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Designing Integrated High Quality Linux Applications

Avi Alkalay

avi at
avi at
Senior IT and Software Architect :: Linux Market Developer
IBM Linux Impact Team :: []

Copyright � 2002 by Avi Alkalay

v2.1, 2002-08-24
Revision History                                                             
Revision 2.1             24 Aug 2002            Revised by: avi              
Rewrite of the /opt /usr/local section.Cosmetics on graphical user interface 
and plugins sections.Fixed screens and programlistings width.                
Revision 2.0             07 May 2002            Revised by: avi              
Final XML conversion. Files reorganization.                                  
Revision 1.9.9           20 Apr 2002            Revised by: avi              
Included other document locations.                                           
Revision 1.98            14 Apr 2002            Revised by: avi              
Title changed from "Creating" to "Designing".                                
Revision 1.97            09 Apr 2002            Revised by: avi              
Converted to XML 4.1.2, and started to use real XSLT. Spell checked the      
english version.                                                             
Revision 1.96            23 Mar 2002            Revised by: avi              
Better HTML style sheets.                                                    
Revision 1.95            17 Mar 2002            Revised by: avi              
Last chapter: One Body, Many Souls. Created appendix. Still have to translate
some words here and there.                                                   
Revision 1.9             16 Mar 2002            Revised by: avi              
Added universal software table with FHS.                                     
Revision 1.7             16 Mar 2002            Revised by: avi              
Everything is now translated except some words.                              
Revision 1.3             27 Feb 2002            Revised by: avi              
Translated and reviewed the most important section of the article: The /opt  
and /usr/local section.                                                      
Revision 1.2             23 Feb 2002            Revised by: avi              
English translation at 65%. Doing some corrections to potuguese version also.
Revision 1.1             17 Feb 2002            Revised by: avi              
Started english translation.                                                 
Revision 1.0             16 Feb 2002            Revised by: avi              
First final version of proposed skeleton.                                    
Revision 0.9.6           16 Feb 2002            Revised by: avi              
Finished Plugin chapter.                                                     
Revision 0.9.5           15 Feb 2002            Revised by: avi              
Finished chapter about boot and subsystems.                                  
Revision 0.9.4           14 Feb 2002            Revised by: avi              
Finished chapter describing the boot process.                                
Revision 0.9.3           08 Feb 2002            Revised by: avi              
Text and style updates.                                                      
Revision 0.9.2           07 Feb 2002            Revised by: avi              
Text updates.                                                                
Revision 0.9             06 Feb 2002            Revised by: avi              
First translation to DocBook.                                                

Table of Contents
1. Introduction
2. User Friendly: Guaranteed Success
    2.1. Embrace the Install-and-Use Paradigm
3. The Four Universal Parts of Any Software
    3.1. Practical Examples
    3.2. The Importance of Clear Separation Between Four Parts
    3.3. One Body, Many Souls
4. Linux Directory Hierarchy: Oriented to the Software Parts
    4.1. FHS Summary
    4.2. Examples Using the FHS
    4.3. Developer, Do Not Install in /opt or /usr/local !
5. Provide Architecture for Extensions and Plugins
    5.1. Abstracting About Plugins
6. Allways Provide RPM Packages of Your Softwares
    6.1. Software Package Modularization
7. Security: The Omnipresent Concept
8. Graphical User Interface
    8.1. KDE, GNOME, Java or Motif?
    8.2. Web Interface: Access from Anywhere
    8.3. Wizards and Graphical Installers
9. Starting Your Software Automatically on Boot
    9.1. From BIOS to Subsystems
    9.2. Runlevels
    9.3. The Subsystems
    9.4. Turning Your Software Into a Subsystem
    9.5. Packaging Your Boot Script
A. Red Hat, About the Filesystem Structure
B. About this Document

1. Introduction

Linux is becoming more and more popular, and many Software vendors are
porting their products from other platformas. This document (article) tries
to clarify some issues and give tips on how to create Linux applications
highly integrated to the Operating System, security and easy of use.

The examples run on [] Red Hat Linux, and should be
compatible with other distributions based on Red Hat ([http://] Conectiva, [] Turbolinux,
[] Caldera, [] PLD, [http://] Mandrake, etc).

2. User Friendly: Guaranteed Success

The user-friendly concept is missassociated with a good GUI (graphical user
interface). In fact, it is much more than that. In systems like Linux (with
more server-like characteristics), the user measures how easy a Software is,
mainly in the installation and initial configuration. He can even forget how
easy were to install and use a certain product, but it will never forget that
a Software package has a complex configuration and installation process. A
migration or new installation allways will be a nightmare, making the user
avoid it.

2.1. Embrace the Install-and-Use Paradigm

Imagine you'll install that expansive product your company bought from ACME,
and realized you'll have to do the following:

 1. To have a manual that shows the installation process step-by-step. We
    know that a manual is the last thing the user reads
 2. Read some README files
 3. Uncompress huge files in your disk (after downloading them from net our
    CD), to create the installation environment
 4. Read more README files that appeared in the installation environment
 5. Comprehend that the installation requires you to execute in a special way
    some provided script (the inconvenient ./
 6. Uncomfortably answer some questions that the script does, like target
    directory, user for the installation, etc. To make it worse, it
    frequently happens in a terminal that has a missconfigured backspace
 7. After the installation, configure some environment variables in your
 8. Edit OS files to include the presence of the new product (e.g. /etc/
    inetd.conf, /etc/inittab)
 9. And the worse: Change security permissions of OS directories and files to
    let the product run OK

Sounds familiar? Who never faced this sad situation, that inducts the user to
make mistakes? If your products' installation process sound like 
Uncompress-Copy-Configure-ConfigureMore-Use, like this one, you have a
problem, and the user won't like it.

