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

Bash Problem Overview

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.

Bash Solutions

Solution 1 - Bash

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.

Solution 2 - Bash

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

Solution 3 - Bash

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

Solution 4 - Bash

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

Solution 5 - Bash

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

Solution 6 - Bash

Try:

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

Solution 7 - Bash

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

Solution 8 - Bash

To connect to a server with multiple interfaces

ssh -o ConnectTimeout=1 -q Necktwi@192.168.1.61;[ $? = 1 ] || ssh -o ConnectTimeout=1 -q Necktwi@192.168.1.51

Solution 9 - Bash

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"

Solution 10 - Bash

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 ...)".

Solution 11 - Bash

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

Solution 12 - Bash

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.

Solution 13 - Bash

I wrote this script to check both netcat and SSH connectivity to all hosts in my servers /etc/hosts

reads /etc/hosts line by line and then tries netcat port 22, and then ssh as "sshuttle" user

quick way to check network sanity

script uses a "sshuttle" user, this is an account that has pub/priv keys on all my hosts and can ssh anywhere (non root account), we use this acct to spin up sshuttle VPN tunnels, but you can add any account that has SSH access to servers

https://gist.github.com/perfecto25/8687d563716ba4923c77162be724beda

output,

./conncheck.sh


netcat is installed, proceeding..
--------------------------------------
tm-us1 (127.0.0.1): ssh OK | nc OK
--------------------------------------
localhost (127.0.0.1): ssh OK | nc OK
--------------------------------------
atlas (192.168.142.21): ssh ERROR | nc OK
--------------------------------------
hydra (192.168.142.22): ssh OK | nc OK
--------------------------------------
nemesis (192.168.140.23): ssh OK | nc OK
--------------------------------------
vulcan (192.168.140.24): ssh OK | nc OK
--------------------------------------
athena (192.168.140.27): ssh OK | nc OK
--------------------------------------
nas1 (192.168.100.101): ssh ERROR | nc OK
--------------------------------------
tm-dev (192.10.23.71): ssh ERROR | nc ERROR
--------------------------------------
WARNING: Your password has expired.
Password change required but no TTY available.
infra01 (192.10.23.186): ssh ERROR | nc OK
--------------------------------------
ns-us1 (192.10.23.252): ssh ERROR | nc OK
--------------------------------------
ns-us2 (192.10.23.182): ssh ERROR | nc OK
--------------------------------------
proxy-us1 (192.10.23.120): ssh OK | nc OK
--------------------------------------
simtm-us1 (192.10.23.236): ssh OK | nc OK
--------------------------------------
tm-us1 (192.10.23.104): ssh OK | nc OK
--------------------------------------
tm-us2 (192.10.23.215): ssh OK | nc OK
--------------------------------------
tm-dev (192.10.23.77): ssh OK | nc OK
--------------------------------------
WARNING: Your password has expired.
Password change required but no TTY available.
tm-uat (192.10.23.225): ssh ERROR | nc OK
--------------------------------------
vpn-us1 (192.10.23.193): ssh OK | nc OK
--------------------------------------

Solution 14 - Bash

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.

Content Type	Original Author	Original Content on Stackoverflow
Question	chutsu	View Question on Stackoverflow
Solution 1 - Bash	user156676	View Answer on Stackoverflow
Solution 2 - Bash	Adrià Cidre	View Answer on Stackoverflow
Solution 3 - Bash	Brian Ott	View Answer on Stackoverflow
Solution 4 - Bash	iarroyo	View Answer on Stackoverflow
Solution 5 - Bash	matt	View Answer on Stackoverflow
Solution 6 - Bash	GUESSWHOz	View Answer on Stackoverflow
Solution 7 - Bash	MeadowMuffins	View Answer on Stackoverflow
Solution 8 - Bash	Necktwi	View Answer on Stackoverflow
Solution 9 - Bash	Mathieu C.	View Answer on Stackoverflow
Solution 10 - Bash	Jonathan H	View Answer on Stackoverflow
Solution 11 - Bash	Mike Q	View Answer on Stackoverflow
Solution 12 - Bash	Dheeraj Kumar	View Answer on Stackoverflow
Solution 13 - Bash	perfecto25	View Answer on Stackoverflow
Solution 14 - Bash	Sam Bisbee	View Answer on Stackoverflow