A.P. Lawrence

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

Unix, Xenix and ODT General FAQ

When should I use kill -9?

When you can't kill it with -15.

The idea here is that properly written programs will respond to a -15 by cleaning up anything they need to do before dying. Understand that this is not a matter of priorities, or the system giving more time with a -15; it's simply that a program can catch the -15, do what it needs to do and then voluntarily exit. It could also choose to ignore the -15 all together. If it hasn't made any provisions at all, the -15 works exactly like the -9: the process dies immediately.

A "kill -9" just causes the process to die; it gets no chance to do any cleanup. Therefore, if you don't know how a program was written, you should try the -15 first, in case it does need to clean up files, flush logs or whatever. If the -15 doesn't work, then use the -9.

It's the unfortunate choice of the word "kill" for this command that has caused all this confusion. It should have been called "signal", because that's what it really does- it sends a signal to a process. It's no different than you waving at someone to get their attention or tapping them on the shoulder.

What the process does when signal taps it on the shoulder depends. Many signals have the default action of causing the process to die. That's the default, but the programmer can choose to write the code differently.

A process can choose to ignore signals (with exceptions) or to catch them and take some other action. You've probably used "kill -1" to restart inittab or something else before. There's nothing magic about signal 1; it's just that those processes have arranged to pay attention to that and re-read or restart when they get it. Some other process could interpret a -1 in a different way- that's up to whoever wrote the code.

The shell "trap" command handles signals. You might have used this to ignore interrupt for certain scripts.

trap "" 2

says "if you get a 2, ignore it"

trap 'echo nope!' 2

causes the "nope!" to be echoed when you interrupt.

Every process has a default action. You can get a listing of those actions with "man 7 signal" (Linux) or "man signal" (SCO).

A "kill -9" cannot be ignored or trapped. If a process sees signal 9, it has no choice but to die. It can't do anything else- not even gracefully clean up its files.

A -15 (which is the same as just doing a kill without minus anything) is a signal with the default action of killing the process. However, it can be caught (that's the technical term for not ignoring) or ignored. Typically, a process that does arrange to catch a 15 will clean up open files, do whatever else it needs to, and then die.

That's why it is a good idea to just try an ordinary kill first. There are three possible outcomes:

• The process has done nothing to prepare for this and will immediately die.
• The process will write its buffers, close its files etc. and then die. What it does is up to the programmer.
• The process will ignore the signal and you'll have to send a -9 to get rid of it.

See also Why can't I kill a process with -9?

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