OpenSSL Parameterized Configuration

This is a common question that comes up in ##openssl with regards to handling openssl req, and here is a strategy to make values in the configuration parameters:

openssl genrsa -out ca.key 2048
config_file=some_file
cn="example.com"
echo "
[ req ]
prompt = no
distinguished_name = req_distinguished_name
x509_extensions = v3_ca

[ req_distinguished_name ]
C                = AB
ST               = CD
L                = EF
O                = G
OU               = HI
CN               = $cn

[ v3_ca ]
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer:always
basicConstraints = CA:true
" > $config_file
openssl req -new -x509 -days 3650 -key ca.key -out ca.crt -config $config_file