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$ ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
$ eval "$(ssh-agent -s)"
Agent pid 59566

If you're using macOS Sierra 10.12.2 or later, you will need to modify your ~/.ssh/config file to automatically 
load keys into the ssh-agent and store passphrases in your keychain.

Host *
 AddKeysToAgent yes
 UseKeychain yes
 IdentityFile ~/.ssh/id_rsa
 
$ ssh-add -K ~/.ssh/id_rsa

$ ssh-copy-id -i <file> -p <port> user@123.45.56.78