为什么在 UNIX 中 kill -9命令为9?

我知道这离题了,我在网上找不到任何地方,我想也许社区里的编程大师知道这一点。
我经常用

kill -9 pid

我一直想知道9的起源。我在网上查了一下,上面说

“9表示不可捕捉或忽略的杀戮信号。换句话说,它将向进程(一些正在运行的应用程序)发出信号,要求它立即退出”(来源: http://wiki.answers.com/Q/What_does_kill_-9_do_in_unix_in_its_entirety)

但是,为什么是9?其他号码呢?是否有任何历史意义或因为 Unix 的架构?

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There were 8 other signals they came up with first.

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

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.

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.

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.

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.

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

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: enter image description here

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.

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.

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.

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>.

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>

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".