I couldn't find a way to get my or other people's public key from Github, but the Ubuntu Server installer can.
How can it and how could I get an ssh public key via GitHub username in a script myself?
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In this particular instance ssh-import-id retrieves an SSH key from GitHub for you. The tool can import from both GitHub and Launchpad. Normally the tool takes the retrieved key and adds it to your authorized keys file which is what Ubuntu Server would have been doing. The manual page outlines the mechanics of the specific API calls it makes to retrieve the key.
The exact usage is
See the manpages