Note that you can limit the output of -text to just the extensions by adding the following option:
-certopt no_subject,no_header,no_version,no_serial,no_signame,no_validity,no_issuer,no_pubkey,no_sigdump,no_aux
i.e.:
openssl x509 -text -noout -in cert.pem \
-certopt no_subject,no_header,no_version,no_serial,no_signame,no_validity,no_issuer,no_pubkey,no_sigdump,no_aux
However, you’ll still need to apply some text parsing logic to get just the Subject Alternative Name.
If that isn’t sufficient, I think you’ll need to write a small program that uses the openssl library to extract the specific field you are looking for. Here are some example programs that show how to parse a cert, including extracting extension fields such as Subject Alternative Name:
https://zakird.com/2013/10/13/certificate-parsing-with-openssl
Note that you don’t have to use openssl and C if you go the programming route… you can pick your favorite language and ASN.1 parser library, and use that. For example, in Java, you could use http://jac-asn1.sourceforge.net/, and many others.