Saturday, February 28, 2009

RHEL4 vs RHEL5

 

Major differences between RHEL4 & RHEL5

1.Virtualization added in RHEL5

2.Package installation method changed to YUM(Yellow-dog Updater Modifier) in RHEL5

3.Redhat Clustring added in RHEL5

4.RHEL5 now supports unlimited RAM and Hard disk

5.Now varients in RHEL5 are

* Red Hat Enterprise Linux Advanced Platform (former AS)

* Red Hat Enterprise Linux (former ES) (limited to 2 CPU-s)

* Red Hat Enterprise Linux Desktop with Workstation and Multi-OS option

* Red Hat Enterprise Linux Desktop with Workstation option (former WS)

* Red Hat Enterprise Linux Desktop with Multi-OS option

* Red Hat Enterprise Linux Desktop (former Desktop)

Where as in RHEL4

* Red Hat Enterprise Linux AS for mission-critical/enterprise computer systems

* Red Hat Enterprise Linux ES for supported network servers

* Red Hat Enterprise Linux WS for technical power-user desktops or high- performance computing

* Red Hat Desktop Co for multiple deployments of single-user desktops

Minor differences

1.Newer kernel(2.6.18)

2.Newer softwares packages

3.Office removed from Server edition

4.More Hardware support

5.By default netconfig command is not there, we have to install it manually

6.SELinux improved a lot in RHEL5

7.We have to provide key at the time of installation so that we can choose to install most of the packages such as Clustring/Virtualization.

8.Some of the commands changed like in LVM

References

https://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/release-notes/package-changes.html

http://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux

Please Comment your thoughts regarding this post:-)

Wednesday, February 25, 2009

Linux Definition

Linux is a high performance, yet completely free, Unix-like operating system that is suitable for use on a wide range of computers and other products. Most distributions (i.e., versions) consist of a kernel (i.e., the core of the operating system) together with hundreds of free utilities and application programs in a coordinated package.

A narrower, and somewhat less common, meaning of the term Linux is just the kernel itself. However, when referring to just the kernel, usually the expression the Linux kernel is used.

Linux was started as a hobby in 1991 by Linus Torvalds while a student at the University of Helsinki (in Finland) because he was unhappy with the MS-DOS operating system that came with his new personal computer. He greatly preferred the much more powerful and stable UNIX that he had been using on the university's computers, but he was not able to afford the high licensing fees for any of the commercial versions then available. Today, Torvalds remains the spiritual leader of the Linux movement, and he still coordinates the development of the Linux kernel.

Rapid Growth

The use of Linux by individuals, corporations, government agencies and academic institutions around the world has been growing swiftly1, and many computer experts think that it will eventually become the most widely used operating system for many or most types of applications.

This rapid growth is a result of several factors including (1) the major advantages that Linux has over other operating systems (including over the other Unix-like operating systems and the Microsoft Windows family of operating systems), (2) the rapid progress that is being made on further improving the performance and increasing the functions of Linux, (3) the expanding array of high-quality application programs, (4) the growing awareness by individuals, businesses and other organizations throughout the world of the advantages of Linux and (5) an increase in the number of people who are familiar with installing, administering and using Linux.

Well in excess of a hundred (and possibly more than two hundred) Linux distributions have been developed by a diverse range of companies, non-commercial organizations and individuals. Some of the most popular are Red Hat, SuSE, Mandrake, Debian, Slackware, Linspire and Ubuntu. In addition to these mainstream distributions, numerous specialized distributions are also available, including those optimized for specific types of computers or applications (e.g., for use on notebook computers or routers), those for specific languages or countries (e.g., Polish or Chinese) and ultra-miniature distributions (some of which can even fit on just a single floppy disk, such as muLinux).

UNIX Clone

UNIX was originally developed by Ken Thompson in 1969 at Bell Labs, the highly innovative research arm of AT&T (the former U.S. telecommunications monopoly). Much subsequent work was carried out at the University of California at Berkeley (UCB).

Linux is a clone of UNIX; that is, it was developed to mimic the form and function of UNIX but its source code was written completely independently (i.e., none of it was copied from UNIX source code). Source code is the version of an operating system or other software as it is originally written (i.e., typed into a computer) by a human in a programming language (e.g., the C language in the case of the Linux kernel).

