Create a public SSH key from the private key

The option -y outputs the public key:

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

As a side note, the comment of the public key is lost. I've had a site which required the comment (Launchpad?), so you need to edit ~/.ssh/id_rsa.pub and append a comment to the first line with a space between the comment and key data. An example public key is shown truncated below.

ssh-rsa AAAA..../VqDjtS5 ubuntu@ubuntu