Say you have a private key in PEM format, and you want to use that key for SSH into another server, by adding an entry to your
~/.ssh/authorized_keys file with the public key of such PEM file. The following command will parse your PEM file and output the required RSA format used in
ssh-keygen -y -f path/to/file.pem
This will output a
ssh-rsa AAAA… string that is safe to append to your
ssh-keygen uses the
-f flag to specify the input file name, and the
-y flag to read a private file and output the OpenSSH public key to standard output.