Users like to feel that your Product integrates well with the OS. You should
not demand that the OS adapt himself to your Product (changing environment
variables, etc). It must let the user Install-and-Use.

The Install-And-Use glory is easily achieved using a 3 ingredients receipt:

 1. Understanding the Four Universal Parts of Any Software
 2. Understanding how they are related to Linux's directory hierarchy
 3. Aggressively use a package system, for process automation and leverage
    first items. In our case is RPM.

We'll discuss here what are these ingredients and how to implement them.

3. The Four Universal Parts of Any Software

The file set of any Application Software, graphical, server-side, commercial,
open/free, monolithic etc, has allways four universal parts:

1st :: The Software on its own: the body

The executables, libraries, static-data files, examples, manuals and
documentation, etc. Regular users must have read-only access to these files.
They are changed only when the system administrator makes an upgrade in this

2nd :: Configuration Files: the soul

These are files that define how the Software will run, how to use the Content
, security, performance etc. Without them, the Software on its own is usually

Depending on your Software, specific privileged users may change these files,
to make the Software behave as they want.

It is important to provide documentation about the configuration files.

3rd :: Content

Is what receives all the user attention. Is what the user delegated to be
managed by your Product. Is what makes a user throw away your product and use
the competitors', if it gets damaged.

Are the tables of a database system, the documents for a text editor, the
images and HTML pages of a web-server, the servlets and EJBs of an
Application Server, etc.

4th :: Logs, Dumps etc

Server Software use to generate access logs, trace files problem
determination, temporary files etc. Other types of softwares also use this
files, but it is less common.

It is the last class of file, but many times they are the most problem
generator for a system administrator, because their volume can surpass even
the content size. Due this fact, it is important for you to think in some
methodology or facility for this issue, while you are in design time.

3.1. Practical Examples

Let's see how universal is this concept analyzing some types of softwares:

Table 1. Universality of 4 Parts
|�        |Software on its|Configurations|Content          |Logs, Dumps etc |
|         |Own            |              |                 |                |
|Data Base|Binaries,      |Files that    |Table files,     |For DBs, there  |
|Server   |libraries,     |define the    |index files, etc.|are the backup, |
|         |documentations.|directory of  |This software use|generated in a  |
|         |               |the data      |to have whole    |daily basis. And|
|         |               |files. For    |trees under the  |the logs are    |
|         |               |this type of  |same directory.  |used by the DBA |
|         |               |Software, the |And many times   |to define       |
|         |               |remaining     |they need several|indexing        |
|         |               |configurations|filesystems to   |strategy. His   |
|         |               |usually are in|guarantee        |local on the    |
|         |               |special tables|performance.     |system is also  |
|         |               |inside the    |Their local in   |defined by the  |
|         |               |database.     |the system is    |Configurations. |
|         |               |              |defined by they  |                |
|         |               |              |Configurations.  |                |
|Text     |The same,      |As a          |The documents    |They show as    |
|Processor|templates,     |user-oriented |generated by the |temporary files |
|         |modular file   |Software, its |user, and they go|that can be     |
|         |format filters,|configurations|some place in his|huge. User can  |
|         |etc            |must be put in|$HOME            |define their    |
|         |               |each user's   |                 |location with a |
|         |               |$HOME         |                 |user-friendly   |
|         |               |directory, and|                 |dialog (that    |
|         |               |are files that|                 |saves it in some|
|         |               |defines       |                 |Configuration   |
|         |               |standard fonts|                 |file)           |
|         |               |and           |                 |                |
|         |               |tabulation,   |                 |                |
|         |               |etc.          |                 |                |
|MP3      |Same, audio    |Each user has |Similar to Text  |Similar to Text |
|generator|modular filters|a             |Editor           |Editor          |
|         |               |configuration |                 |                |
|         |               |file in his   |                 |                |
|         |               |$HOME, and    |                 |                |
|         |               |contains      |                 |                |
|         |               |bitrate       |                 |                |
|         |               |preferences   |                 |                |
|         |               |etc           |                 |                |
|Web      |Similar to Data|Files that    |Directories where|Preciouses      |
|Server   |Base           |define the    |the webmaster    |access logs,    |
|         |               |Content       |deposits his     |vital for       |
|         |               |directory,    |creativity. Again|Marketing       |
|         |               |network and   |defined by the   |Intelligence,   |
|         |               |performance   |Configurations   |that are        |
|         |               |parameters,   |                 |generated in a  |
|         |               |security, etc |                 |location and    |
|         |               |              |                 |format defined  |
|         |               |              |                 |by              |
|         |               |              |                 |Configurations  |
|e-Mail   |Similar to     |Files that    |The preciouses   |Mail transfer   |
|Server   |Database and   |define how to |users mail boxes.|log, virus      |
|         |Web-Server     |access user   |Again defined by |detection log,  |
|         |               |database, mail|the              |etc. Again      |
|         |               |routing rules,|Configurations   |defined by the  |
|         |               |etc           |                 |Configurations  |

Pay attention that the Software on its Own contains all your product business
logic, which could be useless if you hadn't a Configuration to define how to
work with a data bundle, provided by the user. So, Configurations are what
connects your product to the user.

We can use a metaphor about a Sculptor (business logic), that needs Bronze (
content) and a Theme or Inspiration (configuration) from a Mecenas (user), to
produce a beautiful work (content). He make annotations in his Journal (logs)
about his day-by-day activities, to report to his Mecenas (user).

3.2. The Importance of Clear Separation Between Four Parts

OK, so let's be more practical. The fact is, if we correctly use the 
universal parts concept, we greatly improve the quality of our Product. We'll
do that simply separating, encapsulating, each one of these parts in
different system directories (having only different files for each part is
not sufficient). There is a standard called []
FHS that defines the Linux directories for each part, and we'll discuss it
later in Section 4.

