How to create a bash script to check the SSH conne

Question

I am in the process of creating a bash script that would log into the remote machines and create private and public keys.

My problem is that the remote machines are not very reliable, and they are not always up. I need a bash script that would check if the SSH connection is up. Before actually creating the keys for future use.

Answer 1

I feel like you're trying to solve the wrong problem here. Shouldn't you be trying to make the ssh daemons more stable? Try running something like monit, which will check to see if the daemon is running and restart it if it isn't (giving you time to find the root problem behind sshd shutting down on you). Or is the network service troublesome? Try looking at man ifup. Does the Whole Damn Thing just like to shut down on you? Well, that's a bigger problem ... try looking at your logs (start with syslog) to find hardware failures or services that are shutting your boxen down (maybe a temperature monitor?).

Making your scripts fault tolerant is great, but you might also want to make your boxen fault tolerant.

Answer 2

Just in case someone only wishes to check if port 22 is open on a remote machine, this simple netcat command is useful. I used it because nmap and telnet were not available for me. Moreover, my ssh configuration uses keyboard password auth.

It is a variant of the solution proposed by GUESSWHOz.

nc -q 0 -w 1 "${remote_ip}" 22 < /dev/null &> /dev/null && echo "Port is reachable" || echo "Port is unreachable"

Answer 3

Try:

echo quit | telnet IP 22 2>/dev/null | grep Connected

Answer 4

Complementing the response of @Adrià Cidre you can do:

status=$(ssh -o BatchMode=yes -o ConnectTimeout=5 user@host echo ok 2>&1)

if [[ $status == ok ]] ; then
  echo auth ok, do something
elif [[ $status == "Permission denied"* ]] ; then
  echo no_auth
else
  echo other_error
fi

Answer 5

You can check this with the return-value ssh gives you:

$ ssh -q user@downhost exit
$ echo $?
255

$ ssh -q user@uphost exit
$ echo $?
0

EDIT: Another approach would be to use nmap (you won't need to have keys or login-stuff):

$ a=`nmap uphost -PN -p ssh | grep open`
$ b=`nmap downhost -PN -p ssh | grep open`

$ echo $a
22/tcp open ssh
$ echo $b
(empty string)

But you'll have to grep the message (nmap does not use the return-value to show if a port was filtered, closed or open).

EDIT2:

If you're interested in the actual state of the ssh-port, you can substitute grep open with egrep 'open|closed|filtered':

$ nmap host -PN -p ssh | egrep 'open|closed|filtered'

Just to be complete.