openssl genrsa req -rand new -genkey -out $DOMAIN.key newkey rsa:2048 openssl req -new nodes -key keyout $DOMAIN.key -out $DOMAIN.csr

Your certificate vendor will give you a bunch of files. You need to concatenate them in order to use them in your web server. Order is essential. For example, for ComodoDigiCert:

cat xw_is.crt COMODORSADomainValidationSecureServerCADigiCertCA.crt COMODORSAAddTrustCATrustedRoot.crt >../xw.is.crt