Linux incorporates all of the features that have made Unix-like systems the longest-lived and what many consider to be the best operating systems still in widespread use. That is, it is a multiuser (i.e., allows multiple simultaneous users), multitasking, highly flexible (with regard to configuration), inherently secure (including high resistance to viruses, spyware and other malware) and extraordinarily robust (i.e., resistance to crashing and needing rebooting) operating system. A multitasking operating system is one in which multiple programs or processes (also referred to as tasks) can execute (i.e., run) on a single computer seemingly simultaneously and without interfering with each other.

As is the case with most of the Unix-like operating systems, Linux is a highly mature (and very sophisticated) work of engineering that has been skillfully crafted by the collective efforts of thousands of the best minds in computer science. There is no planned (and little unplanned) obsolescence.

Yet Linux is much more than just a clone of another highly successful operating system. It also represents a philosophy, one which not only incorporates the simple but elegant Unix philosophy but which also has also taken it a big step further and made it a truly free operating system.

Moreover, Linux is a product of the Internet era. In contrast to proprietary (i.e., commercial) operating systems, which have been developed mostly by paid programmers employed at corporations, Linux has been developed virtually since its inception by an informal, world-wide network of unpaid (but highly skilled and motivated) volunteers who communicate via the Internet.

Advantages as Compared With Proprietary Unix-like Systems

Linux has several important advantages over the proprietary Unix-like operating systems (e.g., AIX, HP-UX and Solaris). One is that it is free software. This means that it is free both in a monetary sense and with regard to use. That is, everyone is permitted to download Linux from the Internet (or obtain it from other sources, including from friends) at no cost And everyone is also permitted to use it for any desired purpose, including studying, modifying, extending, installing on as many computers as desired, making copies as many copies as desired and redistributing. This is possible because Torvalds wisely released it under a free software license, the GNU General Public License (GPL).

Yet another advantage of Linux as compared with proprietary Unix-like operating systems is that it can generally run on a much wider range of hardware, including both system types and processor types. For example, it can run on cell phones, game machines, notebook computers, desktop computers, workstations, mainframes, supercomputers -- and even some wristwatches.

Linux is no longer the only Unix-like operating system that available under a free software license. There are several others, most notably the BSD (Berkeley Software Distribution) systems, which descended from work done on UNIX at UCB. Each of these systems, which include FreeBSD, NetBSD, NetBSD and Darwin (which is used by Mac OS X), has its own advantages and disadvantages. However, the number of users of Linux is much greater than that of the BSDs, mainly because it is easier to use, particularly for the less technically proficient.

Advantages as Compared With Microsoft Windows

Linux also has some very big advantages as compared with the Microsoft Windows family of operating systems2. The most obvious is that businesses and other organizations can save vast sums of money because there are no licensing fees nor is their any pressure for costly (and often disruptive) upgrades (so-called forced upgrades).

Linux can also cut administration and maintenance costs as compared with the Microsoft Windows operating systems because it is considerably more stable (it rarely crashes or needs rebooting) and is highly resistant to viruses and other malicious attacks.

In addition, Linux has the advantage that it can operate on older hardware that is unsuitable for newer versions of Microsoft Windows. This is because it is much more compactly written. Whereas upgrading to newer versions of Microsoft Windows generally requires costly outlays for new hardware, it is often possible to upgrade to newer versions of Linux without buying any new equipment.

The availability of the source code for Linux can also offer substantial benefits to users as compared with the closed (i.e., secret) source code for the Microsoft Windows operating systems. For example, corporations, government agencies and other organizations can monitor the code for security holes, including secret backdoors that allow others (e.g., government agencies) to access or change data. Having the source code also allows users to customize Linux to a far greater extent than can be done with closed source operating systems.

Thousands of application programs are available for Linux. Many of them offer performance and functions at least equal to those available for Microsoft Windows and other operating systems. Moreover, most of them are also free software, and many are included on the same CDROMs that contain Linux and can be installed automatically during Linux installation.

________

1For some recent examples of the spreading use of Linux, see Linux Success Stories, The Linux Information Project, updated January 2006.

