how to check if a host is in your known_host ssh

Question

I have the following command works in my script that adds the host to the known hosts in ssh.

VAR2=$(expect -c '
 spawn ssh -o StrictHostKeyChecking=no '"$REMOTE_HOST_USER@$REMOTE_HOST_IP"'
 expect "*?assword:*"
 send "'"$REMOTE_HOST_PASSWD"'\r"
 expect { 
 "Permission denied, please try again." {
 exit '"$WRONG_PASSWORD"' 
 }
 }
 ')

Works fine, but I need to control before the command if the host is already in known_hosts and not execute command if it is already in known_hosts. How can i check if an host is in known_hosts?

Solution 1

Try: ssh-keygen -F <hostname>

Will show the known_hosts line(s) if the hostname fingerprint is found and the command returns 0, otherwise nothing is shown and the command returns 1.

Solution 2

According to ssh-keygen(1) man page

-F hostname Search for the specified hostname in a known_hosts file, listing any occurrences found. This option is useful to find hashed host names or addresses and may also be used in conjunction with the -H option to print found keys in a hashed format.

Comments

[secondman]

This doesn't seem to do anything, or even print anything to the screen. Strange.

[complex857]

@VinceKronlein, That should happen when no match found for your hostname. Only full matches returned.

[secondman]

Yeah that was my bad. I wasn't using just the hostname ie: google.com, I was trying to look for a key for a git repo using the git host ie: [email protected] ... once I used just the hostname alone it worked great.

[maoizm]

if you like I trying to find out if there exists the key of SSH server listening to non-standard port (other than 22), use the following command: ssh-keygen -F "[gerrit.example.com]:29418"