HTCIA - 2003 - Introduction to Linux

Table of Contents

• What is Linux?

• Brief Introduction to Linux
  • The Boot Sequence
  • Command-Line Interface
  • Graphical User Interface
  • Login
  • Fundamental Applications

• Linux Architecture

• The Kernel
  • Modules
  • Re-compiling

• Filesystem Information
  • Hierarchical
  • Components
    • file
    • directory
    • links
    • special files
    • inodes
  • Everything is a file
  • Mounts (disk, NFS)
  • Filesystem Types
  • Common UNIX Directory Structure

• Users and Groups
  • User == uid
  • Group == gid
  • authentication
  • /etc/passwd
  • /etc/group
  • Permissions

• Distributions

• Comparison to Windows
  • Software Instalaltion / Maintenance
  • Registry
  • Services

• A Brief history of Time...

• Resources
  • LSU's redhat.lsu.edu
  • Linux Documentation
  • Other Important Resources

What is Linux?

Operating System

• kernel
• modules (drivers)
• filesystem(s)
• CLI and/or GUI
• GNU
• collection of utilities, applications, and services

Flexible System

• Distributions
• Multi-platform / Multi-architecture
• Desktop / Server

Why?

• It's free (most distributions)
• It's mature (it will do whatever you want and is stable)
• It's secure (and is maintainable)
• It's efficient (extend life of hardware)
• It's growing in popularity (15% of server market, 8% of desktop)
• It's interoperable (among other major OSs)
• Many choices of distributions, applications, and services

Return to top of page

Brief Introduction to Linux

The Boot Sequence

Restart the Windows system and choose Red Hat Linux instead of the default Windows

After the BIOS checks the hardware, it reads the Master Boot Record (MBR) from the first Hard Drive (HD) in the machine. GRUB, the GRand Unified Bootloader, is the preferred boot loader for Red Hat Linux. LILO, the LInux LOader, was the dominant predecessor to GRUB. When booting, the first thing you see after the BIOS is done is the GRUB menu. The menu allows you to select which operating system, Windows or Linux including kernel versions (SMP, UP, hand-compiled, etc.), to load. GRUB then initially loads and transfers control to the operating system.

For Linux, GRUB then loads the kernel which immediately starts detecting hardware and loading drivers. As soon as the kernel is fully loaded and initialized it starts the init process which displays the Welcome to Red Hat Linux message. Newer versions of init allow you to press "I" to enter an interactive mode, letting you selectively choose which services to start (or not start), similar to jumping into Extension Manager on a Macintosh system, or pressing F5 at boot on a DOS/Windows system.

As each service is started, its name is echoed on the left of the screen. If the service starts successfully, a message [  OK  ] appears on the right. If the service failed to start successfully, a message [FAILED] appears on the right.

Once all of the services have loaded (init has finished) the login prompt appears.

Command-Line Interface

Like the family of UNIX-like operating systems before it, the primary interface to Linux is the command-line. The command-line is still the most powerful and direct way to interact with the system. Shells are how command-line interfaces are implemented in Linux. Each shell has varying capabilities and features and the user should choose the shell that best suits their needs. The shell is simply an application running on top of the kernel and provides a powerful interface to the system.

Graphical User Interface

Graphics were not a native part of the original UNIX environment. The X-Window system is a client / server based protocol for displaying graphics. The system where the graphics are viewed runs an X-Window server. The system where the graphics are generated runs an X-Window client. They can be the same machine (just like a traditional Windows environment) or they can be two different systems an Internet apart (as implemented by Remote Desktop in Windows).

The X-Window system grew out of the Athena project at MIT.

Most commercial UNIXes provide proprietary X-Window servers for their version of UNIX. The free UNIX world has concentrated on XFree86 as the primary X-Window server (despite the "86" referring to the Intel x86 architecture, XFree86 has been ported to numerous architectures).