2For a more complete list of Linux's advantages as compared with the Microsoft Windows systems, see 25 Reasons to Convert to Linux, The Linux Information Project, January 2006.

Monday, February 23, 2009

25 Reasons to Convert to Linux

Businesses, educational institutions, governmental agencies and other organizations around the world are converting1 their computer operating systems from Microsoft Windows to Linux at an increasing pace. They are likewise converting their application programs from commercial software to free software (also referred to as open source software). There are at least 25 reasons for this situation, including:

(1) Because it is licensed under a free software license2, Linux (as well as other free software) is available at no cost. It can be downloaded from the Internet for free, and it can be purchased in disk or box form at trivial cost. One copy can be used on as many computers as desired with no restrictions. This is in sharp contrast to Microsoft Windows, which can easily cost US$100 or more per computer.

(2) Because it is free software, Linux is also free in the sense that anybody is permitted to modify it, including its source code, in any way desired. If modified versions are not redistributed (i.e., given away or sold outside of an organization), they can be kept secret. This is also in sharp contrast to Microsoft Windows, for which modification of the software is generally not permitted. Source code is the original version of a program as it is written by a programmer using a programming language and before being converted by a compiler into a form such that its instructions can be understood directly by a computer's CPU (central processing unit); it is generally necessary to have the source code in order to be able to make changes to a program. This ability to freely experiment with and modify the source code, and to do so without disclosing the modifications to outsiders, has been a very important consideration for a number of large organizations3.

(3) High quality support for Linux is available for free on the Internet, including in newsgroups and other forums. Some people claim that this support is at least as good as that provided for proprietary (i.e., commercial) operating systems for a fee. Linux support can also be purchased on a commercial basis if desired. Among the types of support that can be required for operating systems are help with customization, assistance in installing new programs, patches to cope with new security threats and patches to fix newly discovered bugs (i.e., defects). Fortunately, the need for the last two types is relatively infrequent for Linux.

(4) There is little possibility that support for Linux will be discontinued at some future date due to planned obsolescence or for any other reason. This is because the source code will always be available to anyone who wants it, including individuals who provide support for free over the Internet and businesses which provide it for a fee. In contrast, with Microsoft Windows and other proprietary software for which the source code is usually kept secret, obtaining support becomes difficult (from both a technical and a legal point of view) if the developer decides to withdraw it (for example in order to try to force users to pay for upgrades to newer versions).

(5) There is little or no fear of major obsolescence, planned or otherwise, with Linux. This is because the UNIX architecture on which it is based has been exhaustively tested and refined for more than 35 years and has proven to be extremely efficient, robust and secure. Improvements continue at a rapid pace, but new versions remain basically compatible with the underlying UNIX architecture.

(6) There are no forced upgrades4 for Linux users. This because older versions continue to be supported (e.g., with the development of new security patches and device drivers) and because newer versions, if desired, are available for free (as is all free software) and are typically highly compatible with older versions. The developers of proprietary software, however, have strong financial incentives to engage in planned obsolescence, namely, in order to induce users of earlier versions to spend money to buy or lease new versions.

(7) Should a user decide to upgrade to a newer version of Linux, there are no licensing fees or other software costs if the user selects a free distribution (i.e., version). Moreover, the training, program modification/conversion, hardware acquisition and other costs associated with upgrading to a new version are also relatively low due to the compatibility with earlier versions.

(8) Linux has no onerous requirements for keeping track of licenses. In a company with hundreds or thousands of computers, a number of full time personnel can be required just to make sure that all of the computers in use are in compliance with the complex licensing terms of the EULAs (end user license agreements) for Microsoft Windows, Microsoft Office and other proprietary software. And for Linux users there is no fear of surprise audits by the BSA (Business Software Alliance)5, with possible severe penalties for minor license violations.

(9) Linux features superior security, including a very low rate of infection by viruses, trojans, worms, spyware and other malware. This is because UNIX and all of its descendants (including Linux) been designed from the ground up with security in mind, rather than having attempts at security tacked on as an afterthought. For example, users do not routinely use the system as the root (i.e., administrative) user, thereby protecting key system files even in the event of a break-in by a malicious intruder. Also, a robust firewall is included in major distributions and it is enabled by default. Another important factor is the free availability of the source code, which allows thousands of people around the globe to search for security vulnerabilities in it6.

