I wanted to offer that (if you are "C handy") writing your own shell is a super informative exercise. We had to write our own shell in my operating system class at GT and I actually got it working well enough that I could "exec ./myshell" and use it for some day to day stuff. I felt empowered

I tried to dig up the course but naturally things are wwaaaaaaay different now than back in my day. But OCW has something similar https://ocw.mit.edu/courses/6-828-operating-system-engineeri... and does ship the source files https://ocw.mit.edu/courses/6-828-operating-system-engineeri... although I have no idea why that's only present in a graduate level class