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

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.

Typically, one runs ssh-keygen to generate a keypair on the local machine, then ssh-copy-id to copy the public key to remote machines. It seems that you are doing things differently. Why, what is your goal? — ephemient
– ephemient, Commented Sep 10, 2009 at 19:43
Since you’re obviously changing how the remote machines establish connections, consider deploying mosh. mosh.mit.edu It is intended to supplement SSH on unstable connections. I have very good experiences with it. — Daniel
– Daniel, Commented Aug 13, 2013 at 11:08
@ephemient I know it's a bit late, but it seems pretty straightforward that they key was not for the local machine or not for the local user. — Abandoned Cart
– Abandoned Cart, Commented Apr 27, 2019 at 13:10

Josh Correia · Accepted Answer · 2023-11-14 21:33:18Z

237

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.

edited Nov 14, 2023 at 21:33

Josh Correia

4,4103 gold badges42 silver badges67 bronze badges

answered Sep 10, 2009 at 13:33

user156676

Sign up to request clarification or add additional context in comments.

10 Comments

Henley Wing Chiu Over a year ago

To be complete, can u indicate which return code means success and which means failure to SSH?

Sibbs Gambling Over a year ago

Wondering what if the SSH attempt just hangs there?

ssc Over a year ago

Great answer! However, you do not mention that attempting to ssh into a down host fails only after a timeout of e.g. 60 seconds - which might be prohibitive for some usages. Also, if a hostname if defined in ~/.ssh/config, the first ssh approach works while the second nmap way fails with Failed to resolve "<hostname>".

Toskan Over a year ago

no explanation at all about the commands... or what you are actually doing.. what is $? ? etc

grim_i_am Over a year ago

@ssc you can add the -o option to limit the timeout duration. The following example reduces it to 5 seconds: ssh -o ConnectTimeout=5 -q user@downhost exit

|

Adrià Cidre · Accepted Answer · 2013-08-13 10:54:42Z

35

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

answered Aug 13, 2013 at 10:54

Adrià Cidre

4124 silver badges3 bronze badges

2 Comments

Matthaeus Gaius Caesar Over a year ago

This will fail if a password is required and incorrectly not output ok, despite the remote being connectable.

Luca Fagioli Over a year ago

Good answer but you should remove the BatchMode option in order to make it work even on systems that require a password.

Littm · Accepted Answer · 2012-10-11 01:51:50Z

23

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

edited Oct 11, 2012 at 1:51

Littm

4,9454 gold badges33 silver badges40 bronze badges

answered Jul 19, 2012 at 14:13

Brian Ott

2312 silver badges2 bronze badges

1 Comment

Xdg Over a year ago

One line output: (ssh -q -o "BatchMode=yes" -o "ConnectTimeout=3" [email protected] "echo 2>&1" && echo SSH_OK || echo SSH_NOK) | tail -n1

iarroyo · Accepted Answer · 2015-10-22 09:04:48Z

22

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

answered Oct 22, 2015 at 9:04

iarroyo

2,45427 silver badges23 bronze badges

Comments

matt · Accepted Answer · 2020-07-06 13:23:59Z

10

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

answered Jul 6, 2020 at 13:23

matt

4,7541 gold badge32 silver badges34 bronze badges

1 Comment

Daniel Over a year ago

Great! I recomment also adding -o ConnectTimeout=5, for faster exit in cases where the destination port is filtered.

Community · Accepted Answer · 2012-02-03 11:24:09Z

7

Try:

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

edited Feb 3, 2012 at 11:24

CommunityBot

11 silver badge

answered Apr 23, 2010 at 5:19

GUESSWHOz

811 silver badge1 bronze badge

1 Comment

Ding-Yi Chen Over a year ago

A problem of this approach is it does not recognize the hosts defined in ssh_config (i.e /etc/ssh/config or ~/.ssh/config)

MeadowMuffins · Accepted Answer · 2021-02-22 06:21:08Z

4

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

answered Feb 22, 2021 at 6:21

MeadowMuffins

5371 gold badge5 silver badges21 bronze badges

Comments

gcb · Accepted Answer · 2023-02-01 21:48:27Z

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"

-q 0 means leave as soon as idle.

-w 1 wait for connection for 1second.

edited Feb 1, 2023 at 21:48

gcb

14.5k9 gold badges72 silver badges97 bronze badges

answered Mar 23, 2018 at 14:30

Mathieu C.

8796 silver badges10 bronze badges

Comments

Necktwi · Accepted Answer · 2019-02-10 02:52:59Z

1

To connect to a server with multiple interfaces

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

answered Feb 10, 2019 at 2:52

Necktwi

2,6758 gold badges43 silver badges70 bronze badges

Comments

4b0 · Accepted Answer · 2019-08-20 08:45:55Z

1

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.

edited Aug 20, 2019 at 8:45

4b0

22.4k30 gold badges97 silver badges143 bronze badges

answered Aug 20, 2019 at 8:42

Dheeraj Kumar

113 bronze badges

2 Comments

R J Over a year ago

This will only ping the remote IP. Does not guarantee if ssh connection is possible.

Dheeraj Kumar Over a year ago

Thanks for pointing that out, here's the code for ssh xlinu.blogspot.com/2019/09/… export user="username" export pass="password" export i="hostname" export SSHPASS=$pass sshpass -e ssh $user@$i -q "echo $i is Accessible"

Jonathan H · Accepted Answer · 2018-07-17 17:57:12Z

0

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 ...)".

answered Jul 17, 2018 at 17:57

Jonathan H

8,0125 gold badges55 silver badges84 bronze badges

1 Comment

Rakib Fiha Over a year ago

This does not answer the question. The OP wants to check whether SSH connection can be established not checking a file in a remote ssh location.

Mike Q · Accepted Answer · 2019-05-06 17:42:31Z

0

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

answered May 6, 2019 at 17:42

Mike Q

7,4955 gold badges62 silver badges69 bronze badges

Comments

perfecto25 · Accepted Answer · 2021-11-03 23:02:38Z

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
--------------------------------------

Please provide the script in your answer instead of (or in addition to) a link to it.

Sam Bisbee · Accepted Answer · 2009-09-13 18:32:49Z

-7

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.

answered Sep 13, 2009 at 18:32

Sam Bisbee

4,44123 silver badges25 bronze badges

1 Comment

stwissel Over a year ago

Sam: there are valid use cases to have the script checking it. E.g (like me): I have a cron job running on my machine to backup my data via rsync to my home nas. Now I'm outside or even disconnected quite often and need to reschedule if the connection wasn't available. My boxen runs quite well, but as the saying goes: it is always the cable (a.k.a network)

Collectives™ on Stack Overflow

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

14 Answers 14

10 Comments

2 Comments

1 Comment

Comments

1 Comment

1 Comment

Comments

Comments

Comments

2 Comments

1 Comment

Comments

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

14 Answers 14

10 Comments

2 Comments

1 Comment

Comments

1 Comment

1 Comment

Comments

Comments

Comments

2 Comments

1 Comment

Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related