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

Question

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?

Internal

I think, that this question better belongs to superuser.com – Chandelle

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

Plentiful

Back in the day (by which I mean 4.xBSD or so) you couldn't use the mnemonics with the shell command, which is why the number 9 is written into an awful lot of old dusty-deck shell scripts. – Jennings

Answer 2

There were 8 other signals they came up with first.

Steno

Answer 3

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.

Uric

Answer 4

First you need to know what are Signals in Unix-like systems (It'll take just few minutes).

Signals, are software interrupts sent to a (running) program to indicate that an important event has occurred.

The events can vary from user requests to illegal memory access errors. Some signals, such as the interrupt signal, indicate that a user has asked the program to do something that is not in the usual flow of control.

There are several types of Signals we can use - to get a full list of all the available/possible Signals use "$ kill -l" command:

In the above output it's clearly visible, that each Signal has a 'signal number' (e.g. 1, 2, 3) and a 'signal name' (e.g. SIGUP, SIGINT, SIGQUIT) associated with it. For a detailed look up what each and every Signal does, visit this link.

Finally, coming to the question "Why number 9 in kill -9 command":

There are several methods of delivering signals to a program or script. One of commonly used method for sending signal is to use the kill command - the basic syntax is:

$ kill -signal pid

Where signal is either the number or name of the signal, followed by the process Id (pid) to which the signal will be sent.

For example - -SIGKILL (or -9), signal kills the process immediately.

$ kill -SIGKILL 1001

and

$ kill -9 1001

both command are one the same thing i.e. above we have used the 'signal name', and later we have used 'signal number'.

Verdict: One has an open choice to whether use the 'signal name' or 'signal number' with the kill command.

Pycnidium

Answer 5

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.

Dugout

Really can't tell if you're pulling this out of your ass, or if there's a chance that it's true. – Steno

The end of that topic truly is epic, particularly the two comments before mine. I also stumbled upon that article about kill and which sequence to use: pthree.org/2012/08/14/appropriate-use-of-kill-9-pid – Mantling

I so much want this to be true... :) – Orgeat

The person saying 'number 9' is not Lennon. I suspect there's just as much truth in the rest of the story (i.e. none) but who knows :) – Olein

Answer 6

There’s a very long list of Unix signals, which you can view on Wikipedia. Somewhat confusingly, you can actually use kill to send any signal to a process. For instance, kill -SIGSTOP 12345 forces process 12345 to pause its execution, while kill -SIGCONT 12345 tells it to resume. A slightly less cryptic version of kill -9 is kill -SIGKILL.

Emylee

The underlying system call that sends signals is also called kill. Probably because the default behavior for most of the original set of signals (numbers 1 through 15) was to terminate the process. – Jennings

Answer 7

I don't think there is any significance to number 9. In addition, despite common believe, kill is used not only to kill processes but also send a signal to a process. If you are really curious you can read here and here.

Rawden

Answer 8

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".

Eugine

Answer 9

SIGKILL use to kill the process. SIGKILL can not be ignored or handled. In Linux, Ways to give SIGKILL.

kill -9 <process_pid> 
kill -SIGKILL <process_pid> 
killall -SIGKILL <process_name>
killall -9 <process_name>

Geanticlinal

Answer 10

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.

Dezhnev

Answer 11

The -9 is the signal_number, and specifies that the kill message sent should be of the KILL (non-catchable, non-ignorable) type.

kill -9 pid

Which is same as below.

kill -SIGKILL pid

Without specifying a signal_number the default is -15, which is TERM (software termination signal). Typing kill <pid> is the same as kill -15 <pid>.

Ferule

Answer 12

Both are same as kill -sigkill processID, kill -9 processID. Its basically for forced termination of the process.

Smog

Answer 13

there are some process which cannot be kill like this "kill %1" . if we have to terminate that process so special command is used to kill that process which is kill -9. eg open vim and stop if by using ctrl+z then see jobs and after apply kill process than this process will not terminated so here we use kill -9 command for terminating.

Duple