By now let's see the value of this separation to the user:

 1. He gains a clear vision about where is each part, specially his 
    Configurations and Content, and he feels your Product as something
    completely under control. The clareza brings ease of use, security and
    confidence in your Product. And in practice it permits him manipulate
    each part independently
 2. It is clear now that, for instance, when backing up, user action is
    needed only for Configurations and Content (the puritans will also backup
    some logs). The user don't have to care about Software on its Own,
    because it is safe, original, on the product CD, in his shelf.
 3. For upgrades, the new package will overwrite only the business logic,
    leaving intact the user's precious Configurations and Content. Here is
    very important to keep old content and configuration compatible, or to
    provide some tools help migration of data
 4. The logs being kept in a separate filesystem (obviously suggested in your
    documentation), avoids that their exaggerated growth interfere with the 
    Content, or with the stability of the whole system
 5. If your Software follows some directory standards, the user don't have to
    reconfigure his system or environment to use it. He will simply 

Let's make some exercise with separation using as example a system called 
MySoftware, in which the business logic is in Example 1 and the configuration
is in Example 2.

Example 1. A Shell program referring an external configuration file
## /usr/bin/MySoftware                                                        
## Business logic of MyProgram system.                                        
## Do not change nothing in this file. All configuration can be               
## made on /etc/MySoftware.conf                                               
## We'll not support any modifications made here.                             
# Default configuration file                                                  
CONF=/etc/MySoftware.conf       (1)                                           
# Minimal content directories                                                 
MIN_CONTENT_PATH=/var/www:/var/MySoftware/www   (2)                           
if [ -r "$CONF"]; then                                                        
        . "$CONF"       (3)                                                   
# All the content I'll serve are the "minimal" plus the ones provided         
# by the user in the configuration file $CONF                                 

(1) Definition of the configuration file name.
(2) Definition of some static parameters.
(3) The configuration is readed from an external file, if exists.
(4) After reading the configuration file, all content directories -- user's +
    product's -- goes together in the $CONTENT_PATH, that will be used from
    now on.

Example 2. File containing only the configurations for MySoftware
## /etc/MySoftware.conf                                                        
## Configuration parameters for MySoftware.                                    
## Change as much as you want.                                                 
# Content directory.                                                           
# A ':' separated list of directories for your content.                        
# The directories /var/www and /var/MySofware are already there, so            
# include here your special directories, if any.                               
CONF_CONTENT_PATH=/var/NewInstance:/var/NewInstance2    (1)                    
# Your e-mail address, for notifications.                                     (2)                                            
# Logs directory                                                               
LOG_DIR=/var/log/myInstance     (3)                                            

(1) (2) (3)
    These are user defined parameters.

3.3. One Body, Many Souls

When I was a system administrator for IBM e-business Hosting Services, I was
fascinated by [] Apache's flexibility letting us do
things like this:
bash# /usr/sbin/httpd &                                                      
bash# /usr/sbin/httpd -f /etc/httpd/ &                       
bash# /usr/sbin/httpd -f /etc/httpd/ &                       
bash# /usr/sbin/httpd -f /etc/httpd/ &                       

If we don't pass any parameter (like the first example), Apache loads its
default, hardcoded configuration file from /etc/httpd/conf/httpd.conf. We
built other configs, one for each customer, with a completely different
structure, IP address, loaded modules, content directory, passwords, domains,
log strategy etc.

This same concept is used by a text editor of a multiuser desktop (like
Linux). When the code is loaded, it looks for a configuration file on the
user's $HOME, and depending who invoked him (user A or B), it will appear
differently because each user has its own personal configuration.

The obvious conclusion is that the Software's body (business logic) is pure e
completely oriented by his manipulator's spirit (configuration). But the
competitive advantage lays on how easy we switch from one spirit to another,
like in Apache's example. It is very healthy to promote it to your user.
You'll be letting him create intimacy, reliability, confort with your

We used this approach with many different Softwares in that e-business
Hosting time, and it was extremely usefull for maintenance etc. In a version
migration we had total control over where were each of its parts, and
upgraded and downgraded Software with no waste of time, with obvious success.

But there were some Products that refused to work this way. They had so many
hardcoded parameters, that we couldn't see what divided the body from their 
spirit (or other parts). These Softwares were marked as bad guys and
discarded/replaced as soon as possible.

We concluded that the good guys Softwares were intuitively blessed by their
developer's four parts vision. And they made our life easyer. In fact, in
that time we formulated this theory, that continues to prove itself.

Do you want to deploy bad guy or good guy Software?

4. Linux Directory Hierarchy: Oriented to the Software Parts

By now, all discussion are OS independent. On Linux, the Four Software Parts
theory is expressed in his directory structure, which is classified and
documented in the [] Filesystem Hierarchy
Standard. The FHS is part of the LSB (Linux Standard Base), which makes him a
good thing because all the industry is moving thowards it, and is a constant
preoccupation to all distributions. FHS defines in which directories each
peace of Apache, Samba, Mozilla, KDE and your Software must go, and you don't
have any other reason to not use it while thinking in developing your
Software, but I'll give you some more:

 1. FHS is a standard, and we can't live without standards
 2. This is the most basic OS organization, that are related to access levels
    and security, where users intuitively find each type of file, etc
 3. Makes user's life easyer

This last reason already justifies FHS adoption, so allways use the FHS !!!

More about FHS importance and sharing the same directory structure can be
found in [
ch-filesystem.html] Red Hat website.

