Managing Multiple SSH Identities

This is definitely a good place to start:

• https://docs.github.com/en/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent

As for setting up multiple SSH identities, assume you have accounts with the following emails:

• GitLab -> foxmascot@gmail.com
• GitHub -> octocat@gmail.com

First, you'd generate one key for your GitLab email:

$ ssh-keygen -t ed25519 -C "foxmascot@gmail.com"

And then will be prompted to specify a file path

Generating public/private ed25519 key pair.
Enter file in which to save the key (/Users/your-name/.ssh/id_ed25519):

You can either simply hit enter or if you’d like, specify it’s for GitLab: /Users/your-name/.ssh/id_ed25519_gitlab. For the sake of the rest of the example, we’re going to assume you opted for the default path and filename of /Users/your-name/.ssh/id_ed25519.

Then, generate another one for your GitHub email:

$ ssh-keygen -t ed25519 -C "octocat@gmail.com"

But when prompted for a path this time, specify the path and append _github to the file:

Generating public/private ed25519 key pair.
Enter file in which to save the key (/Users/your-name/.ssh/id_ed25519): /Users/your-name/.ssh/id_ed25519_github

This should create a bunch of files in the ~/.ssh/ folder:

• id_ed25519
• id_ed25519_github
• id_ed25519.pub
• id_ed25519_github.pub

To make sure that the correct ones are used for the corresponding domains, create a new (or edit the existing) file at ~/.ssh/config with at least the following:

Host gitlab
  HostName gitlab.com
  IdentityFile ~/.ssh/id_ed25519


Host github
  HostName github.com
  IdentityFile ~/.ssh/id_ed25519_github

And if you added them to your machine's keychain, you likely want to include these other options as well:

Host gitlab
  UseKeychain yes
  AddKeysToAgent yes
  HostName gitlab.com
  IdentityFile ~/.ssh/id_ed25519


Host github
  UseKeychain yes
  AddKeysToAgent yes
  HostName github.com
  IdentityFile ~/.ssh/id_ed25519_github

Finally, create new SSH key entries in your accounts (via GitLab, GitHub, etc.) using the content from each .pub file for its corresponding domain (for example, id_ed25519_github.pub for GitHub).

Bonus

Adding this to your ~/.bash_profile (or ~/.zshrc) will automatically add the ssh keys to the keychain on your machine when you open a new terminal window or reload your profile

function addSshKeysToAgent {
    # -K === --apple-use-keychain
    # -A === --apple-load-keychain
    /usr/bin/ssh-add --apple-use-keychain ~/.ssh/id_ed25519
    /usr/bin/ssh-add --apple-use-keychain ~/.ssh/id_ed25519_github
}

function maybeStartSshAgentAndAddKeys {
    # from https://stackoverflow.com/a/45841963
    # if ps -p $SSH_AGENT_PID > /dev/null
    if [ $(ps ax | grep [s]sh-agent | wc -l) -gt 0 ]
    then
        echo "ssh-agent is already running"
        # Do something knowing the pid exists, i.e. the process with $PID is running
    else
        eval "$(ssh-agent -s)"
    fi

    addSshKeysToAgent
}

maybeStartSshAgentAndAddKeys