ssh-add keys in your bashrc

agentrc

:<<'DOCS'
* This script ensures that your shell always has an agent with your preferred keys added.
  On macos you do not need to start the agent explicitly. Just add the keys. If your key
  requires a password, and it should, you will not be prompted for it again as long as
  agent maintains your key. Simply putting ssh-add ~/.ssh/id_rsa in your .bash_profile
  causes you to get prompted for your password for every shell and encourages people to
  remove the password, which they shouldn't. This was created to solve that properly.

* Place this file at ~/.ssh/agentrc use it in ~/.bash_profile with:
[[ -f ~/.ssh/agentrc ]] && source ~/.ssh/agentrc

* You can copy-paste the next 5 lines to get and use this script:
curl -sLo ~/.ssh/agentrc https://gist.githubusercontent.com/RichardBronosky/cfdc69ec0f3edd941dccdacbab45c4e2/raw/agentrc
cat >> ~/.bash_profile <<'EOF'

[[ -f ~/.ssh/agentrc ]] && source ~/.ssh/agentrc
EOF

DOCS

for key in ~/.ssh/id_rsa # Add additional keys here
do
  ssh-add -l | \
    grep -q "$key" || \
      ssh-add $key
done

# vim: ft=sh et sw=2 ts=2 sts=2

Yes, I know I specify .bash_profile and not .bashrc as stated in the description. Modern macOS doesn't have a .bashrc by default, though I still google search for it.