why not kill -9 a process?

Question

I am always very hesitant to run kill -9.

But I see other admins do it almost routinely.

I figure there is probably a sensible middle ground.

So:

1. When should kill -9 be used?
2. What should be tried before doing it?
3. Is there any kind of debugging a "hung" process that could cause further problems?
4. When should kill -9 never be used?
5. What bad things can happen if kill -9 is used?

Answer 1

Generally, you should use kill -15 before kill -9 so as to give the program a chance to clean up after itself. (Programs can't catch or ignore SIGKILL, but they can and often do catch SIGTERM.) If you don't give the program a chance to finish what it's doing and clean up, it may leave corrupted files around that it won't be able to understand once restarted.

strace/truss, ltrace (truss -u on Solaris, where it's a bit more helpful; ltrace I find too often doesn't present arguments to library calls in a usable format), and gdb are generally good ideas for looking at why a stuck program is stuck. There are also useful /proc-based tools on Solaris, some of which have been ported to Linux (pstack is often helpful).

Answer 2

Randal Schwartz used to frequently post "Useless use of (x)" on lists. One such post was about kill -9. It includes reasons and a recipe to follow. Here is a reconstructed version (quoted below).

(Quote abomination)

No no no.  Don't use kill -9.

It doesn't give the process a chance to cleanly:

1) shut down socket connections

2) clean up temp files

3) inform its children that it is going away

4) reset its terminal characteristics

and so on and so on and so on.

Generally, send 15, and wait a second or two, and if that doesn't
work, send 2, and if that doesn't work, send 1.  If that doesn't,
REMOVE THE BINARY because the program is badly behaved!

Don't use kill -9.  Don't bring out the combine harvester just to tidy
up the flower pot.

Just another Useless Use of Usenet,

(.signature)

Answer 3

Never never do a kill -9 1. Also avoid doing a kill on certain processes like mount. When I have to kill a lot of processes ( say for example an X session gets hung and I have to kill all the processes of a certain user ), I reverse the order of the processes: eg.

ps -ef|remove all processes not matching a certain criteria| awk '{print $2}'|ruby -e '$A=stdin.readlines; A.reverse.each{|a| puts "kill -9 #{a}"}'|bash

Keep in mind that kill does not stop a process and release it's resources. All it does is send a SIGKILL signal to the process, you could wind up with a process that's hung.