4.1. FHS Summary

So let's summarize what the FHS has to say about Linux directories:

Linux system directories

    Directory for the executables that are accessed by all users (everybody
    have this directory in their $PATH). The main files of your Software will
    probably be here. You should never create a subdirectory under this
    Like /usr/bin, but here you'll find only boot process vital executables,
    that are simple and small. Your Software (being high-level) probably
    doesn't have nothing to install here.
    Like /usr/bin, but contains only the executables that must be accessed by
    the administrator (root user). Regular users should never have this
    directory in their $PATH. If your Software is a daemon, This is the
    directory for some of executables.
    Like /usr/sbin, but only for the boot process vital executables, and that
    will be accessed by sysadmin for some system maintaining. Commands like 
    fsck (filesystem check), init (father of all processes), ifconfig
    (network configuration), mount, etc can be found here. It is the system's
    most vital directory.
    Contains dynamic libraries and support static files for the executables
    at /usr/bin and /usr/sbin. You can create a subdirectory like /usr/lib/
    myproduct to contain your helper files, or dynamic libraries that will be
    accessed only by your Software, without user intervention. A subdirectory
    here can be used as a container for plugins and extensions.
    Like /usr/lib but contains dynamic libraries and support static files
    needed in the boot process. You'll never find an executable at /bin or /
    sbin that needs a library that is outside this directory. Kernel modules
    (device drivers) are under /lib.
    Contains configuration files. If your Software uses several files, put
    them under a subfolder like /etc/myproduct/
    The name comes from "variable", because everything that is under this
    directory changes frequently, and the package system (RPM) doesn't keep
    control of. Usually /var is mounted over a separate high-performance
    partition. In /var/log logfiles grow up. For web content we use /var/www,
    and so on.
    Contains the user's (real human beings) home directories. Your Software
    package should never install files here (in installation time). If your
    business logic requires a special UNIX user (not a human being) to be
    created, you should assign him a home directory under /var or other place
    outside /home. Please, never forget that.
/usr/share/doc, /usr/share/man
    The "share" word is used because what is under /usr/share is platform
    independent, and can be shared among several machines across a network
    filesystem. Therefore this is the place for manuals, documentations,
    examples etc.
/usr/local, /opt
    These are obsolete folders. When UNIX didn't have a package system (like
    RPM), sysadmins needed to separate an optional (or local) Software from
    the main OS. These were the directories used for that.

You may think is a bad idea to break your Software (as a whole) in many
pieces, instead of keeping it all under a self-contained directory. But a 
package system (RPM) has a database that manages it all for you in a very
professional way, taking care of configuration files, directories etc. And if
you spread your Software using the FHS, beyond the user friendliness, you'll
bring an intuitive way to the sysadmin configure it, and work better with
performance and security.

4.2. Examples Using the FHS

Now that we know where each part of our software must be installed, lets
review the Universal Parts Table applied to the FHS.

Table 2. Same Software, applying FHS
|�        |Software on its  |Configurations |Content  |Logs, Dumps etc      |
|         |Own              |               |         |                     |
|Data Base|/usr/bin/, /usr/ |/etc/mydb/     |/var/db/ |/var/db/instance1/   |
|Server   |lib/, /usr/share/|               |instance1|transactions/, /var/ |
|         |doc/mydb/, /usr/ |               |/, /var/ |log/db/              |
|         |share/doc/mydb/  |               |db/      |access-instance1.log,|
|         |examples/        |               |instance2|/var/log/db/         |
|         |                 |               |/, etc   |access-instance2.log |
|Text     |/usr/bin/, /usr/ |$HOME          |$HOME/   |$HOME/.myeditor-tmp/ |
|Editor   |lib/, /usr/lib/  |/.myeditor.conf|Docs/    |                     |
|         |myeditor/plugins |               |         |                     |
|         |/, /usr/share/   |               |         |                     |
|         |myeditor/        |               |         |                     |
|         |templates/, /usr/|               |         |                     |
|         |share/doc/       |               |         |                     |
|         |myeditor/        |               |         |                     |
|MP3      |/usr/bin/, /usr/ |$HOME          |$HOME/   |$HOME/.mymp3-tmp/    |
|Generator|lib/, /usr/lib/  |/.mymp3.conf   |Music/   |                     |
|         |mymp3/plugins/, /|               |         |                     |
|         |usr/share/doc/   |               |         |                     |
|         |mymp3/           |               |         |                     |
|Web      |/usr/sbin/, /usr/|/etc/httpd/, / |/var/www |/var/logs/httpd/, /  |
|Server   |bin/, /usr/lib/  |etc/httpd/     |/, /var/ |var/logs/httpd/      |
|         |httpd-modules/, /|instance1/, /  |www/     |instance1/, /var/logs|
|         |usr/share/doc/   |etc/httpd/     |instance1|/httpd/instance2/    |
|         |httpd/, /usr/    |instance2/     |/, /var/ |                     |
|         |share/doc/httpd/ |               |www/     |                     |
|         |examples/        |               |instance2|                     |
|         |                 |               |/        |                     |
|E-Mail   |/usr/sbin/, /usr/|/etc/mail/, /  |/var/mail|/var/spool/mailqueue |
|Server   |bin/, /usr/lib/, |etc/           |/        |/, /var/logs/mail.log|
|         |/usr/share/doc/  |  |         |                     |
|         |mymail/          |               |         |                     |

4.3. Developer, Do Not Install in /opt or /usr/local !

If you are a systems administrator, this section is not for you. This is a
subject for developers and packagers, to make sysadmin's life easyer.

The /opt and /usr/local directories are used by sysadmins to manualy
non-packaged files (without RPM) of a software, precisely to not loose
control over those files. Notice how separated this folder are from the rest
of the system.

