Why number 9 in kill -9 command in unix

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

User · Answer

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

User · Answer

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

User · Answer

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  lt pid gt  is the same as kill -15  lt pid gt

User · Answer

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

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There were 8 other signals they came up with first

User · Answer

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

User · Answer

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

User · Answer

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

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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

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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

User · Answer

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

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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

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SIGKILL use to kill the process  SIGKILL can not be ignored or handled  In Linux  Ways to give SIGKILL    kill -9  lt process pid gt   kill -SIGKILL  lt process pid gt   killall -SIGKILL  lt process name gt  killall -9  lt process name gt

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

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