(10) Linux is highly resistant to system crashes and rarely needs rebooting (i.e., restarting). This can be very important for large organizations for which even a few minutes of down time can result in a substantial cost. The reason is that Linux has been designed from the ground up to be an extremely stable and robust operating system, incorporating all that has been learned about attaining these goals from the more than 35 years of history of Unix-like operating systems.

(11) Although the number and variety of application programs for Linux are not yet as large as those available for the Microsoft Windows operating systems, an extensive selection already exists and it is continuing to increase rapidly as more and more developers start creating programs for Linux. Most Linux applications are also free software (including nearly all of the most popular ones), and many have features and performance equal or superior to those of comparable applications for use with Microsoft Windows. In fact, users often find that all the applications that they want are freely available on the Internet and that it is no longer necessary to purchase any commercial software.

(12) There is a choice of numerous distributions (several hundred) of Linux, each with its own unique set of characteristics but all basically compatible with each other. This allows users to select the versions which best meet their specific requirements. It also means that if one provider of Linux were to go out of business, there would still be many others from which to choose. Moreover, it fosters a healthy competition among them, thereby contributing to the continuous improvements in Linux's quality and performance. If the choice seems overwhelming, it is usually difficult to make a mistake by selecting one of the most popular distributions, such as Red Hat or SuSE.

(13) Linux features a high degree of flexibility of configuration, and a great deal of customization can be accomplished very easily and without having to modify the source code. For example, it is a simple matter to configure Linux during installation so that it will be optimized for use as a workstation, desktop computer, notebook computer, web server, database server or a router. Likewise, the appearance and behavior of the desktop, including icons and menus, can be configured in an almost infinite number of ways, according to user tastes or requirements. They can even be made to resemble Microsoft Windows. Should this not be enough, the ability to freely access, revise and recompile the source code allows virtually unlimited flexibility of configuration.

(14) Linux and other free software uses open format file formats. These are formats for word processing, spreadsheet and other file types that conform to industry-wide standards and which can be used by any developer of software to create compatible programs, in contrast to the closed formats commonly used by some proprietary software. This eliminates the problem of lock-in to proprietary standards, with the consequent difficulty and expense of switching to other software in the future. It allows the user to have complete control of its data, particularly in the event that at some future date the developer who originally created the software goes out of business or stops supporting its earlier software.

(15) Linux is generally faster for a given set of hardware specifications. This is due to greater optimization of the source code, including far less code bloat.

(16) Linux features a high degree of compatibility with other operating systems. For example, it can read, write, copy, erase and otherwise manipulate data that resides on Microsoft Windows partitions on the same hard disk drive (HDD), act as a Windows server for a network containing Windows clients, format disks for use with Windows, and even run Windows programs directly if necessary. In contrast, the Microsoft Windows operating systems cannot access HDD partitions that contain other operating systems, cannot format disks for other operating systems, etc.

(17) Very high ethical standards are maintained for Linux and other free software, in large part due to the very openness of their development process and the free availability of the source code. Linux has never been convicted in a Federal court of violation of U.S. antitrust laws or other crimes, nor has it had to pay penalties for the unauthorized copying of technology developed by other companies. Antitrust refers to government policy to regulate or break up monopolies in order to promote free competition and attain the benefits that such competition can provide to the economy and to society as a whole.

(18) Linux reduces the need to upgrade or replace hardware when upgrading to newer versions. This is because its code is very efficient and compact, thus allowing it to work effectively on older computers that are not suitable for the newest versions of Microsoft Windows.

(19) Linux is capable of operating on a wide variety of platforms (i.e., processor and system types), rather than just being limited to Intel-compatible processors and computers. It scales well and is well suited for use on a diverse array of equipment ranging from supercomputers to industrial robots to electronic medical equipment to cell phones (and can even run on a wristwatch).

