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.
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.
HTH, flokra
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.