Besides the X-Window server, the X-Window protocol requires a window manager to display the windows. The window manager controls the overall look and feel of the windowing environment (minimize, maximize, close, scrooll-bars, borders, colors, title bars, etc.). The two currently dominant window managers in the free UNIX world are KDE and GNOME (but there are dozens of others to choose from like fluxbox, fvwm, etc.). KDE was developed by an independent group of programmers and is a loose approximation of the commercial CDE environment. GNOME development was started under a free license by Red Hat. Red Hat, as of their v8.0 release, has taken a giant step forward, according to some people. They have developed a common, consistant, interface called Blue Curve. Regardless of whether the user picks GNOME or KDE, the GUI looks 95% identical.

Running an X-Window server and window manager on a machine provides a familiar graphical interface. Luckily, in the free UNIX world the user has a choice (kind of).

The computer mouse was invented in 1968 by Douglas Englebart while at Stanford University.

Login

The login process authenticates the user and is the mechanism allowing them to interact with the system, if authentication is successful. The user's login ID will subsequently be used to determine authorization later on.

The initial login screen is a text screen, however, since these systems are configured to automatically start X-Windows, they quickly switch to a graphical login. The login screen allows users to chose which style of desktop they want, provides for the ability to restart or shutdown the computer, and allows users to change their default language.

In order to interact with the machine users must have a valid ID and password. As you will learn, this ID and password may or may not be stored on the local machine. The name space (users and groups) can be maintained on another server somewhere on the network.

Login as student

Fundamental Applications

The following applications provide a basic interface to the system. Since they will be used frequently in this class, we take time to introduce them now.

Console

Since the command-line interface (CLI) is the primary interface to a UNIX system, having access to the command-line while in a GUI is critical. A console application serves this purpose, similar to command.com or cmd.exe in Windows. Each desktop manager typically has its own version of a console application. Under GNOME the application is gnome-terminal and under KDE the application is konsole.

Open konsole by clicking on the LCD-screen icon

Other things you can do:
• resize the window: the default is 80x24
• right-click | Settings: font, schemas, configure konsole
• tabs or multiple windows

Konqueror

Konqueror started as a browser application, but has grown in scope (like Internet Explorer in Windows) to include many other functions such as file management, application framework, and general I/O application.

Open konqueror by double-clicking the "Home" icon

Other things you can do:
• Profiles: web browser, filemanager, etc.
• As an application framework: console
• General I/O: Audio CD, network access
• Tabs: multiple windows in one

Return to top of page

Linux Architecture

Linus Torvalds set out to write his own operating system as a result of his dissatisfaction with Minix and its creator, Andy Tanenbaum.

In 1987 Andrew S. Tanenbaum -- a professor in Amsterdam, the Netherlands -- created MINIX as a teaching aid for his Operating Systems class.

Linus soon realized that the project was a huge undertaking so he decided to start by writing a kernel, utilizing the tools being developed by the GNU project (shells, compilers, editors, etc.).

In 1984 and 1985, Richard M. Stallman started GNU and founded the Free Software Foundation.

As Linus made early progess, he made copies of Linux available to other people via the Internet. Suggestions for improvement and fixes for bugs started flooding in and full Internet collaboration on the kernel project started.

Today, Linux is composed of a kernel and other supporting applications layered around it. The development of Linux would not have been possible without the software provided by the GNU project and developers from all over the world and the use of the Internet.

Return to top of page

Kernel

The kernel is at the heart of the operating system. It controls all of the hardware in the machine and provides coordinated access to that hardware. It starts and stops processes (programs), allocates memory to programs, manages virtual memory (swap), associates drivers with hardware, controls interrupts (hardware and software), provides TCP/IP network capabilities, and provides a multi-tasking environment by cycling through all running applications.

Because of its modularity and open source nature, anybody can add functionality to the kernel. However, Linus has final authority over what is officially added to a kernel release. The "official" kernel can be found at http://www.kernel.org/.

