How to setup SSH keys for multiple Github accounts

Problem

A local Git repository can push and pull from a remote Github account using SSH authentication. This is typically configured by adding your SSH key to your Github account. If you have two or more Github accounts and try to use the same SSH public key for them, Github rejects the key with the error Key is already in use.

Solution

This problem can be solved by following these steps:

• Generate a second SSH key as described here. But instead of overwriting the default ~/.ssh/id_rsa file, write this key to a separate file. For example: ~/.ssh/second_id_rsa.

• Login to the Github website with the second Github account credentials. Add the second SSH public key ~/.ssh/second_id_rsa.pub generated in the above step to this account.

• Add host entries in ~/.ssh/config for the main Github host and a second Github host name of your own creation. For example, let us name the second host: github.com-second. We point the main Github host to the default SSH key file and the second host to the second SSH key file. For example, you might need to add these lines:

# Main Github host
Host github.com
    HostName github.com
    User git
    AddKeysToAgent yes
    IdentityFile ~/.ssh/id_rsa

# Second Github host
Host github.com-second
    HostName github.com
    User git
    AddKeysToAgent yes
    IdentityFile ~/.ssh/second_id_rsa

• Git clone, push or pull in repositories from the main Github host should operate normally as before now.

• To clone the second repo, use github.com-second instead of github.com in the SSH remote URL:

$ git clone git@github.com-second:joe/some_repo.git

This stores the git@github.com-second:joe/some_repo.git URL as the origin in .git/config in the newly created local clone. So all subsequent fetch and push commands will work seamlessly.

For an existing local clone, you can also modify the SSH URL in its .git/config in the same way to obtain the same result.

Tried with: Git 2.17.1 and Ubuntu 18.04