(20) Linux is a superior choice for use in academic institutions for a number of reasons. Among them is the fact that there are no secrets (in sharp contrast to proprietary software), thereby providing students the opportunity to study how computers really work rather than to just learn how to use them. Many educators are convinced that it is far more important for students to study computer science fundamentals than to practice specific applications (such as Microsoft Word or Microsoft PowerPoint). One reason is that computer science fundamentals will still be valid many years from now, whereas the specific application programs, especially the proprietary ones that do not conform to industry-wide standards, are constantly changing and those currently in use will likely become obsolete in a few years7.

(21) For governmental agencies, Linux and other free software allows for transparency of data because it stores the data in formats consistent with industry-wide standards. This is in contrast to the proprietary, closed formats characteristic of commercial software. Such transparency is important for maintaining an effective democracy. Keeping non-secret data in standards-compliant formats allows anyone to access it without having to purchase expensive proprietary software. Also, storing secret data in standards-compliant formats is widely regarding as being more secure than keeping it in proprietary formats.

(22) With Linux and other free software there is little reason to fear the existence of backdoors, in large part because all of the source code is available for inspection. A backdoor is a secret method for obtaining remote access to a computer. There is a (not unjustified) concern by many foreign governments and corporations that back doors have been covertly inserted into proprietary software that could allow the software developer and agencies of other governments to snoop into their most confidential data.

(23) Using and advocating Linux helps foster a healthy diversity and increased competition throughout the software industry. Such competition can promote technological advance, improve performance and lower costs for open source software and proprietary software alike. Both economic theory and hundreds of years of real-world experience clearly show that monopolies have little incentive to innovate, tend to produce shoddy products, charge inflated prices and tend to corrupt the political system.

(24) Linux and other free software have not only caught up with, or some cases surpassed, their proprietary counterparts, but they are also developing at a faster pace8. This trend will accelerate as demand for such software continues to grow and more and more individuals and organizations become actively involved it its development.

(25) Linux and other free software provide the opportunity for users to contribute to the advance of software technology because the source code is freely available to study, improve, extend and redistribute. This has been fairly common, and the most notable corporate example has been IBM. In addition to giving back to the software community and being a virtue in itself, such contributions can have great public relations value.

(26) There are actually more than 25 reasons that organizations around the world are converting to Linux and other free software. A twenty-sixth reason is that with Linux, in contrast to the Microsoft Windows operating systems, there is no need to defragment HDDs. Fragmentation, which is the spreading of data into non-contiguous locations on a hard disk drive, can reduce the efficiency of data storage and slow down computer operation. Defragmentation is not difficult, but it can be an annoyance to have to do it periodically, and it is unnecessary with a well designed operating system.

There are also several situations in which businesses and other organizations might benefit from converting their operating systems from Linux to Microsoft Windows:

(1) An organization that is downsizing its operation and already has valid but unused licenses for Microsoft Windows might be able to reduce personnel costs by replacing its Linux experts with Windows administrators, as the latter can generally be hired for significantly less than Linux administrators.

(2) Were Microsoft to offer an organization an extremely low cost incentive package, including providing long-term licenses (and upgrades) at nominal cost, paying for new hardware and providing free training and support, this could be a very attractive proposition. In fact, it could be a win-win situation because of the great publicity value that could accrue to Microsoft from a company or other organization converting from Linux to Windows.

________

1For links to articles about some recent conversions to Linux, see Linux Success Stories, The Linux Information Project, December 2005.

2Linux and most other free software is licensed under the GNU General Public License (GPL). This highly successful license has been specifically designed to provide as much freedom as possible for users, both in a monetary sense and with regard to use. The GPL allows anyone to freely access the source code for software licensed under it in order to study, use, modify, extend and redistribute it as desired with only the very minimal requirements of always making the source code for redistributions of modified versions freely available and including a copy of the GPL text with the software.

It should be pointed out in this context that there are also compiled versions of Linux that are not free in a monetary sense, which is also permitted by the GPL. However, they are not necessarily any better than the free versions. Businesses and other organizations have the option of selecting whether they want a free or non-free version, and if they select the former, they have many distributions from which to choose.

3This is a major reason that Google selected Linux, according to industry sources.

4Forced upgrades occur when the developer stops supporting an earlier version of its software, and thus security patches for the newest viruses and other malware become unavailable and drivers are no longer developed for new hardware. Thus, many users have little choice but to purchase or lease the newest version. This can be very costly because of the licensing fees or other payments that must be made to the software developer or vendor. Further adding to the cost is the frequent need to purchase new hardware to accommodate the bloated code in the new versions of the software. In addition, installing the new software and troubleshooting it can be very time-consuming for system administrators and disruptive for business operations.

