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

Answer 1

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.

Answer 2

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 3

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.

Answer 4

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.

Answer 5

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.

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.