How to permanently add a private key with ssh-add on Ubuntu?

A solution would be to force the key files to be kept permanently, by adding them in your ~/.ssh/config file:

IdentityFile ~/.ssh/gitHubKey
IdentityFile ~/.ssh/id_rsa_buhlServer

If you do not have a 'config' file in the ~/.ssh directory, then you should create one. It does not need root rights, so simply:

nano ~/.ssh/config

...and enter the lines above as per your requirements.

For this to work the file needs to have chmod 600. You can use the command chmod 600 ~/.ssh/config.

If you want all users on the computer to use the key put these lines into /etc/ssh/ssh_config and the key in a folder accessible to all.

Additionally if you want to set the key specific to one host, you can do the following in your ~/.ssh/config :

Host github.com
    User git
    IdentityFile ~/.ssh/githubKey

This has the advantage when you have many identities that a server doesn't reject you because you tried the wrong identities first. Only the specific identity will be tried.

I solved that problem on Mac OSX (10.10) by using -K option for ssh-add:

ssh-add -K ~/.ssh/your_private_key

For macOS 10.12 and later you need to additionally edit your ssh config as described here: https://github.com/jirsbek/SSH-keys-in-macOS-Sierra-keychain