This blog post on LispCast is a pretty good start to get thinking about the intricacies of the interaction between Lisp (Machine) ideas and the current Unix environment. Of course that includes plan9 on the one side and Emacs on the other.

Lisp shell

There is scsh, but it's not really what I'm looking for. Using emacs as login shell (with the eshell package) comes closest to it regarding both with existing commands and integration of Lisp-based ones. However, while pipes work as expected with eshell, data is still passed around as (formatted) text. There doesn't seem to be an easy way to pass around in-memory objects, at least while staying in Emacs itself. That would of course mean to reimplement some (larger?) parts of that system.

This all ties in to the idea that unstructured text isn't the best idea to represent data between processes. Even though Unix pipes are extremely useful, the ecosystem of shell and C conventions means that the obvious way isn't completely correct, meaning that there are edge cases to consider. The best is something as innocent as ls | wc -l, which will break, depending on the shell settings, with some (unlikely) characters in filenames, i.e. newlines.

Common formats

One of the problems is obviously that in order to pass around structured data, i.e. objects, all participants have to understand their format. Passing references won't work without OS support though.

Instead of having unstructured streams, use streams of (data) objects. The distinction here is Plain Old Objects (PODs) instead of objects with an associated behaviour.

Let's take a look at standard Unix command line tools (I'm using GNU Coreutils here) in order to reproduce the behaviour and/or intent behind them:

Output of entire files

The first command here is cat. Although GNU cat includes additional transformations, this command concatenates files. Similar to the description, we can image a CAT to perform a similar operation on streams of objects.

It doesn't make much sense to concatenate a HTML document and an MP3 file (hence you won't do it in most cases anyway). However, since files are unstructured, cat can work on them.

Registering functionality

Although you can call commands individually on files, some of them form an ad-hoc service interface already: The C compiler, along with the toolchain forms one such interface, where you're required to use the same interface if you want to seamlessly replace one part of the toolchain.

Same goes for the Coreutils: As long as you honour the interface, programs can be replaced with different implementations.

Interactive commands

Emacs has a special form interactive to indicate whether a command can be directly called via the command prompt. There is also special handling there to accomodate the use of interactive arguments. This is something that can be generalised to the OS. An example of where this already happens is the .mailcap file and the .desktop files from desktop suites.

Threading and job control

Unfortunately getting proper job control to work will be a bit of a problem in any Lisp implementation, since the best way to implement the concurrent jobs is using threads, which are not particularly suited for handling the multitude of signals and other problems associated with them. Even without job control pipelines implemented in Lisp require shared in-memory communication channels, so something like (object-based) streams, mailboxes, or queues are necessary to move IO between different threads.