Why number 9 in kill -9 command in unix? [closed]

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Closed 8 months ago.

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 1

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

Answer 2

It's a reference to "Revoulution 9" by the Beatles. A collection of strung together sound clips and found noises, this recording features John Lennon repeating over and over "Number 9, Number 9..." Further, this song drew further attention in 1969 when it was discovered that when played backwards, John seemed to be saying "Turn me on, dead man..."

Therefore the ninth signal was destined to be the deadliest of the kill signals.

Answer 3

why kill -9 : the number 9 in the list of signals has been chosen to be SIGKILL in reference to "kill the 9 lives of a cat".

Answer 4

Type the kill -l command on your shell

you will found that at 9th number [ 9) SIGKILL ], so one can use either kill -9 or kill -SIGKILL

SIGKILL is sure kill signal, It can not be dis-positioned, ignore or handle. It always work with its default behaviour, which is to kill the process.

Answer 5

There were 8 other signals they came up with first.

Answer 6

I think a better answer here is simply this:

mike@sleepycat:~☺  kill -l
 1) SIGHUP   2) SIGINT   3) SIGQUIT  4) SIGILL   5) SIGTRAP  
 6) SIGABRT  7) SIGBUS   8) SIGFPE   9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG  24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF 28) SIGWINCH    29) SIGIO   30) SIGPWR
31) SIGSYS  34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX    

As for the "significance" of 9... I would say there is probably none. According to The Linux Programming Interface(p 388):

Each signal is defined as a unique (small) integer, starting sequentially from 1. These integers are defined in with symbolic names of the form SIGxxxx . Since the actual numbers used for each signal vary across implementations, it is these symbolic names that are always used in programs.