value  -->$?"

In the above code, I am trying to SSH to the remote server, and tries to check if I can connect or not. I have few servers, which is password less is activated and few servers for which passwords are still not yet deactivated. So my concern, if there is a password, it will get stuck in the same screen asking for the password and it will be there for infinite time, without Exit logic.

Question : How to implement the timers for the above code, and if it stays in the same screen asking for the password. and exit with some error code

2) When I execute the above code, i get the following error codes:

127 -- > I guess its for success
225 -- > for any error.

are there any other error codes other than above?

Answer 1

You could wrap the call to ssh using the timeout command. The timeout command exits with code 124 if a timeout occurs.

timeout 10s ssh -q [email protected] exit
if [ $? -eq 124 ]; then
    echo "Timeout out"
fi

Or, as Vorsprung has commented on your question (as I was looking up the man page!):

ssh -oPasswordAuthentication=no -q [email protected] exit

which will disallow interactive password authentication. You'd then have to check the exit code.

Answer 2

It sounds like the question is asking about 'batch mode' processing against a number of hosts at once (e.g., in a loop). The following ssh command will simply fail if PKA is not configured for a given host (public key authentication -- the "password-less" login mentioned the original question), so it is good for scripts that you just want to continue running regardless of problems connecting to one or two hosts. It also won't bother with all the queries about "are you sure you want to continue connecting" to previously unknown hosts (or you can just add StrictHostKeyChecking no to your ~/.ssh/config for that). For example,

for host in host1 host2 host3  # etc
do
  ssh -o BatchMode=yes -o StrictHostKeyChecking=no -o ConnectTimeout=5 \
      user@$host  'uptime' || echo "problem: host=$host"
done

You'll have to check your (client) ssh man page to see if these options (BatchMode, ConnectTimeout) are supported, though.