Recently on my Mac I wanted to add a second SSH key to my authentication agent. Using the ssh-add command I was able to do just that until I restarted my computer. To add the key each time I start up a new terminal session I just added
ssh-add ~/.ssh/ec2.pem > /dev/null 2>&1
to my .profile file located at ~/.profile
The last part of the command is to route the output of the command to dev null so I don’t see it when I open up terminal.
> /dev/null 2>&1