Two GitHub accounts, one computer · step 4 of 7

macOS: SSH keys and Keychain

macOS's ssh-agent remembers the passphrase in Keychain — one flag and reboots stop being a problem.

1. Generate two SSH keys

In Terminal:

ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_user1 -C "user1@example.com"
ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_user2 -C "user2@example.com"

Set a passphrase for each key — on macOS, “don’t ask every time” is built in and more convenient than on Windows (see step 6).

2. Add the public keys to GitHub

cat ~/.ssh/id_ed25519_user1.pub | pbcopy

Paste it under Settings → SSH and GPG keys on the user1 account. Repeat for user2.

The key file name and the “Title” label on GitHub are arbitrary — just descriptions for you. What must match exactly is the public key contents and which account you add it to. More in the “How it works” step.

3. Wire the keys into .gitconfig-userX

This is the file you created in step 1 of the tutorial (in your home directory, ~, e.g. /Users/yourname/.gitconfig-user1). Now you add a [core] section to it pointing at a specific key.

Open the file with the nano terminal editor (simpler than vim for beginners):

nano ~/.gitconfig-user1

If the file already exists from step 1 (with a [user] section), you’ll see its content. Use the arrow keys to move to the end of the file and add the new section. The whole file should look like this:

[user]
    name = User1
    email = user1@example.com
[core]
    sshCommand = "ssh -i ~/.ssh/id_ed25519_user1 -o IdentitiesOnly=yes"

Save and exit nano: Ctrl+O (write out), Enter (confirm the file name), Ctrl+X (exit). Repeat the same for .gitconfig-user2, swapping user1 for user2 in both places (the file name and the key path inside sshCommand).

Alternative without opening an editor — append the section directly from the terminal:

cat >> ~/.gitconfig-user1 << 'EOF'
[core]
    sshCommand = "ssh -i ~/.ssh/id_ed25519_user1 -o IdentitiesOnly=yes"
EOF

This appends the [core] section to the end of the file without touching what’s already there. Verify:

cat ~/.gitconfig-user1

You should see both the [user] and [core] sections, one after the other.

Paths starting with ~ generally work fine in core.sshCommand on macOS/Linux (unlike Windows, where it’s safer to give the full path).

4. Switch the repository remote to SSH

git remote set-url origin git@github.com:user1/repo-name.git
git remote -v

5. Test the connection

ssh -T -i ~/.ssh/id_ed25519_user1 -o IdentitiesOnly=yes git@github.com

You should see Hi user1! You've successfully authenticated....

6. Skip typing the passphrase on every push

macOS has built-in ssh-agent integration with Keychain. Add an entry to ~/.ssh/config:

Host github.com-user1
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_ed25519_user1
    UseKeychain yes
    AddKeysToAgent yes

Host github.com-user2
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_ed25519_user2
    UseKeychain yes
    AddKeysToAgent yes

UseKeychain yes plus AddKeysToAgent yes means that after entering the passphrase once during the first ssh-add, it’s stored in Keychain and survives a reboot — unlike on Windows.

Add the keys once:

ssh-add --apple-use-keychain ~/.ssh/id_ed25519_user1
ssh-add --apple-use-keychain ~/.ssh/id_ed25519_user2

Check:

ssh-add -l

If you use Host github.com-userX aliases from ~/.ssh/config together with core.sshCommand in gitconfig, pick one mechanism so they don’t fight each other. core.sshCommand alone (step 1 of the tutorial), without aliases in ~/.ssh/config, is easier to maintain. If you prefer aliases instead, in .gitconfig-userX skip core.sshCommand and just point the remote at git@github.com-user1:user1/repo.git.

Note: Apple’s Git vs Homebrew Git

macOS ships with Apple’s Git (Xcode Command Line Tools) by default. If you’ve also installed Git via Homebrew, check which one is active:

which git
git --version

includeIf behaves identically in both, but it’s worth knowing which install you’re using when something doesn’t behave as expected.