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In order to add an SSH key to an existing user, all you need to do is append their public key to the file ~/.ssh/authorized_keys A quick one liner to do that is:
On Ubuntu, you can also use the handy ssh-import-id if the user has a public key registered on GitHub or Launchpad. To automatically import the key, you can run:
ssh-import-id gh:username
For all the info on how to use SSH keys, check out this article:
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