Use Multiple GitHub Accounts on One Machine

Two separate concerns: SSH decides which credential authenticates to GitHub; Git configuration decides which name/email is written into your commits.

1 — Create a separate SSH key

ssh-keygen -t ed25519 -C "personal@example.com" -f ~/.ssh/id_ed25519_personal

Add the generated .pub key to the corresponding GitHub account. Keep the private key private.

2 — Configure SSH host aliases

Host github-work
  HostName github.com
  User git
  IdentityFile ~/.ssh/id_ed25519_work
  IdentitiesOnly yes

Host github-personal
  HostName github.com
  User git
  IdentityFile ~/.ssh/id_ed25519_personal
  IdentitiesOnly yes

The aliases github-work and github-personal both connect to GitHub, but each selects a different identity. IdentitiesOnly yes prevents SSH from offering unrelated agent identities before the configured key.

3 — Test authentication before cloning

ssh -T git@github-work
ssh -T git@github-personal

Confirm each alias authenticates as the account you expect before changing repository remotes.

4 — Clone with the correct alias

git clone git@github-personal:YOUR-USER/YOUR-REPO.git

For an existing repository, inspect and update the remote if necessary:

git remote -v
git remote set-url origin git@github-personal:YOUR-USER/YOUR-REPO.git

5 — Set commit identity by directory

Git supports conditional includes based on the repository directory. For example, in ~/.gitconfig:

[user]
    name = Work Name
    email = work@example.com

[includeIf "gitdir:~/Projects/personal/"]
    path = ~/.gitconfig-personal

Then create ~/.gitconfig-personal:

[user]
    name = Personal Name
    email = personal@example.com

A gitdir: pattern ending in / applies recursively beneath that directory.

6 — Verify inside a repository

git config user.name
git config user.email
git remote -v
ssh -G github-personal | grep -E "hostname|identityfile|identitiesonly"

Verify both pieces: the commit identity and the SSH remote/identity selection.

Common mistakes

Changing user.email and assuming that changes SSH authentication.
Creating two keys but cloning every repository through plain github.com.
Forgetting to update an existing repository remote.
Letting ssh-agent offer many keys without controlling identity selection.