In Linux I know "echo -n passphrase | sha256sum" will give me a string which then you convert to base58 to get a private key, and then you need to do something to that to get the corresponding public key.

What are the Linux commands to do this beyond the first one "echo -n passphrase | sha256sum" to get the private/public key pairs? Or how would I do that? I am trying to verify pairs given to me by bitaddress.org or brainwallet.org...