Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
What are .pid files?

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)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P223 - Perl - Interprocess Communication
  [604] Perl - multiprocess applications - (2006-02-13)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2694] Multiple processes (forking) in Python - (2010-03-25)
  [2970] Perl - doing several things at the same time - (2010-09-25)
  [3010] Children, zombies, and reaping processes - (2010-10-23)
  [3412] Handling binary data in Perl is easy! - (2011-08-30)
  [3940] Run other processes from within your Perl program - (2012-12-03)

A164 - Web Application Deployment - Services and Regular Jobs
  [544] Repeating tasks with crontab - (2005-12-27)
  [907] Browser -> httpd -> Tomcat -> MySQL. Restarting. - (2006-10-28)
  [1028] Linux / Unix - process priority and nice - (2007-01-10)
  [1288] Linux run states, shell special commands, and directory structures - (2007-08-03)
  [1553] Automatic startup and shutdown of Tomcat - (2008-02-24)
  [1633] Changing a screen saver from a web page (PHP, Perl, OSX) - (2008-05-06)
  [1700] FTP server on Fedora Linux - (2008-07-06)
  [1731] Apache httpd, MySQL, PHP - installation procedure - (2008-08-01)
  [1733] memcached - overview, installation, example of use in PHP - (2008-08-02)
  [1765] Dialects of English and Unix - (2008-08-21)
  [1903] daemons - what is running on my Linux server? - (2008-11-23)
  [2145] Using the internet to remotely check for power failure at home (PHP) - (2009-04-29)
  [2182] What Linux run level am I in? - (2009-05-15)
  [3143] On time - (2011-01-23)
  [3791] The Kernel, Shells and Daemons. Greek Gods in computing - (2012-07-01)
  [3792] Managing daemons from a terminal session - (2012-07-01)
  [4487] Starting MySQL. ERROR! The server quit without updating PID file - how we fixed it. - (2015-05-06)


Back to
Children, zombies, and reaping processes
Previous and next
or
Horse's mouth home
Forward to
Exception handling in Perl - using eval
Some other Articles
Logging the performance of the Apache httpd web server
Well House Manor - the next six years
Audio equipment
Exception handling in Perl - using eval
What are .pid files?
Expect in Perl - a short explanation and a practical example
Dulwich College Preparatory, and Sevenoaks, Schools
Setting up a matrix of data (2D array) for processing in your program
Santa announcement, 5th December 2010, Melksham
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2024: 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/3011_Wha ... iles-.html • PAGE BUILT: Sun Oct 11 16:07:41 2020 • BUILD SYSTEM: JelliaJamb