The classic one I always heard of was a shell. You learn about input parsing, fork/exec, and usually some syscalls. Lots of features can be added on, like run

The classic one I always heard of was a shell. You learn about input parsing, fork/exec, and usually some syscalls. Lots of features can be added on, like running processes in the background, autocompletions etc. Writing a shell in C (you can use Readline if you don't care to learn much about parsing) teaches you lots of stuff about the operating system you're working on.