install .p12 or .cer in console macos

This will import the bundle to the default keychain:

security import ./bundle.p12 -P secretPassword

secretPassword is the p12 file encryption password.

While the answer by Stuart should work, it is not required to create another keychain first.

It looks like you can do this using the import command. I've managed to do the following:

security create-keychain -p password bobbins.keychain
security add-certificates ./MyCertificate.cer

security unlock-keychain -p password bobbins.keychain
security import ./MyPrivateKey.p12 -k bobbins.keychain -P privateKeyPassword

I found I had to unlock the keychain, otherwise it prompted for the keychain password.

Hope this helps.