One thing I found pretty useful is to port very early Linux command line programs to modern platforms. I just "ported" (nerfed the number of switches) the 1991

One thing I found pretty useful is to port very early Linux command line programs to modern platforms. I just "ported" (nerfed the number of switches) the 1991 version of `ls.c` to modern Ubuntu and learned a lot of about `stat()`, `lstat()`, the `struct state`, file types in Linux and some programming tricks. It only has 1.2K LoC so it's easy to sort out the logic and write your own from scratch if needed.