Kernels are released in "stable" and "development" versions. Kernel versions are numbered so that the second part (between the first and second decimal point) determines if the kernel is stable or development. Stable versions are always even numbered: 2.0.7-3, 2.2.12, and 2.4.9-13 are all stable kernels. Development versions are always odd numbered: 2.1.4, 2.3.5-2, and 2.5.1 are all development kernels. After freezing enhancements and bug fixes to a stable kernel, a new development kernel tree starts. When a development tree becomes stable enough, it gets renumbered as a new stable release.

A final benefit of Linus' open source, modular kernel is the user's ability to remove or include only the parts of the kernel they want or need. This has resulted in full blown router engines and firewalls that fit on a floppy disk. Also, as a system administrator for a server, you can fully optimize the kernel code for what your server does.

Modules

The Linux kernel includes device drivers allowing it to communicate with various hardware and virtual devices. Initially all drivers were built into the kernel. Modern kernels now support the dynamic loading and unloading of device drivers by the use of modules. This is similar to Windows but significantly different.

Insert a floppy disk, then in the console window, type lsmod, type mount /mnt/floppy, then type lsmod again. Notice the addition of the fat and vfat modules.

Recompiling

Besides device drivers, the Linux kernel may contain various other functionality. The administrator of a system can choose to have these components either be an integral part of the kernel, loadable as a module, or excluded entirely. The process of selecting these components and regenerating a new kernel and / or modules is called recompiling the kernel, translating the human readable source code into machine understandable code. Invoking (running) a newly compiled kernel is one of the few times a Linux system needs to be rebooted.

Return to top of page

Filesystem Information

Hierarchical

UNIX uses a true hierarchical filesystem, which is very different from the Windows structure. The UNIX filesystem starts at a single point: / (pronounced "root" [not to be confused with the root user or /root, the root user's home directory]).

Components

The UNIX filesystem is comprised of several parts:

file

Files contain data, either for users or the system. The data can be text or binary.

directory

A directory is a collection of files, like a folder in Windows.

links

Links are pointers to existing files or directories, similar to shorcuts in Windows.

special file

Special files include:
• sockets
• pipes
• character
• block
These special files are used by the kernel and running applications to control the flow of data in the system.

i-nodes

i-nodes contain all of the metadata about any of the above components such as the date and time they were created, who created them, when they were accessed, their size, where they are stored, etc.

Everything is a file

One of the most difficult thing to grasp about the UNIX filesystem is that everything in UNIX is treated as a file. EVERYTHING! This includes directories, special files, partitions, disk drives, mice, serial ports, printers, displays, network interfaces, etc. Everything!

Open the Konqueror window and navigate to the /dev directory

Mounts (disk, NFS)

The UNIX filesystem is analogous to a tree. The root, /, is the root of the hypothetical tree and directories are the branches. Real trees can have new branches grafted onto them. Similarly, the UNIX filesystem can have other filesystems grafted onto them. As such, the UNIX filesystem can be comprised of many different filesystems.

The mount command is used to perform the grafting or to show existing grafts.

In the console window, type mount to see a list of currently mounted (grafted) filesystems. Notice the /mnt/floppy item you mounted (grafted) earlier.

Even if several hard disks are in a system, all access to the disks will be through /. This is unlike the Windows world where separate disks/partitions are accessed via different drive letters (C:, D:, etc.).

Filesystem Types

Linux systems can access many different physical filesystem types. Recall that Windows can use either FAT, FAT32, or NTFS. The default physical filesystem under Red Hat Linux is ext2 or the newer ext3. Additionally, Linux can access many different physical filesystems.

