Quote:shell's job is mainly to process output from different programs

I disagree with that. The shell's job is to handle IPC, and that's it. Text processing is done by the processes themselves, the shell only connects outputs to the next process input.
Otherwise, you're creating a full blown text processor, that happens to have a way to start processes.

I love the structural expression approach, but I think you should focus on that rather than making a complete shell. That wouldn't make much difference:


would then be (reusing sed's flags):