Game developer and digital artist Krzysztof Jankowski has built a compact operating system, designed to run atop MicroPython and provide an easy-to-use POSIX-style command line interface — and it's called, appropriately, smolOS.
As the name implies, smolOS is a small operating system — small enough to run as a MicroPython program on the low-cost Espressif ESP8266 microcontroller family, in fact. Designed to be easy to use, the operating system will be immediately familiar to anyone who's used POSIX-style platforms like Linux, UNIX, or BSD in the past — though, as it's still in its very early stages of development, the feature set is admittedly limited.
Flashing the OS to an Espressif ESP8266 device and connecting over UART provides a command-line interface for the smolOS platform — while a built-in help function reveals the currently-available commands:
ls, for listing files;
rm, for deleting them;
cat, for printing them to the console;
cls, for clearing the screen; and
ed, a simple text editor. A bonus program,
welcome, prints a welcome screen which includes an ASCII art logo, the smolOS and MicroPython version numbers, the firmware version, and the microcontroller's CPU speed.
Jankowski's OS is, admittedly, bare-bones — but, then, it is only in version 0.2 at the time of writing. It also provides a surprisingly polished platform for future expansion — and there's enough memory left on the device to offer additional commands, while other functionality can be integrated from existing MicroPython projects alongside smolOS itself.
The smolOS source code and installation instructions are available on GitHub under the permissive MIT license.