Common UNIX Directory Structure

The UNIX filesystem is traditionally divided into a number of common directories. The series of directories, seperated by a slash ("/") and often ending with a file, is called a path. Each directory path has a specific function. Below is a table listing some of the common directories and their normal use:

Path	Windows Equivalent	Description
/	C:	The start of the filesystem (or main drive in Windows)
/home	My Documents or Profiles or D:	User's private files
/usr or /usr/local	Program Files	Installed software
/dev	Windows or Windows\System	Device drivers
/etc	The Registry or .ini files	Application and operating system configuration information
/tmp	Windows\Temp	Temporary system files
/bin	Windows	System executable files

Open the Konqueror window and navigate to the /etc/sysconfig/network-scripts/ifcfg-eth0 file

Like Windows, Linux can access remote filesystems across the network. Windows calls this "Sharing". UNIX calls it NFS (Network File System). Again similar to Windows, which can both share and access Shares, Linux using NFS can export and mount filesystems. Samba is a software project to allow Linux (and other UNIX-like OSes) to mount and export filesystems the same way a Windows system "shares" directories.

Return to top of page

Users and Groups

In Linux a user is identified by their username. Linux usernames are similar to Windows logon IDs. Linux maintains a list of users, called a name space, either locally on each individual machine or across the network via NIS for a group of machines.

User == uid

Each user on a Linux machine has a unique username that corresponds to a unique uid. The uid is a numeric value used to determine if the user is authorized to perform a particular task. It is important to note that the Linux operating system is concerned only with the numeric value of the uid and not the text that describes the user's name.

Group == gid

Groups are also associated with unique numbers called gids. Users may belong to more than one group, although they can be associated with only one group at a time. Groups may have zero, one, or more members. Like uids, the Linux operating system is concerned only with the numeric value of the gid and not the textual name of the group.

Authentication

Each time a user tries to login, Linux attempts to authenticate that user against a name space. The login process prompts for a username and, usually, a password. Linux then encrypts the entered password and checks to see if the encrypted text matches the stored version of the user's encrypted password. If there is a match, the user is authenticated; if not, the user is denied access.

/etc/passwd

Linux keeps a table of information about each user in the file /etc/password. Each line of this file represents a different user and has seven different fields:
1. username
2. encrypted password
3. uid
4. gid
5. identification
6. path to home directory
7. initial program

In the console window, type cat /etc/passwd

/etc/group

Linuux maintains a list of groups as a table in the file /etc/group. This file has three fields:
1. group name
2. gid
3. list of usernames

In the Konqueror window navigate to the /etc/group file and open it

Permissions

Since Linux treats everything as a file, learning about file permisions is very important. Linux allows access to a file based upon affiliation as defined by user, group, and other. The possible permissions for each affiliation are read (r), write (w), and execute (x). Linux actually uses one bit to indicate these three permissions for each of the three affiliations.

In the console window, type ls -l /etc/passwd

Your output should look similar to this:

-rw-r--r--    1 root     root         1755 Mar 10 13:48 /etc/passwd

The first dash is a flag specifying the type of file. The next three characters are the permissions for the user who owns the file. The next three characters are the permissions for the group associated with the file. The final three characters are the permissions for any user who is not the owner of the file nor belongs to the group associated with the file.

Each of the permissions has a different meaning depending on whether it is set for a file or a directory:

Type	r	w	x
File	Can read the file	Can change the contents or delete the file	Can execute the file
Directory	Can see a listing of files in the directory	Can create new files in the directory or delete files from the directory	Can change into that directory

Return to top of page

Distributions

The kernel is the basis of a Linux system. Fairly early in the evolution of Linux, people other than Linus started putting the kernel together with various software packages. The different combinations of kernel and software packages became known as a distribution. All of the distributions start with one of the Linux kernels and then combine various other software to produce a usable system.

This early development resulted in a large number of Linux distributions being created. Suddenly, one version of Linux was not required to be everything to everyone, unlike with Windows. This flexibility allowed Linux to grow and prosper.

Early distributions included (but weren't limited to) Slackware, Yggdrasil, and Debian. The primary distribution in the U.S. today is Red Hat. The primary distribution in Europe today is SuSE. Other major distributions include (but again aren't limited to) Mandrake (which is based on Red Hat), Gentoo, YellowDog (for PPCs), Debian, and Slackware. Visit Distro Watch for a nearly complete list of Linux distributions.

