Examples of Unix/Linux Signals

1. Pressing the Control_C key usually causes a running process to terminate. Here is why. The Control_C key generates a keyboard hardware interrupt. The keyboard interrupt handler converts the Control_C key to a SIGINT(2) signal to all processes on the terminal and wake up such processes if they are waiting for keyboard inputs. While in kernel mode, every process is required to check and handle outstanding signals. For most signals, the default action of a process is to call the kernel’s kexit(exitValue) function to terminate. In Linux, the low byte of exitValue is the signal number that caused the process to terminate.
2.  The user may use the nohup a.out & command to run a process in the background. The process will continue to run even after the user logout. The nohup command causes the sh to fork a child to execute the program as usual, but the child ignores the SIGHUP(1) signal. When the user logout, the sh sends a SIGHUP signal to all processes associated with the terminal. Upon receiving such a signal, the background process simply ignores it and continues to run. To prevent the background process from using the terminal for I/O, the background process usually disconnects itself from the terminal (by redirecting its file descriptors 0,1,2 to /dev/null), making it totally immune to any terminal oriented signals.
3. Perhaps a few days later the user login again and finds (by ps -u UID) that the background process is still running. The user may use the sh command

kill pid (or kill -s 9 pid)

to kill it. Here is how. The process executing kill sends a SIGTERM(15) signal to the target process identified by pid, requesting it to die. The targeted process will comply with the request and terminate. If the process has chosen to ignore the SIGTERM signal, it may refuse to die. In that case, we may use kill -s 9 pid, which will kill it for sure. This is because processes cannot change their actions for the number 9 signal. The reader may wonder, why number 9? In the original Unix, there were only 9 signals. The number 9 signal was reserved as the last resort to kill a process. Although later Unix/ Linux systems expand the signal numbers to 31, the meaning of signal number 9 is still retained.

Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.