ssh user@host "cat >> .ssh/authorized_keys2" < .ssh/id_rsa.pub
This will append the ssh public key on the local machine to the authorized_keys file in the remote machine so that the local machine will in the future be able to ssh to the remote without a password.
If you are setting up multiple machines this way, this one liner is faster than having to ssh into each remote to update the authorized_keys file.
You could build on this to setup multiple machines with a single command:
for m in host1 host2 host3 host4; do ssh user@$m "cat >> .ssh/authorized_keys2" < .ssh/id_rsa.pub; done