A manual installation process (without RPM, or based on simple file copy) is
documented in forgoten document inside a drawer (if it was documented), and
inside the head of who made installation. If he moves to another job, that
installations becomes obscure to the rest of the team, and is a time bomb.

With RPM is different. RPM (or any other package system) is an installation
"process" by itself. It is self-documented in his database and pre and
post-install actions, which permits total control. Turns installations
independent from who did it, turning installtions in a business process.

Installations based on coping files into /opt or /usr/local are far from
providing the organization, system visibility and control that RPM provides.
I can say /opt and /usr/local would be obsoleted when all softwares become

It is very important to Linux evolution and popularization (especially in the
desktop battlefield), that developers stop using this hell directories, and
start using the FHS. After reading this section, if you still think this
folders are good business, please drop me an e-mail.

Products that are entirely installed under one directory, use the 
self-contained approach, that has several problems:

 1. Forces the user to change environment variables like $PATH and
    $LD_LIBRARY_PATH to use your product easily.
 2. Puts files in non-standard places, complicating system integration, and
    future installation of extensions to your product.
 3. The sysadmin probably didn't prepared disk space in these partitions,
    generating problems in installation time.
 4. It is an accepted approach only for pure graphical application, without
    the command line concept. This is why it were well accepted in Windows.
 5. ...even using this approach, you can't avoid installing or changing files
    in standard locations to, for instance, make your icons appear in the
    user desktop.

Many developers believe that the "self-contained" approach let them work with
several versions of the same product, for testing purposes, or whatever. Yes,
agree, with this or any good reason in the planet. But remember that a High
Quality Software (or Commercial Grade Software) objective is to be practical
for the final user, and not to be easy to their developers and testers.
Invite yourself to visit an unexperienced user (but potential customer) and
watch him installing your product.

Developer, don't be afraid of spreading your files according to FHS because
RPM will keep an eye on them.

If you have a business reason to let the user work with several versions of
your Product simultaneously (or any other reason), make a [
/max-rpm/ch-rpm-reloc.html] relocatable package, which is described in the
[] Maximum RPM book. Be also aware about the
implications of using this feature, [
s1-rpm-reloc-wrinkles.html] described in the same book.

Red Hat and derivated distributions allways use the directory standard,
instead of /opt or /usr/local. Read what Red Hat says about this subject, and
think about it.

Note The Makefiles of an OpenSource Software that is portable to other UNICES
     must have the standard installation in /usr/local for compatibility     
     reasons. But must also give the option, and induct the packager, to     
     create the package using FHS specifications.                            

5. Provide Architecture for Extensions and Plugins

You'll probably let other Software vendors plug extensions to your product.
Since you are the author of the initial Software, is your responsability to
organize it in such a way that the user can simply install the extension RPM
and use it, without forcing him modify any configuration file. It is again
the famous Install-and-Use that guarantees ease-of-use.

Well, and extension is nothing more that some files in a right format (DLLs
that implements the API your Software defined), put in the right folders
(directories your Software looks for extensions).

We can see many applications requesting the user to change configuration
files to "declare" the presence of a new plugin. This is a bad approach that
must be avoided because makes user's or plugin provider's life harder.

The most important thing to consider in your plugin architecture is to not
share files between plugins and your Software. You should provide an
architecture where plugins will be able to fully install and uninstall
themselves by simply putting and removing files in specific directories,
documented in you Software. Good candidates are /usr/lib/myproduct/plugins as
the plugins directory, and /etc/myproduct/plugins as the plugins
configuration files directory. Your Software and plugins must be sufficient
intelligent to know how to find files, specially configurations, in these

Using this approach, no post-install procedures is required from the user,
and from the plugin provider.

5.1. Abstracting About Plugins

I would like to close this subject inviting the reader a se abstratir and
think about any Software can be treated as an extension to the lower level
Software. In the same way a third party plugin is an extension to your
Software, your Software is also an extension to the OS (lower level). This is
where all the Integration (from the title of this document) magic lives. So
we can apply all the ease-of-use concepts we discussed before to the plugin
architecture design of your Software.

6. Allways Provide RPM Packages of Your Softwares

This is extremely important for many reasons:

 1. Ease-of-use. This is allways the primordial motivation.
 2. Automates some tasks that must be made before and after the installation
    of your Software. Again bringing ease-of-use.
 3. Intelligently manages configuration files, documentation etc, providing
    more control in an upgrade
 4. Manages interdependencies with other packages and versions, guaranteeing
    good functionality.
 5. Lets you distribute Software with your company's digital signature, and
    makes integrity checks (MD5) in each file, guaranteeing precedence, and
    reporting unwanted file modification.
 6. Provides tools to let interact with your graphic installer.

But a good package is not only put together your files in a RPM. FHS must be
followed, configuration and documentation files must be marked as is, and
pre- and post-install scripts must be robust, to not let them damage the
system (remember that installation processes is done by root).

Know well RPM because it can bring much power and facilities to you and your
user. There are a lot of documentation available about RPM on the Internet:

��*�The book [] Maximum RPM, also
    available [] on-line and in printable [http://] PostScript format.
��*�[] RPM-HOWTO which is smaller and more

6.1. Software Package Modularization

You should give user the option to install only the part of your Software he
wants. Imagine your Software has a client part and a server part, and both
use files and libraries in common. You should break them in 3 RPMs. For
instance, lets say the name of your product is MyDB, so you'll provide the

 1. MyDB-common-1.0-3.i386.rpm
 2. MyDB-server-1.0-3.i386.rpm
 3. MyDB-client-1.0-3.i386.rpm

and last 2 packages depends on the first. If the user is installing a client
profile, he will use:

 1. MyDB-common-1.0-3.i386.rpm
 2. MyDB-client-1.0-3.i386.rpm

If he is installing a server profile:

 1. MyDB-common-1.0-3.i386.rpm
 2. MyDB-server-1.0-3.i386.rpm

This approach will help the user save disk space, and be aware of how your
Software is organized.

