$ jmod hash --hash-module B --module-path target/jmods target/jmods/A.jmod

The current implementation of jmod hash ignores the target jmod file and therefore the above command does not work because jmod hash does not find the module with no outgoing edge that matches the given pattern.

See http://mail.openjdk.java.net/pipermail/jigsaw-dev/2019-January/014094.html

and
https://stackoverflow.com/questions/54269994/how-should-i-use-jmod-hash