I needed a "whoami" command to debug Karaf/JAAS integration, so I made it.
The Maven "site" for this project should be visible here: https://dfjdejulio.github.io/karaf-whoami/
Installing this adds the "jaas:whoami" command to Karaf. Internally, it uses standard JAAS methods to retreive the current Subject, and pulls all the Principals out of it. By default, it only pulls out versions of Karaf's UserPrincipal. If you give it the "-g" or "-r" switch, it will pull out Karaf's group or role principals instead, and if you use the "-a" switch, it will pull out all principals -- even the ones Karaf doesn't have a direct understanding of, like Kerberos principals -- and adds a column to the table so you can see the class of each principal.
I've already submitted this feature upstream, so this is only of use with older versions of Karaf from before the submission was accepted.