5 The highly controversial BSA is an international organization that was set up by major developers of proprietary software. It is empowered by the mandatory EULAs accompanying such software to make surprise audits on users and to levy heavy fines in the event of violation.

6 The is the same principle that is employed by public key cryptography (PKC), which is the most secure form of data encryption. And it is the opposite of the approach used with proprietary software, for which an attempt is made to keep the source code secret as a means of hiding security vulnerabilities.

7 For a more detailed look at the reasons that Linux is the best choice for use in educational institutions, see Linux and Education, The Linux Information Project, March 2004.

8Numerous examples could be cited. For example, Apache is the dominant web server, hosting far more web sites than any of its proprietary counterparts. Likewise, it is widely agreed that the free Firefox web browser is far more advanced (in terms of security, usability, etc.) than Microsoft's Internet Explorer. Internet Explorer is free in a monetary sense, but it is not free software because the source code is not made available and users are prohibited from modifying it.

Saturday, February 21, 2009

Mounting CD/DVD-ROM

Because a DVD-ROM drive is a "removable" storage device, you may find that you can't access a DVD after inserting it. You have to manually enter a command to "mount" the DVD-ROM drive before you can access it. On my system, the DVD-ROM drive is the first drive (master) on the secondary IDE channel. As a result, the command I use to mount my DVD-ROM drive is:

mount -t iso9660 /dev/hdc /cdrom

I know this looks a little cryptic at first but it's really quite simple.

  • mount makes a device part of the file system.
  • -t iso9660 specifies the format of the file system being mounted. (The iso9660 is the standard format for data CDs (and most DVDs) but would be msdos if we were mounting a floppy drive with a DOS-formatted floppy in it.)
  • /dev/hdc is the path to the DVD-ROM drive's device driver file. The c in the hdc indicates the first hard-drive on the secondary IDE channel. With SCSI hard-drives the third hard-drive would be sdc.)
  • /cdrom is the directory to "map" the device to in the file system so it can be accessed. This has to be an existing directory but it can actually be any directory you want. You could use the mkdir command to create a directory called "shiny-spinning-thing" off the root of the file system and replace /cdrom with /shiny-spinning-thing in the above command if you wanted to.

Using the above mount command simply maps the DVD-ROM drive to the /cdrom directory (which was created during the installation). The directory a device gets mapped to is called the "mount point". As such, in order to access the files on the DVD-ROM once it's been mounted you just go to the mount point its been mapped to by entering

cd /cdrom

and use the ls command to view a list of the files on it. If you get an error along the lines of:

kernel does not recognize /dev/hdc

it's likely your DVD-ROM drive is connected as the slave on the primary IDE channel (i.e. it's /dev/hdb).

Tip: The mount command to access a DOS formatted floppy in the first floppy drive would be:

mount -t msdos /dev/fd0 /floppy

Note that Debian creates the /cdrom and /floppy directories off of the root of the file system during the installation. Other Linux distros and UNIX more often put them under the /mnt directory. In order to mount a DVD drive on these systems you simply change the target directory in the command:

mount -t iso9660 /dev/hdc /mnt/cdrom

Just as you mounted the removable disk to access it, you have to unmount it when you are done. Pressing the eject button on the DVD drive won't open the tray until you do unmount the drive. For this you just use the umount and specify its mount point in the file system:

umount /mnt/cdrom

Thursday, February 19, 2009

What happens When you boot your system?

