Sunday, January 11, 2009
Test or check ssh connection in bash script
Many times I have come across situations where I need to test a SSH connection to a remote server before performing some operation using ssh in that box. Here is a bash script function to test a ssh connection.
....
....
_TestSSH() {
local user=${1}
local host=${2}
local timeout=${3}
ssh -q -q -o "BatchMode=yes" -o "ConnectTimeout ${timeout}" ${user}@${host} "echo 2>&1" && return 0 || echo "Make sure you have access to ${host}"
}
_TestSSH root 172.24.0.102 5 && _somefunction
...
...
Where:
From man pages of ssh command:
-q (Quiet mode) - Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed. If a second -q is given then even fatal errors are suppressed.
echo 2>&1 - I have used this command, but this can be any command to execute in the remote host as a test command.
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7 comments:
The simplest command to execute on the remote machine for test purposes is ":" command. see the below example
[root@localhost ~]# :
[root@localhost ~]#
Some times, when you do ssh you may end up with the following message
# ssh 192.168.10.1
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
5c:9b:16:56:a6:cd:11:10:3a:cd:1b:a2:91:cd:e5:1c.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts:1
RSA host key for 192.168.10.1 has changed and you have requested strict checking.
Host key verification failed.
To fix this problem, here is the tip
# ssh-keygen -R 192.168.10.1
Use the -R option to removes all keys belonging to hostname from a known_hosts file.
Sample output:
/root/.ssh/known_hosts updated.
Original contents retained as /root/.ssh/known_hosts.old
Now, you can connect to the host without a problem.
@Adithya, true, as colon : does nothing, its one of the simplest command we can use to test. Thanks.
There are a lot of times this would fail. You need to add some logic.
Change:
ssh -q -q -o "BatchMode=yes" -o "ConnectTimeout ${timeout}" ${user}@${host} "echo 2>&1"
to:
[ "$(ssh -q -q -o "BatchMode=yes" -o "ConnectTimeout ${timeout}" ${user}@${host} echo up 2>&1)")" == "up" ]
but like Adithya Kiran said, this is still dependent on your key being copied or your password being typed correctly
@curtlee2002, thanks a lot.
Found on stackoverflow:
ssh -q user@downhost exit
Would that be the shortest way?
If you need ssh on a regular basis keys and an entry in ~/.ssh/config make your live way easier
Found on StackOverflow suggesting to use:
ssh -q user@downhost exit
which would return 0 when successful. Would that be the shortest version?
A complete sample script (e.g. rsync via ssh if the host is there) would be nice.
I use SSH a lot and I'm way to lazy to type passwords. So I generate keys and enter the hosts in my ~/.ssh/config file. This way I save a lot of headache.
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