$ ssh-keygen --help ssh-keygen: illegal option -- - usage: ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa | rsa1] [-N new_passphrase] [-C comment] [-f output_keyfile] ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile] ssh-keygen -i [-m key_format] [-f input_keyfile] ssh-keygen -e [-m key_format] [-f input_keyfile] ssh-keygen -y [-f input_keyfile] ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile] ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile] ssh-keygen -B [-f input_keyfile] ssh-keygen -D pkcs11 ssh-keygen -F hostname [-f known_hosts_file] [-l] ssh-keygen -H [-f known_hosts_file] ssh-keygen -R hostname [-f known_hosts_file] ssh-keygen -r hostname [-f input_keyfile] [-g] ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point] ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines] [-j start_line] [-K checkpt] [-W generator] ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals] [-O option] [-V validity_interval] [-z serial_number] file ... ssh-keygen -L [-f input_keyfile] ssh-keygen -A ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file ... ssh-keygen -Q -f krl_file file ...
$ ssh-keygen -f "~/.ssh/known_hosts" -R <IPアドレス>