Generate a private key
openssl genrsa -out <private-dkim-key-file>
Extract the public key
openssl rsa -pubout -outform PEM -in <private-dkim-key-file> -out <public-dkim-key-file>
Get the public contents
cat <public-dkim-key-file> | grep -v -- --- | while read L ; do echo -n $L ; done ; echo
or if this is going into BIND9
cat <public-dkim-key-file> | grep -v -- ---