Given a private SSH key, how do I recover my public key?

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http://unix.stackexchange.com – I've somehow lost my id_rsa.pub file. I still have my id_rsa file. How do I recover the public key from the private key? (This question came up after ssh-copy-id was reporting "ERROR: No identities found" while ssh-keygen reported "/root/.ssh/id_rsa already exists". ls ~/.ssh pointed out the problem was having id_rsa but not id_rsa.pub, and only finding the solution buried at the bottom of a long post on an Ubuntu support forum.) (HowTos)