wizard:run graham$ ls *.pid
DirectoryService.pid
configd.pid
diskarbitrationd.pid
hdiejectd.pid
httpd.pid
mds.pid
ntpd.pid
racoon.pid
syslog.pid
wizard:run graham$

When a process that needs to be contacted / alerted by other processes is started, it often records its process id (PID) into a ".pid file" so that those other processes know where to find it. The files above are form the /var/run directory on my OS X machine and point to system daemons, but it's a scheme that you may use for your own service processes (daemons) too. Have a look at these files ... and you'll just find a number in each of them.

Here's an example of how a newly starting daemon can log its pid (in Perl):
  open(FH,">","/var/run/nonfat.pid") and print FH "$$\n" and close FH;

And then how another process can check its most recent process ID, and whether it's running:
  open(FH,"/var/run/nonfat.pid");
  chomp($nfpid = <FH>);
  if ( kill 0,$nfpid ) { ...

Perl's kill function isn't as dramatic as it sounds - I sometime find that my classes are surprised to learn that killing is not always fatal. It should, really, have been called the "signal" command and with a zero first parameter it doesn't even send a signal - it just checks to see whether the process of the particular ID is there and running.

Once you've established that a process you want to alert is running, you can send it a proper signal:
  kill "USR1",$nfpid;

Source code examples ... [sending signal] and [receiving and handling signals].
(written 2010-10-23)

 P223 - Perl - Interprocess Communication
  [3412] Handling binary data in Perl is easy! - (2011-08-30)
  [3010] Children, zombies, and reaping processes - (2010-10-23)
  [2970] Perl - doing several things at the same time - (2010-09-25)
  [2694] Multiple processes (forking) in Python - (2010-03-25)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [604] Perl - multiprocess applications - (2006-02-13)

A164 - Web Application Deployment - Services and Regular Jobs
  [3143] On time - (2011-01-23)
  [2182] What Linux run level am I in? - (2009-05-15)
  [2145] Using the internet to remotely check for power failure at home (PHP) - (2009-04-29)
  [1903] daemons - what is running on my Linux server? - (2008-11-23)
  [1765] Dialects of English and Unix - (2008-08-21)
  [1733] memcached - overview, installation, example of use in PHP - (2008-08-02)
  [1731] Apache httpd, MySQL, PHP - installation procedure - (2008-08-01)
  [1700] FTP server on Fedora Linux - (2008-07-06)
  [1633] Changing a screen saver from a web page (PHP, Perl, OSX) - (2008-05-06)
  [1553] Automatic startup and shutdown of Tomcat - (2008-02-24)
  [1288] Linux run states, shell special commands, and directory structures - (2007-08-03)
  [1028] Linux / Unix - process priority and nice - (2007-01-10)
  [907] Browser -> httpd -> Tomcat -> MySQL. Restarting. - (2006-10-28)
  [544] Repeating tasks with crontab - (2005-12-27)

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