Writing a service in Linux

As Khem's request, today I will (try to) explain the simple way to write a linux service.

The linux services are nothing but programs running in the background (usually daemon) to provide their service. For example, the httpd (HTTP Daemon) provide HTTP server service which made your system be a web server.

Writing a Linux service is easy, it require only two major steps.

1. Implement your program as Linux daemon

2. Write the service script, usually simple shell script, and place it in "/etc/init.d" directory

Ah, and you should be the root to start/stop or add services.

Briefly, the daemon is a program that run in the background and had practically no parent (literally it had only init process - the pid #1 - be its parent). You may implement Linux daemon program using C by simply calling daemon() function (in unistd.h). After daemon() called, your program will be a daemon :)

Service script is somewhat easy to write, too. Check the code below for example. (Suppose that this code was in the file /etc/init.d/testservice).

Sample service script called "testservice"

----------------------------

 1: #!/bin/bash

 2: . /etc/init.d/function #Include some necessary function

 3: case $1 in

 4:   start)

 5:     echo -n "Starting test service";

 6:     daemon /program/directory/test arg1 arg2;

 7:     echo;

 8:   ;;

 9:   stop)

10:     echo -n "Exiting test service";

11:     killproc test;

12:     echo;

13:   ;;

14:   status)

15:     status test;

16:   ;;

17:   *)

18:     echo " Usage: service testservice {start|stop|status}";

19:   ;;

20: esac

--------------------------------------

The "daemon" in line 6, "killproc" in line 11, and "status" in line 15 are bash functions written in /etc/init.d/functions. We include that file from . command in line 2. Alternatively, "source" command might be used instead of ".", for the code readability. The arguments in line 6 will be passed in the program you write.

Actually, you don't have to write the service script this way. You can make your own style, write your own function, define your command, or what not, since this is only the script being invoked via command "service".

You can even write perl script, or python, and put it in "/etc/init.d". Don't forget to change its permission to be 755 (executable). That's it. 

When root invoke command "service your-service ", the service program will pass argument to "your-service" script.

By the way, as I shown you, Linux (I tested on red-hat based Fedora Core 5 and Cent OS 4) had already provided some convenient function to start/stop or check service status. It would be more convenient to write in the simple style I posted :)

***NOTE***: For some reason (maybe pattern matching problem), the daemon program name should not include "_" (underscore). My service (shell) script didn't work when I had a underscore in a file name.

Hope this might benefit you a bit :)

Fug.U.Man