上篇文章里列举出了常用的kill signal ，但是没有对于signal 0的描述，这里总结下
首先看下这篇文章 http://www.linuxjournal.com/content/monitoring-processes-kill
Monitoring Processes with Kill
If you have a process ID but aren't sure whether it's valid, you can use the most unlikely of candidates to test it: the kill command. If you don't see any reference to this on the kill(1) man page, check the info pages. The man/info page states that signal 0 is special and that the exit code from kill tells whether a signal could be sent to the specified process (or processes).
So kill -0 will not terminate the process, and the return status can be used to determine whether a process is running. For example:
$ echo $$ # show our process id 12833 $ /bin/bash # create new process $ echo $$ # show new process id 12902 $ kill -0 12902 $ echo $? # exists, exit code is 0 0 $ exit # return to previous shell $ kill -0 12902 bash: kill: (12902) - No such process $ echo $? # doesn't exist, exit code is 1 1
Many UNIX dæmons store their process IDs in a file in /var/run when they are started. Using kill -0 to test the pid is a lot easier than parsing ps output. For example, to test whether cron is running, do the following:
# kill -0 $(cat /var/run/cron.pid) # echo $? 0
下面的评论说出了重点：
Signal 0 not only tests for existance, it also tests for permissions, so to speak. "Am I allowed to send a singal to a given PID?"
再来看下这篇帖子
http://www.linuxquestions.org/questions/linux-newbie-8/what-does-kill-0-pid-do-651484/
it does nothing, but the exit code of your "kill -0 PID" command just returns 0 if you can send a signal to PID, and returns 1 if you can't (don't have access or invalid PID)
最后看下kill的文档
http://linux.die.net/man/2/kill
http://pubs.opengroup.org/onlinepubs/009695399/functions/kill.html
man kill page
0 0 n/a exit code indicates if a signal may be sent