Systems
Why building your own shell changes how you see a computer
A terminal looks like a text box. Underneath, it coordinates processes, files, and signals. Building one reveals the operating system that was there all along.
The smallest interface to the operating system
A shell begins with an almost trivial loop: show a prompt, read a line, and run what was entered. That apparent simplicity is exactly what makes it such a powerful project. Every new feature opens a different door into the operating system.
Launching a command means creating a process, replacing its program, and waiting for it to finish. Add pipes, redirections, and signals, and isolated concepts begin to form one coherent system.
Errors become the map
The first time a descriptor stays open or a signal interrupts something unexpectedly, the bug is not a detour from learning. It is the learning. It forces you to build a precise model of resources, ownership, and lifetimes.
That model returns later in servers, containers, and distributed systems. Not because they are all shells, but because they rest on the same foundations.
Build it so you can explain it
Being able to define fork or exec is not the same as knowing how they work together. A project makes you turn vocabulary into decisions. In the end, you can explain not only what a terminal does, but why it works that way.