this command:

$ cat my-app.yaml

should be like this to actually create the file on the local host:

cat > my-app.yaml<<EOF

apiVersion: wildfly.org/v1alpha1

kind: WildFlyServer

metadata: name: my-app

spec:

applicationImage: 'my-app:latest'

replicas: 1

EOF

Here: https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.3/html-single/getting_started_with_jboss_eap_for_openshift_online/index#building-an-application-image-using-eap-s2i-build-template_default