Kernel interface to shell ring
In the previous section, we explained that the shell ring in a Unix system exposes the interfaces defined in the SUS or POSIX standard. There are mainly two ways to invoke a certain logic in the shell ring, either through the libc or using shell utility programs. A user application should either get linked with libc libraries to execute shell routines, or it should execute an existing utility program that's available in the system.
Note that the existing utility programs are themselves using the libc libraries. Therefore, we can generalize and state that all shell routines can be found in libc libraries. This gives even more importance to standard C libraries. If you want to create a new Unix system from scratch, you must write your own libc after having the kernel up and ready.
If you have followed the flow of this book and have read the previous chapters, you'll see that pieces of the puzzle are coming together. We needed to have a compilation...