bash/ssh test for public key authentication

Question

Is there a way to programmatically test whether ssh can authenticate using a public key? I would like to do something like this (preferably in bash, but am open to a python solution):

ssh-test-thingy user@host || echo "could not authenticate using publickey"

where ssh-test-thingyreturns a non-zero exit status if no public key matches on the remote host.

Answer 1

I'd pass the option -o BatchMode=yes to ssh and see if that works. It will disable prompting for a password, which I think is equivalent in practice to your desire to find out if authentication via keys is possible. ssh-test-thingy could be written as a bash script like so:

exec ssh -o BatchMode=yes "$@" true

This will simply pass the user@host (and any other arguments) along, and try to run true on the remote host, which if it works will immediately return a status code of success (0).