[bash] How to create a bash script to check the SSH connection?

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.

https://onpyth.blogspot.com/2019/08/check-ping-connectivity-to-multiple-host.html

Above link is to create Python script for checking connectivity. You can use similar method and use:

ping -w 1 -c 1 "IP Address" 

Command to create bash script.

ssh -q -o "BatchMode=yes" -i /home/sicmapp/.ssh/id_rsa <ID>@<Servername>.<domain> "echo 2>&1" && echo $host SSH_OK || echo $host SSH_NOK

Below ssh command should have an exit code of 0 on a successful connection and a non-zero value otherwise.

ssh -q -o BatchMode=yes [email protected] exit

if [ $? != "0" ]; then
    echo "Connection failed"
fi

Following @user156676, to check a range of ips:

#!/bin/sh
IP='192.168.0.'
PWD='your_password'
USR='your_usr'

for i in $(seq 229 255);do
    sshpass -p $PWD ssh -q -o ConnectTimeout=3 ${USR}@${IP}${i} exit
    let ret=$?
    if [ $ret -eq 5 ]; then
        echo $IP$i "Refused!"  $ret
    elif [ $ret -eq 255 ] ; then
        echo $IP$i "Server Down!" $ret
    elif [ $ret -eq 0 ] ; then
        echo $IP$i "Connnected!" $ret
    else
        echo $IP$i "Unknown return code!" $ret
    fi  
done

Try:

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

To connect to a server with multiple interfaces

ssh -o ConnectTimeout=1 -q [email protected];[ $? = 1 ] || ssh -o ConnectTimeout=1 -q [email protected]

Example Using BASH 4+ script:

# -- ip/host and res which is result of nmap (note must have nmap installed)
ip="192.168.0.1"
res=$(nmap ${ip} -PN -p ssh | grep open)

# -- if result contains open, we can reach ssh else assume failure) --
if [[ "${res}" =~ "open" ]] ;then
    echo "It's Open! Let's SSH to it.."
else
    echo "The host ${ip} is not accessible!"
fi

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

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"

You can use something like this

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

This will output "ok" if ssh connection is ok

If you would like to check a remote folder exists, or any other file-test really:

if [ -n "$(ssh "${user}@${server}" [ -d "$folder" ] && echo 1; exit)" ]; then
    # exists
else
    # doesn't exist
fi

Do not forget the quotes in "$(ssh ...)".

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.