7. Security: The Omnipresent Concept

From a very general perspective, security is synonym of order, conscience.
And insecure is everything that makes a system stop without the user wish. So
besides open network ports, or weak cryptography (that are beyond the scope
of this document), applications that inducts the user to use it only as root,
or make him change files in inappropriate places, is considered insecure. We
can say the same for the apps that fills a filesystem that is vital to the

Many standards appeared from good practices discussed and developed in
conjunction for a long time. So you should know and use them when you'll
package your software, because they are key for you to achieve a good
organization (security) level.

8. Graphical User Interface

Everybody loves graphical interfaces. Many times they make our life easyer,
and this way helps to popularize a Software, because the learning curve get
smaller. But for the everyday use, a command with many options and a good
manual becomes much more practical, making scripts easy, remote access, etc.
So the suggestion is, whenever is possible, to provide both interfaces:
graphical for the beginners, and the powerfull command line for the expert.

8.1. KDE, GNOME, Java or Motif?

Better then a simple graphical interface is a consistent integrated desktop.
So developer, please do not reinvent the wheel using proprietary libraries.
Today's Linux desktop is full-featured, complete APIs that makes your life

The desktops today in Linuxland are KDE and GNOME. Try to allways use one of
them, or both.

[] KDE is the most outstanding, offering a true consistent
desktop, flexible, with an extremely elegant architecture, using components
(like Microsoft's COM and COM+), intercommunication, performance etc. It is
constantly evolving, and is developed in C++. Its applications have an
familiar integrated look-and-feel, is light and mature. People say that KDE 3
is shiny diamond, ready to be used, and is my first suggestion to you.

[] GNOME also brings the integrated desktop proposal,
but it is far from the maturity and ease-of-use of KDE. From the other side,
is very well supported by the comunity, and good improvements are appearing.

Motif isn't an integrated desktop. It is a widgets library (button, scrollbar
etc), plus a window-manager. It was born commercial, is mature and popular in
commercial applications. But is considered obsolete in front of KDE and
GNOME, that integrates the desktop. Motif source code was opened by the
[] OpenGroup and because that was renamed to [http:/
/] OpenMotif.

[] Java is being used more and more for graphical
interfaces, specially in server Software, where the graphics are only helpers
to configuration and administration.

8.2. Web Interface: Access from Anywhere

Nowadays every desktop has a browser, and if your Product is a server
application, the Web Interface is the right choice, because it lets a user
administer it from anywhere. But keep in mind the security and organization
of your CGIs, because they use to be front doors for crackers. Web interface
(CGI) is completely different programming paradigm. Try to understand it
conceptually first, starting from "how a web-server works", "what is a URL",
etc, to get on this without compromising your Product's security.

8.3. Wizards and Graphical Installers

Specially for a commercial Product, your Software must provide a graphical
installer. Believe me, they are impressive in a demonstration, and CIOs love

More then just installation, a wizard helps in the initial configuration of
your Product, collects info like activation key etc, and shows the developer

A wizard should not do more than this:

 1. Ask which modules to install, experienced by the user as checkboxes.
 2. Get the necessary info to build an initial configuration (the soul) for
    the Software.
 3. Install the selected modules, that are in fact RPM files. Each checkbox
    must represent one or more RPMs, because each RPM is a indivisible
    (atomic) portion of a Software.
 4. After RPMs installation, change the configuration (soul) files (marked
    this way in the RPMs), or create some content, based on the data the user
    gave to the wizard.

So the wizard hides the RPM installation and writes initial personalization.
RPM is still responsable for putting all your software files in the correct
places. This role should never be of your installer. Think that an
experienced user (there are a lot of them in the Linux world) should be able
to reproduce your Product installation without the graphical help, using only
RPM commands. In fact, in big data centers, where people make mass
installations, a graphical installer only disturbs.

RPM provides tools that help your graphical installer interact with them,
like installation percentage viewer. Documentation for use are allways in the
RPM manual (man rpm) and in the [] Maximum RPM

9. Starting Your Software Automatically on Boot

The way Linux starts (and stops) all its subsystems is very simple and
modular. Lets you define initialization order, runlevels etc

9.1. From BIOS to Subsystems

Lets review what happens when we boot Linux:

 1. The BIOS or a bootloader (lilo, zlilo, grub, etc) loads Linux Kernel from
    disk to memory, with some parameters defined in the bootloader
    configuration. We can see this process watching the dots that appear in
    the screen. Kernel file stays in the /boot directory, and is accessed
    only at this moment.
 2. In memory, Kernel code starts to run, detecting a series of vital
    devices, disk partitions etc.
 3. On of the last things Kernel does is to mount the / (root) filesystem,
    that obrigatoriamente must contain the /etc, /sbin, /bin and /lib
 4. Immediately behind, calls the program called init (/sbin/init) and passes
    the control to him.
 5. The init command will read his configuration file (/etc/inittab) which
    defines the system runlevel, and some Shell scripts to be run.
 6. These scripts will continue the setup of system's minimal infrastructure,
    mounting other filesystems (according to /etc/fstab), activating swap
    space (virtual memory), etc.
 7. The last step, and most interesting for you, is the execution of the
    special script called /etc/rc.d/rc, which initializes the subsystems
    according to a directory structure under /etc/rc.d. The name rc comes
    from run commands.

9.2. Runlevels

The runlevels mechanism lets Linux initialize itself in different ways. And
also lets us change from one profile (runlevel) to another without rebooting.

