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  • Why are we not using 1 or 2 or 3 etc.. we just using 9 always. Is there a stages in between? Commented Jul 8, 2017 at 16:06
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    @Athiri Those aren't "levels" or "stages" of kill power, they are signal numbers Commented Jul 8, 2017 at 16:14
  • I've added an expanded explanation of signal numbers. Commented Jul 8, 2017 at 16:19
  • Of course the process is notified: if it wants to, it can set up a signal handler and catch the signal. If it wants not to die, it can do so. It's only by default that most signals kill the process. But SIGKILL (9) cannot be caught or ignored, which is why people think that kill -9 is necessary: it is not - it depends on what you want to do (and BTW, if the process is sleeping uninterruptibly in the kernel, it cannot be killed: it will only disappear after a reboot). Also all of the "hardware" signals can be generated on modern hardware: which ones do you think are obsolete? Commented Jul 8, 2017 at 17:10
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    When I said the process was never notified, I said 'originally'. I was referring to very early UNIX, Third Edition or earlier. Commented Jul 8, 2017 at 17:22