Signal handling
Most shells let you stop or pause processes with special keystrokes. These special keystrokes, such as Ctrl-C or Ctrl-Z, work by sending signals to the shell’s subprocesses. For example, pressing CTRL-C sends the SIGINT signal, which usually stops the current program; pressing CTRL-Z sends the SIGTSTP signal, which usually sends the current program to the background.
Recall that your terminal window is running a shell program itself. The shell must make sure that these keystrokes do not stop the shell program itself. If you try these keystrokes in your homemade shell right now, the signals are sent directly to the shell process itself. This means that attempting to CTRL-Z a subprocess of your shell, for example, will also stop the shell itself. We want to have the signals affect only the subprocesses that our shell creates, even when your shell is idle.