The interface is not necessarily the content of the message conveyed by the interface. Lines of text std(in|out) is a well-defined interface. Some programs may go further and specify the format of messages passed on that interface. https://github.com/synacor/dtk is one such system that specifies (for most of its commands) tab-delimited records as the message format. It interfaces well with other elements of the UNIX ecosystem, however, because its interface is simply lines of text on std(in|out).