Writing a C compiler in pure shell is one of those projects that sounds absurd until you think about bootstrapping. If you want to compile C on a system where you literally have nothing but a POSIX shell, this is exactly what you need. The fact that the parser itself is BNF-generated from shell modules makes it even more interesting as a study in how far you can push shell scripting before it breaks. Would love to see this evolve into a proper repo with tests so it can actually serve as a minimal bootstrapping tool.
It's not just a toy or a fun hobby project, there's potential for practical use as a step in bootstrapping an entire software stack from human-verifiable artifacts.
A shell is almost always used to setup the bootstrap environment, so the dependency on a shell is more or less always there.
Otherwise, something special with POSIX shell is its large number of independent implementations, making it the ideal starting point for diverse double-compilation (https://arxiv.org/abs/1004.5534). The idea is to bootstrap a toolchain from multiple compilers (shells in this case), and the result compared to verify that no shell introduced a trusting trust attack.
The shell.c ouroboros is really cool. Being able to bootstrap trust through an entirely different language family (shell → C → shell) adds genuine value to the trusting-trust problem beyond just technical novelty.