Another current trend is putting a Linux system on a CD. This approach has the benefit of not destroying data on the systems hard drive and allows users to experiment with Linux without modifying their current installations. One implication is that you can boot a machine and have a working system (from the CD) that can help you recover or analyze data on the local hard drive. The primary Linux-on-CD distribution today is Knoppix.

It is important to note that the kernel is completely seperate from any distribution. Kernel development and releases continue on their own schedule. When a new kernel is released, distributions take time to get their configuration and tools working with the new kernel (by adding and removing code to and from the kernel).

Even though most (usually all) of the software in a distribution of software is free (money-wise), distributors still sell Linux. Purchasing Linux usually provides you with printed manuals (books), installtion support, and sometimes commercial software. However, almost every distribution allows you to download a CD ISO image.

It should be noted that there are three major Windows "clones," namely Lycoris, Lindows, and Xandros. Their goal is to create a Linux system that looks and acts just like a Windows system.

Return to top of page

Comparison to Windows

The combination of the Linux kernel, GNU (and other) tools, and a modern X-Window server with GNOME or KDE provides an experience similar to today's Windows systems. Many Windows-like capabilities have been replicated in the modern Linux/X-Window environment like desktop icons, file browsing, drag-and-drop, GUI configuration tools, etc.

New Software Installation / Maintenance

Modern Windows systems usually install software to a common place: "Program Files". Linux systems put most software under /usr, either in /usr/bin or /usr/local/bin. Windows applications typically use a consistant install program like "Wise" or "InstallShield" so that most software installations look and feel the same.

Red Hat Linux uses RPM, the Red Hat Package Manager. Software on Red Hat systems is bundled in a "package." A package includes all of the binary data for the software, rules detailing prerequisite software, and possibly scripts to be executed before, during, and after installation.

Packages come in either source or binary form. Binary packages are architecture-specific (i.e. Intel uses i386, i486, i586, and i686; PowerPC uses ppc, SPARC uses sparc, etc.) and can only be installed on the appropriate hardware. The package manger command is rpm, and it sports a wide array of features. Here is a table of commonly used rpm commands and what they do:

Command	Description
rpm -qa	List all installed packages
rpm -qi package	Show information about an installed package
rpm -qil package	Show information about an installed package including all files in the package
rpm -qip package	Show information about an uninstalled package
rpm -qilp package	Show information about an uninstalled package including all files in the package
rpm -qf file	Determine which installed package a file belongs to
rpm -Uvh package	Install or upgrade a package
rpm -Fvh package	Update an installed package (freshen)
rpm -e package	Uninstall a package
rpm --verify package	Verify the integrity of an installed package
rpm --help	Get help on rpm (there are many more options available)

Install the tuxracer game

After a Windows installation, the first thing you need to do is update the system. Similarly, after a Linux OS installation you need to apply critical updates. Windows provides a website to obtain updated software. Red Hat also provides a website to do the same. LSU mirrors updates locally for several versions of Red Hat at http://redhat.lsu.edu/.

Registry

While Windows maintains a binary "Registry" to hold system and installed software settings, Linux puts everything in files, most of which are text. As shown in our table above, the /etc directory tree in Linux contains most of the files to configure your system and installed software.

As an example, to change the DNS servers on a Windows machine you have to get to the Network Control Panel applet and navigate through several panels, make the change, and (depending on your Windows version) reboot the machine. In Red Hat, all you have to do is edit the /etc/resolv.conf file.

Use Konqueror to edit the /etc/resolv.conf file

Services

Older Windows v9x use the System Tray to emulate "services" running on the system. Windows NT and above actually implement true services which can be controlled using Server Manager. Red Hat controls services via scripts found in /etc/rc.d/init.d/ and automatically starts or stops these services based on entries in the /etc/rc.d/rcx.d/ directories.

