kill command

[Shivkumar]

Dear Sirs,

While killing a process as a root the command "$kill pid" is preferable or "kill -9 pid" preferable ?

Thanks,
Shiv

[Nguyen Anh Tien]

#kill -9 pid is prefer
tienna

[Senthil Prabu.S_1]

Hi,
kill -9 is preferred always,

[Vibhor Kumar Agarwal]

Kill -9 is used for force killing.

So it depends on your situation.
You might kill an vital process with kill -9.

Otherwise you know hte answer.

[Arunvijai_4]

Hi Shiv,

# kill -9 will immediately kills the process and sometimes it is prefered.

-Arun

[Senthil Prabu.S_1 · Accepted solution]

Shiva,
sometime, -9 does have negative impact. The KILL signal does not allow a process to run any cleanup code, which means blasting away with kill -9 may leave child processes of a parent orphaned, temporary files open, shared memory segments active, and sockets busy. This leaves the system in a messy state, and could lead to unanticipated and hard to debug problems.

[Joseph Loo]

hi,

try "kill pid" first, if do not succeed, use "kill -9". but make sure u get the correct pid.

regards.

[Senthil Prabu.S_1]

Hi,
It depend on your need to use -9 with kill command.

Also, refer this link which talks that -9 option in other way;
http://www.speculation.org/garrick/kill-9.html

[Arunvijai_4]

Hi Shiv, You can refer this link for kill -9

http://sial.org/howto/shell/kill-9/

The KILL signal does not allow a process to run any cleanup code, which means blasting away with kill -9 may leave child processes of a parent orphaned, temporary files open, shared memory segments active, and sockets busy.

-Arun

[Senthil Prabu.S_1]

Hi again,
THe situation when it is preferred to use kill -9 is
"Use of kill -9 by default may seem acceptable where a known problematic application is involved. However, using the KILL signal by default assumes the problematic process will remain so."

[A. Clay Stephenson]

Kill -9 pid should be used as a weapon of last resort. It you do a man signal, you will find that SIGKILL (9) cannot be ignored or caught. This means that it is not possible to do any cleanup (e.g. remove temp files, shared memory, semaphores, etc.)

The best approach is to start to kill in this order: -15, -1, -2, -3, -11, and finally and only if it's really necessary a kill -9. You should test for the existence of a process via kill -0 pid between each of these escalating signal. kill -0 pid sets $? to 0 if the PID is still valid.

kill -9 pid is one of those bad techniques that many admins learn but is almost always a bad idea.

[Arunvijai_4]

Hi Shiv, As from man page of kill, you will get to know what happnes with various signals.

signum signame Name Description
___________________________________________________________________________
0 SIGNULL Null Check access to pid
1 SIGHUP Hangup Terminate; can be trapped
2 SIGINT Interrupt Terminate; can be trapped
3 SIGQUIT Quit Terminate with core dump; can be trapped
9 SIGKILL Kill Forced termination; cannot be trapped
15 SIGTERM Terminate Terminate; can be trapped
24 SIGSTOP Stop Pause the process; cannot be trapped
25 SIGTSTP Terminal stop Pause the process; can be trapped
26 SIGCONT Continue Run a stopped process

As Clay said, "-9" is the last weapon.

-Arun

[Devender Khatana]

Hi Shiv,

As indicated earlier kill -9 should be used at last when other does not seems to work.Sometime even you will find that not even kill -9 works, this will be generally the case when some process has some pending I/O and that is not getting competed due to some failure.

But these sort of scenarios are very rare.

HTH,
Devender

[Shivkumar]

Hi Clay,

While using man signal command how to go to directly to the page SIGKILL (9) ?

currently i go by each line and page until i reach there.

Regards,
Shiv

[Arturo Galbiati]

Hi Shiv,
best way is to kill pid to gracefully release all the resources busy by the pid, while for a while and if it's not die kill -9 pid.
HTH,
Art

[Raj D.]

Shiv ,

First try gracefully

i.e
# kill -1 PID

similar to
# kill -HUP PID



If it is not killed gracefully , try killing with SIGKILL signal i.e
# kill -9 PID.


------------------------------------
Here are the detail Signals for kill :

1) HUP 16) USR1 31) RESERVED
2) INT 17) USR2 32) DIL
3) QUIT 18) CHLD 33) XCPU
4) ILL 19) PWR 34) XFSZ
5) TRAP 20) VTALRM 35) bad trap
6) IOT 21) PROF 36) bad trap
7) EMT 22) POLL 37) RTMIN
8) FPE 23) WINCH 38) RTMIN+1
9) KILL 24) STOP 39) RTMIN+2
10) BUS 25) TSTP 40) RTMIN+3
11) SEGV 26) CONT 41) RTMAX-3
12) SYS 27) TTIN 42) RTMAX-2
13) PIPE 28) TTOU 43) RTMAX-1
14) ALRM 29) URG 44) RTMAX
15) TERM 30) LOST
-----------------------------------------



Cheers,
Raj.

[Arunvijai_4]

Hi Shiv, # man kill and /SIGKILL will reach you to kill -9.

-Arun

[Shivkumar]

"kill pid" uses which signal number by default ?

[Arunvijai_4]

Hi Shiv, kill send SIGTERM (15), the (default) terminate signal.

-Arun

[Devender Khatana]

Hi,

Kill by default uses signal 15. This will be the same when you do not define any signal.

HTH,
Devender

[Raj D.]

Shiv ,

15 is the default, if you do not specify any signal , 15 is TERM , i.e SIGTERM


hth,
Raj.
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