When you boot your system
services and processes are started via shell scripts.
All shell scripts that could possibly get executed when you boot your system are stored in the /etc/init.d sub-directory. (Note that even directories can have periods in their name.)
In order to understand the boot-up process you have to be familier with runlevels.
Linux/UNIX systems can be set to run in different modes of functionality.
They can operate in a single-user mode, such as in the case of strictly being a "workstation" (desktop PC), or they can run in multi-user mode to operate as a server.
Each runlevel is identified by a single-digit number.
The runlevels worth remembering are:
0 - shut down the system
1 - single-user mode
2 - 5 multi-user mode
6 - reboot
Recall that any shell script that might be run at system startup are stored in the /etc/init.d directory. All these different runlevels do is run a different set of the scripts stored in this directory at startup. When the system boots up the first startup file it reads is the /etc/inittab text file. This file basically tells the system what the default run level is via the line

id:2:initdefault:

This is the line in the /etc/inittab file you need to edit if you want to change the default run level. Most distros default to runlevel 3 which is not a secure thing to do for Internet servers.
Recall that any shell script that might be run at system startup are stored in the /etc/init.d directory.
All these different runlevels do is run a different set of the scripts.
These different scripts are stored in/etc/init.d directory.
When the system boots up the first startup file it reads is the /etc/inittab text file.
This file basically tells the system what the default run level is via the line

id:2:initdefault:

This is the line in the /etc/inittab file you need to edit if you want to change the default run level. Most distros default to runlevel 3 which is not a secure thing to do for Internet servers.
Each runlevel also has it's own subdirectory under the /etc directory, and the subdirectory name contains the runlevel number. The naming convention for these subdirectories is:

rc2.d

with the runlevel represented as the number (in blue) above.
These runlevel subdirectories don't actually contain any scripts. Rather, they all contain symbolic links to the scripts in the /etc/init.d subdirectory.
This is so all the different runlevels can share common scripts eliminating the need to have multiple copies of the same script in multiple runlevel subdirectories. (That's why any script that could be run at startup is saved to the /etc/init.d subdirectory.) Here's a diagram of the process. Note that special single-user runlevel is always run at bootup to set up the basic system. The higher runlevel scripts are additionally run to provide the multi-user/server functionality.

Linux startup process

There's another reason for using links in the individual runlevel subdirectories. These links use a special naming convention to indicate when they should be run and in what order they should be run. Links that have names starting with an upper-case S are called when the runlevel is entered (starting). Links that have names starting with an upper-case K are called when the runlevel is exited to Kill services. (Due to this naming convention it is more common to simply rename a symbolic link so that it begins with a dash (-) or underscore (_) rather than delete them when customizing a runlevel.)
The directories for the multi-user runlevels only contain links that start with an S. Only those run levels that deal with shutting down or restricting functionality of the system (0, 1, 6) have links that start with a K to kill services.
The numbers after the leading S or K determine the order in which the links are called, lowest number first. This is important for process dependancy reasons. For example, you wouldn't want to start up the Samba service if the networking service wasn't already running. For example:

S20thisservice
S30thatservice
S40anotherservice
S80yetanother

Tuesday, February 17, 2009

Driver Definition - What is a Driver?

A driver is a small piece of software that tells the operating system and other software how to communicate with a piece of hardware.

For example, all printers come accompanied with drivers to install that tell the operating system exactly how to print information on the page. Sound card drivers tell your software exactly how to translate data into audio signals that the card can output to a set of speakers. The same applies to video cards, keyboards, monitors, etc.

The drivers for each piece of hardware in your computer are centrally managed from Device Manager, available in all versions of Microsoft Windows.

Saturday, February 14, 2009

A Blog on Linux on Valentines Day…… Crazy Idea :)

Hello everyone…………. Hmm I’m not getting from where I have to start my first post. I’m an ordinary guy working in an IT company so for me, my cubical is like home to me

Coz I’ll be spending most of the time of my day in there :) .

After seeing sooo many people blogging around me for long time, finally I have

decided to maintain a blog of my own. As of now I’m new to both LOVE (maintaining a GF) and Blogging (Maintaining a blog), my friends and colleagues insisted me to start any one of the two today. I thought it takes less time to maintain a blog than to maintain a Girl Friend… so choose this one. (Sorry gals it was just a Joke :) :) :) ).

In this blog I mainly concentrate on all the aspects of Linux and Virtualization. The purpose of this blog is to share my knowledge and experiences with you and I hope this blog will be useful for you in knowing about Linux. I’ll try to update this blog regularly, if anyone have any queries or suggestions regarding any posts, please leave a comment and I’ll get back to you as early as possible.

If you like my blog please follow or get subscribed to it  :).

 
Things You Should Know About Linux !!!