Why number 9 in kill -9 command in unix?

unix command history kill
Alby picture Alby · Mar 31, 2012 · Viewed 141.7k times · Source

I understand it's off topic, I couldn't find anywhere online and I was thinking maybe programming gurus in the community might know this.
I usually use 

kill -9 pid

to kill the job. I always wondered the origin of 9. I looked it up online, and it says

"9 Means KILL signal that is not catchable or ignorable. In other words it would signal process (some running application) to quit immediately" (source: http://wiki.answers.com/Q/What_does_kill_-9_do_in_unix_in_its_entirety)

But, why 9? and what about the other numbers? is there any historical significance or because of the architecture of Unix?

Answer

Rafał Rawicki picture Rafał Rawicki · Mar 31, 2012

See the wikipedia article on Unix signals for the list of other signals. SIGKILL just happened to get the number 9.

You can as well use the mnemonics, as the numbers:

kill -SIGKILL pid

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