What does kill -- -0 do?

Question

Yes, I've seen that there's already a similar question, but I came across kill -- -0 and was wondering what -- is doing?

We do already have https://unix.stackexchange.com/questions/11376/ . — JdeBP, Dec 13 '17 at 21:09
@JdeBP That post explains part of the question (the -- part), but doesn't explain -0 though — Sergiy Kolodyazhnyy, Dec 14 '17 at 00:40
Where did you run across this? I've never seen it before, and am curious. Thanks! — cxw, Dec 14 '17 at 01:08
The question isn't asking about the -0. Indeed, the question explicitly points to an existing question that covers that, too. — JdeBP, Dec 14 '17 at 06:45
The question asks about signal zero (what is non-existent), and not about parametrization! Not dupe! — peterh, Dec 14 '17 at 08:21

score 17 · Accepted Answer · answered Dec 13 '17 at 16:36

17

In UNIX/Linux world two dashes one after other mean end of options. For example if you want to search for string -n with grep you should use command like:

grep -- -n file

If you want to get only the line number in above case you should use

grep -l -- -n file

So the command kill -- -0 try to send signal to process with ID -0 (minus zero)

answered Dec 13 '17 at 16:36

Romeo Ninov

17,484

12

As for what process 0 is: "If pid equals 0, then sig is sent to every process in the process group of the calling process." (kill(2)) – ilkkachu Dec 13 '17 at 16:40
Just for the record PID 0 is available in some UNIX OS (like Solaris) – Romeo Ninov Dec 13 '17 at 16:44
As some system/kernel process, I hope? Though FWIW, POSIX has almost the same description of killing pid 0, except that it mentions unspecified system processes. – ilkkachu Dec 13 '17 at 16:49
@ilkkachu: What would be a process group? Do you mean the Cgroup? Please be so kind an give an example where this use case occurs. Thanks. – manifestor Dec 13 '17 at 16:58
@chevallier https://en.wikipedia.org/wiki/Process_group – ilkkachu Dec 13 '17 at 17:12
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Why would one want to pass PID of minus zero? Aren't integers on all CPUs running POSIX-compatible OSes today two's complement, thus lacking any special encoding for -0? – Ruslan Dec 13 '17 at 21:22
@ilkkachu That seems to conflict with the answer here, unless I misread: https://unix.stackexchange.com/q/169898/85039 – Sergiy Kolodyazhnyy Dec 14 '17 at 00:57
@SergiyKolodyazhnyy I don't see the conflict - -0 the option argument is signal specifier 0, aka no signal but process checking (that answer), and -0 the non-option argument is the process group of the current process (this answer and ilkkachu's comment). – muru Dec 14 '17 at 04:01
@muru according to that answer -0 "...will not actually in fact send anything to your process's PID, but will in fact check whether you have permissions to do so", but ilkkachu says it's for sending signal to every proc in process group. So is it actually for sending or for checking ? – Sergiy Kolodyazhnyy Dec 14 '17 at 04:06
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@SergiyKolodyazhnyy -0 the option will not send anything. -0 the non-option will. Hence the -- in the command here. – muru Dec 14 '17 at 04:08
@muru Ah, I see now !!! That makes more sense. – Sergiy Kolodyazhnyy Dec 14 '17 at 04:11

score 7 · Answer 2 · answered Dec 13 '17 at 16:39

In this case it is used to denote that the -0 is not an option being passed in. If you were to do kill -0 it would think of -0 as an option not as the PID being passed in.

For example if you did ls -- -l it will look for a file named -l as opposed to just listing the directory in long form.

What does kill -- -0 do?

2 Answers2

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