The default runlevel is defined in /etc/inittab with a line like this:

Example 3. Default runlevel (3, in this case) line in /etc/inittab

Runlevels are numbers from 0 to 6 and each one of them is used following this

    Halts the system. Turning to this runlevel, all subsystems are softly
    deactivated before the shutdown. Don't use it in the initdefault line of
    Mono-user mode. Only vital subsystems are initialized because it is used
    for system maintenance. No user authentication (login) is required in
    this runlevel. A command line is directly returned to the user.
3, 2
    3 is used when a system is in full production. Take it as the runlevel
    your software will run. 2 is historical and is like 3, but without NFS.
    Not used. You can define it as you want, but is uncommon.
    Like 3 plus a graphical login. It is ideal for a desktop workstation. Use
    3 if the machine will be used as a server, for security and performance
    Like runlevel 0, but after complete stop, the machine is rebooted. Don't
    use it in the initdefault line of /etc/inittab.

You can switch from one runlevel to another using the telinit command. And
you can see the current runlevel and the last one with the runlevel command.
See bellow how we switched from runlevel 3 to 5.
bash# runlevel                                                               
N 3                                                                          
bash# telinit 5                                                              
bash# runlevel                                                               
3 5                                                                          

9.3. The Subsystems

Subsystems examples are a web-server, data base server, OS network layer etc.
We'll not consider a user oriented application (like a text editor) as a

Linux provides an elegant and modular way to organize the subsystems
initialization. An important fact to think is about subsystems
interdependencies. For instance, it makes no sense to start a web-server
before basic networking subsystem is active.

Subsystems are organized under the /etc/init.d and /etc/rc.d/rcN.d

    All installed Subsystems put in this directory a control program, which
    is a script that follows a simple standard described bellow. This is a
    simplified listing of this directory:
    Example 4. Subsystems installed in /etc/init.d
    bash:/etc/init.d# ls -l                                                  
    -rwxr-xr-x  1 root  root   9284 Aug 13  2001 functions                   
    -rwxr-xr-x  1 root  root   4984 Sep  5 00:18 halt                        
    -rwxr-xr-x  1 root  root   5528 Nov  5 09:44 firewall                    
    -rwxr-xr-x  1 root  root   1277 Sep  5 21:09 keytable                    
    -rwxr-xr-x  1 root  root    487 Jan 30  2001 killall                     
    -rwxr-xr-x  1 root  root   7958 Aug 15 17:20 network                     
    -rwxr-xr-x  1 root  root   1490 Sep  5 07:54 ntpd                        
    -rwxr-xr-x  1 root  root   2295 Jan 30  2001 rawdevices                  
    -rwxr-xr-x  1 root  root   1830 Aug 31 09:29 httpd                       
    -rwxr-xr-x  1 root  root   1311 Aug 15 14:18 syslog                      
/etc/rc.d/rcN.d (N is the runlevel indicator)
    These directories must contain only special symbolic links to the scripts
    in /etc/init.d. This is how it looks:
    Example 5. /etc/rc3.d listing
    bash:/etc/rc3.d# ls -l                                                             
    lrwxrwxrwx  1 root  root     18 Jan 14 11:59 K92firewall -> ../init.d/firewall     
    lrwxrwxrwx  1 root  root     17 Jan 14 11:59 S10network -> ../init.d/network       
    lrwxrwxrwx  1 root  root     16 Jan 14 11:59 S12syslog -> ../init.d/syslog         
    lrwxrwxrwx  1 root  root     18 Jan 14 11:59 S17keytable -> ../init.d/keytable     
    lrwxrwxrwx  1 root  root     20 Jan 14 11:59 S56rawdevices -> ../init.d/rawdevices 
    lrwxrwxrwx  1 root  root     16 Jan 14 11:59 S56xinetd -> ../init.d/xinetd         
    lrwxrwxrwx  1 root  root     18 Jan 14 11:59 S75httpd -> ../init.d/httpd           
    lrwxrwxrwx  1 root  root     11 Jan 13 21:45 S99local -> ../rc.local               
    Pay attention that all link names has a prefix starting with letter K
    (from Kill, to deactivate) or S (from Start, to activate), and a 2 digit
    number that defines the boot activation priority. In our example we have
    HTTPd (priority 75) starting after the Network (priority 10) subsystem.
    And the Firewalling subsystem will be deactivated (K) in this runlevel.

So to make your Software start automatically in the boot process, it must be
a subsystem, and we'll see how to do it in the following section.

9.4. Turning Your Software Into a Subsystem

Your Software's files will spread across the filesystems, but you'll want to
provide a simple and consistent interface to let the user at least start and
stop it. Subsystems architecture promotes this ease-of-use, also providing a
way (non obrigatoria) to be automatically started on system initialization.
You just have to create your /etc/init.d script following a standard to make
it functional.

Example 6. Skeleton of a Subsystem control program in /etc/init.d
# /etc/init.d/mysystem                                                           
# Subsystem file for "MySystem" server                                           
# chkconfig: 2345 95 05 (1)                                                      
# description: MySystem server daemon                                            
# processname: MySystem                                                          
# config: /etc/MySystem/mySystem.conf                                            
# config: /etc/sysconfig/mySystem                                                
# pidfile: /var/run/                                                 
# source function library                                                        
. /etc/rc.d/init.d/functions                                                     
# pull in sysconfig settings                                                     
[ -f /etc/sysconfig/mySystem ] && . /etc/sysconfig/mySystem     (2)              
.       (3)                                                                      
start() {       (4)                                                              
        echo -n $"Starting $prog:"                                               
        .       (5)                                                              
        [ "$RETVAL" = 0 ] && touch /var/lock/subsys/$prog                        
stop() {        (6)                                                              
        echo -n $"Stopping $prog:"                                               
        .       (7)                                                              
        killproc $prog -TERM                                                     
        [ "$RETVAL" = 0 ] && rm -f /var/lock/subsys/$prog                        
reload() {      (8)                                                              
        echo -n $"Reloading $prog:"                                              
        killproc $prog -HUP                                                      
case "$1" in    (9)                                                              
                if [ -f /var/lock/subsys/$prog ] ; then                          
                        # avoid race                                             
                        sleep 3                                                  
                status $prog                                                     
        *)      (10)                                                             
                echo $"Usage: $0 {start|stop|restart|reload|condrestart|status}" 
exit $RETVAL                                                                     

