How can I know if a .pem file contains public or private key?

Question

I have a .pem file (base64-encoded cryptography information). What OpenSSL command-line should I use to detect whether it contains a public key or a private key?

As explained in this StackOverflow question, a .pem can contain both.

Since the contents of the file are juste garbled base64, when sending a .pem certificate to some other machine, I would like to make sure I'm exporting the public key and not giving out my private key.

Answer 1

As a practical approximation that should catch most (all?) human mistakes I would recommend just file and grep:

(file $KEYFILE | grep -i private >/dev/null) && echo Private || echo Public-or-unknown

To screen some key material before it goes out:

 for f in *; do (file $f | grep -i private >/dev/null) && file $f; done

Empty output = hopefully no secrets, can proceed.