Author of Vorpal here. You can ask me Qs if you'd like.
Also, a question about piping: in your example the "say" command is hard-coded to use the argument called "words", and the "reverse" command is hard-coded to read from standard input. This seems very limited compared to piping in a real shell, shouldn't each command receive a string/array of input and return output so they can be piped to each other without having to know about it?
On piping, I hard coded those to keep it short. Every single command can receive "args.stdin", which is a stdin stream from previous commands. To pipe out, the "this.log" command does all of the heavy lifting.
So all you would need to do is expect both "args.stdin" and "args.words" - that would cover input both from a directly executed command as well as stdin. This works very well in practice.
How are globals used in the vorpal codebase? Is it going to be easier to write my tests (and actually get code coverage with istanbul working)?
I like the site, and the API overview. The library looks nice!
Yeah, it's got stdout piping methods, etc. which make it easy. It's also got UI manipulation methods so you can simulate user action. I haven't particularly had any trouble writing tests, but really it depends on your use case, which I'm not familiar with.