multiple github accounts, custom ssh hosts, and git url aliases

Because services like GitHub and Bitbucket require each user account to have unique SSH keys, if you have more than one account on those services, it’s necessary to generate multiple SSH keys.

First, we’ll generate the keys.

$ ssh-keygen -t rsa -f github-user1 # key for user1
$ ssh-keygen -t rsa -f github-user2 # key for user2

Then we’ll add each public key to each account in the GitHub or Bitbucket web interface per usual. Now, we’ll have two problems to solve: when we try to pull and push commits to GitHub or Bitbucket, SSH doesn’t know how to select the correct key file. That’s easy: we’ll create an SSH user configuration file (~/.ssh/config) and create an entry like the one below:

Host github.com
IdentityFile ~/.ssh/github-user1

But how do we specify the user2 account? SSH allows us to create “fake” hostnames in its user configuration file. Think of them as domain “aliases.”

Host user1.github.com
HostName github.com
IdentityFile ~/.ssh/github-user1

Host user2.github.com
HostName github.com
IdentityFile ~/.ssh/github-user2

We’ve now mapped each domain alias to each key. This comes with a trade-off: our git clone commands and all other references to our GitHub and Bitbucket repository URLs need a slight modification. Here’s the typical syntax:

$ git clone git@github.com:user1/repo.git

Below is the new syntax.

$ git clone git@user1.github.com:user1/repo.git

Note the “user1.github.com” as the domain.

But this is cumbersome and inconvenient—and unacceptable. Luckily, git’s url aliases come to the rescue. They allow us to substitute some portion of the URL to be cloned for something else. Here’s an example:

[url "user1.github.com:user1"]
    insteadOf = github.com:user1

What this does is substitute “github.com:user1” with “user1.github.com:user1” in the URL parameter of our git commands. A command like this:

$ git clone git@github.com:user1/repo.git

…is automagically translated into this:

$ git clone git@user1.github.com:user1/repo.git

Yup, that’s the same command we were trying to avoid running above—which we’ve successfully done. Git URL aliases live in ~/.gitconfig, and you may add as many as you like.

(Power user tip: if you want to save keystrokes, GitHub and Bitbucket allow you to omit the “.git” extension from the repository’s URL when cloning it.)

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