How to determine if user has ssh keys in a perl script

Question

I have the following script in tcsh that looks like this:

df=`ssh $some_server 2>/dev/null df $2 -k` 
echo $df

Perl script runs this tcsh script and I have the two following questions:

What does the first command do?
How can I determine if I have the ssh keys, before running this command (from the perl script for example, or the tcsh script?

As I understand, If I don't have the ssh keys, it will ask for the password. I would like to print a basic warning to the user if he doesn't have the ssh keys.

Running a perl program that shells out to run ssh manually and try to understand the error conditions will give you a lot of headaches. At least have a look at the system() call where you can pass a list of parameters, so that you do not use the shell. Modules like IPC::Run may also help you write this more cleanly, specially when needing to deal with STDOUT/STDERR. Also, you have various Perl libraries that gives you SSH protocol features, either as rewritten or as a layer on top of it.Have a look at them, even if the question "do we have the proper keys" is complicated(depends on config) — Patrick Mevzek
– Patrick Mevzek, Commented Aug 29, 2018 at 20:15
You can use -o BatchMode=yes to check if passwordless login is possible. For example: ssh -o BatchMode=yes userName@server true >/dev/null 2>&1 || echo "Passwordless login is not possible". See this answer for more information — Håkon Hægland
– Håkon Hægland, Commented Aug 29, 2018 at 20:40

salva · Accepted Answer · 2018-09-04 10:46:48Z

1

When ssh fails to establish a connection to the server it doesn't tell you the reason, but you can run it in verbose mode and analyze the output afterwards.

For instance, use the following command:

ssh -vvv -i $path_to_key -o BatchMode=yes \
    -l $remote_user $remote_host echo ok 2>error.log

And afterwards, if it fails, check error.log for the cause of the problem.

Note that the debugging output may change between different ssh versions, so any solution that depends on it is never going to be very robust. You should consider it just a hint.

answered Sep 4, 2018 at 10:46

salva

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