(1) Although these are comments, they are used by chkconfig command and must
    be present. This particular line defines that on runlevels 2,3,4 and 5,
    this subsystem will be activated with priority 95 (one of the lasts), and
    deactivated with priority 05 (one of the firsts).
(2) Besides your Software's own configuration, this script can also have a
    configuration file. The standard place for it is under /etc/sysconfig
    directory, and in our case we call it mySystem. This code line reads this
    configuration file.
(4) (6) (8)
    Your script can have many functions, but it is obrigatorios the
    implementation of start and stop methods, because they are responsible
    for (de)activation of your Subsystem on boot. Other methods can be called
    from the command line, and you can define as much as you want.
(9) After defining the script actions, the command line is analyzed and the
    requested method (action) is called.
    If this script is executed without any parameter, it will return a help
    message like this:
    bash# /etc/init.d/mysystem                                               
    Usage: mysystem {start|stop|restart|reload|condrestart|status}           
(3) (5) (7)
    Here you put your Software's specific command.

The mysystem subsystem methods you implemented will be called by users with
the service command like this example:

Example 7. service command usage
bash# service mysystem start                                                 
Starting MySystem:                      [ OK ]                               
bash# service mysystem status                                                
Subsysten MySystem is active with pid 1234                                   
bash# service mysystem reload                                                
Reloading MySystem:                     [ OK ]                               
bash# service mysystem stop                                                  
Stopping MySystem:                      [ OK ]                               

You don't have to worry about managing the symbolic links in /etc/rc.d/rcN.d.
The chkconfig command makes it for you, based on the control comments defined
in the beginning of your script.

Example 8. Using the chkconfig command
bash# chkconfig --add mysystem                                               
bash# chkconfig --del mysystem                                               

Read the chkconfig manual page to see what more it can do for you.

9.5. Packaging Your Boot Script

When you'll create the RPM, put your Subsystem script in /etc/init.d and do
not include any /etc/rc.d/rcN.d link, because it is a user decision to make
your subsystem automatic or not. If you include them and the user makes any
change, the RPM file inventory will become inconsistent.

The symbolic links must be created and removed dynamically by the
post-installation and pre-uninstallation process of your package, using the 
chkconfig command. This approach guarantees 100% package and filesystem

A. Red Hat, About the Filesystem Structure

This text was taken from [
RHL-7.2-Manual/ref-guide/ch-filesystem.html] The Official Red Hat Linux
Reference Guide

Why Share a Common Structure?

An operating system's filesystem structure is its most basic level of
organization. Almost all of the ways an operating system interacts with its
users, applications, and security model are dependent upon the way it stores
its files on a primary storage device (normally a hard disk drive). It is
crucial for a variety of reasons that users, as well as programs at the time
of installation and beyond, be able to refer to a common guideline to know
where to read and write their binary, configuration, log, and other necessary

A filesystem can be seen in terms of two different logical categories of

 1. Shareable vs. unsharable files
 2. Variable vs. static files

Shareable files are those that can be accessed by various hosts; unsharable
files are not available to any other hosts. Variable files can change at any
time without system administrator intervention (whether active or passive);
static files, such as documentation and binaries, do not change without an
action from the system administrator or an agent that the system
administrator has placed in motion to accomplish that task.

The reason for looking at files in this way has to do with the type of
permissions given to the directory that holds them. The way in which the
operating system and its users need to utilize the files determines the
directory where those files should be placed, whether the directory is
mounted read-only or read-write, and the level of access allowed on each
file. The top level of this organization (/ directory)is crucial, as the
access to the underlying directories can be restricted or security problems
may manifest themselves if the top level is left disorganized (security=
organization) or without a widely-utilized structure.

However, simply having a structure does not mean very much unless it is a 
standard. Competing structures can actually cause more problems than they
fix. Because of this, Red Hat has chosen the most widely-used filesystem
structure and extended it only slightly to accommodate special files used
within Red Hat Linux.

B. About this Document

This document must be distributed under the terms of [
copyleft/fdl.html] GNU Free Documentation License, which makes him
sufficiently free. Everybody in invited to contribute to his content and

Copyright 2002, Avi Alkalay.

This document is published in the following locations:

��*�[] Main distribution
    pt_BR] [[
    highquality.tar.gz] XML Source]
��*�[] LinuxDoc, as a HOWTO
    HighQuality-Apps-HOWTO.html] single page] [[
    Linux/docs/HOWTO/other-formats/pdf/HighQuality-Apps-HOWTO.pdf] PDF]
��*�[] Linux and Main essay (24th
    March 2002)

It was written originally in brazilian portuguese, and then translated to
english. SGML and the more-then-incredible DocBook was used, that made
possible this document being distributed in other formats, found in website.

It got ready (potuguese+english) in mid march 2002. Everything changed after
this epoch is cosmetics.

I wrote it to help commercial companies and OpenSource developers make
plug-and-play, easy-to-use software for Linux, and this way improve Linux
usability and popularity.

All concepts (from a high level perspective) described here, can be used in
any UNIX flavor, or even other OS, like Windows. Maybe some day I'll write
one of these for Windows....or Mac....

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