[sdkmvx]

The shell needs to print a prompt and read input. This is I/O, which involves the kernel. The shell is (when running interactively) attached to some terminal. These days it's usually a pseudo-terminal with a terminal emulator like xterm on the other end. (With some exceptions) the read syscall on a terminal does not return until there's a newline. The kernel only implements simple line editing. You can kill the whole like (^U) or erase the last character (^H i.e. backspace). Traditionally kill and erase were @ and # which you may see in some old documentation. On a hardcopy terminal you could type daat##te and it would run date. If the right flags are set, instead of outputting the erase character, Unix will back up, print a space, and back up again as described.

However the shell supports more line editing. I bet you can press the left arrow and edit the middle of the line. I bet you can press the up arrow and get the last line typed. I bet you can press ^A and get to the beginning of the line. The kernel doesn't do any of this, and you can't do it in cat. What happens is the shell turns off line editing. Then it gets all input unprocessed, and can decide what to do on it's own. This is why Chris Siebenmann put the PS in there.