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

User · Answer

You can use something like this    ssh -o BatchMode yes -o ConnectTimeout 5 user host echo ok 2 gt  amp 1    This will output  ok  if ssh connection is ok

User · Answer

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

User · Answer

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

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If you would like to check a remote folder exists  or any other file-test really   if   -n    ssh    user    server     -d   folder     amp  amp  echo 1  exit      then       exists else       doesn t exist fi   Do not forget the quotes in    ssh

User · Answer

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  lt   dev null  amp  gt   dev null  amp  amp  echo  Port is reachable     echo  Port is unreachable

User · Answer

Complementing the response of  Adri   Cidre you can do   status   ssh -o BatchMode yes -o ConnectTimeout 5 user host echo ok 2 gt  amp 1   if     status    ok      then   echo auth ok  do something elif     status     Permission denied        then   echo no auth else   echo other error fi

User · Answer

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

User · Answer

Try   echo quit   telnet IP 22 2 gt  dev null   grep Connected

User · Answer

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

User · Answer

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

User · Answer

ssh -q -o  BatchMode yes  -i  home sicmapp  ssh id rsa  lt ID gt   lt Servername gt   lt domain gt   echo 2 gt  amp 1   amp  amp  echo  host SSH OK    echo  host SSH NOK

User · Answer

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  quot Refused  quot    ret     elif    ret -eq 255     then         echo  IP i  quot Server Down  quot   ret     elif    ret -eq 0     then         echo  IP i  quot Connnected  quot   ret     else         echo  IP i  quot Unknown return code  quot   ret     fi   done

User · Answer

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 user remote com exit  if          quot 0 quot     then     echo  quot Connection failed quot  fi

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

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