Use Konqueror to navigate to the /etc/rc.d/rc3.d/ directory

Return to top of page

A Brief History of Time...

UNIX

UNICS (UNiplexed Information and Computing Service) was created at AT&T Bell Laboratories in the fall of 1969 by Ken Thompson (and a few other researchers) after failed attempts working on the MULTICS project. Their work was the result of trying to create a multi-user time-sharing operating system. They started licensing their code to universities in 1974.

"...the number of UNIX installations has grown to 10, with more expected..." - Dennis Ritchie and Ken Thompson, June 1972

While on sabbatical in 1978, Ken Thompson helped student Bill Joy write the Berkeley Software Distribution (BSD). AT&T's UNIX became a stable commercial product while BSD's became a research project and teaching tool. This split between AT&T UNIX and BSD UNIX remains today. However, most commercial and free UNIXes are a blend of both.

It should be pointed out that the first Operating System that Microsoft sold was Xenix, in August of 1980, their UNIX-like OS that ran on both Intel and Motorola processors. Microsoft did not sell their first DOS until 1981 and 1982.

Free UNIX-like OSes

In 1984 and 1985, Richard M. Stallman started GNU and founded the Free Software Foundation. His goal was to create a totally free (libris) UNIX-like operating system. Though many supporting tools were written quickly, the kernel, called the Hurd, was many years in coming. In 1985 Carnegie Mellon created the MACH kernel based on a micro-kernel architecture.

In 1987 Andrew S. Tanenbaum -- a professor in Amsterdam, the Netherlands -- created MINIX as a teaching aid for his Operating Systems class. This was one of the first complete implementations of a "free" UNIX-like OS, although you had to buy Dr. Tanenbaum's book to get it.

Starting in the early 1990's there were several projects that branched off of BSD. The NetBSD and FreeBSD projects both started in 1993. In 1995 a disgruntled NetBSD contributor left and founded the OpenBSD project.

History of GNU/Linux

As part of his schooling at a university in Helsinki, Finland, Linus Torvalds started to write a new filesystem for MINIX. In 1991, after several disputes with Dr. Tanenbaum, Linus decided to write his own operating system. Over time, with the help of others on the Internet, Linus got parts of Stallman's GNU software (shells, compilers, etc.) working with his kernel.

From: torvalds@klaava.Helsinki.FI (Linus Benedict Torvalds)
Newsgroups: comp.os.minix
Subject: What would you like to see most in minix?
Summary: small poll for my new operating system
Message-ID: <1991Aug25.205708.9541@klaava.Helsinki.FI>
Date: 25 Aug 91 20:57:08 GMT
Organization: University of Helsinki

Hello everybody out there using minix -
I'm doing a (free) operating system (just a hobby, won't be big and
professional like gnu) for 386(486) AT clones. This has been brewing
since april, and is starting to get ready. I'd like any feedback on
things people like/dislike in minix, as my OS resembles it somewhat
(same physical layout of the file-system (due to practical reasons)
among other things).
I've currently ported bash(1.08) and gcc(1.40), and things seem to work.
This implies that I'll get something practical within a few months, and
I'd like to know what features most people would want. Any suggestions
are welcome, but I won't promise I'll implement them :-)
Linus (torvalds@kruuna.helsinki.fi)
PS. Yes - it's free of any minix code, and it has a multi-threaded fs.
It is NOT protable (uses 386 task switching etc), and it probably never
will support anything other than AT-harddisks, as that's all I have :-(.

In 1994 he released the kernel v1.0 and the associated GNU tools as the first widespread public GNU/Linux distribution.

Return to top of page

Resources

Many resources exist at LSU and across the Internet to make using GNU/Linux easier. Here are just a few.

LSU's redhat.lsu.edu

OCS's High Performance Computing group maintains the system redhat.lsu.edu to support on-campus users. This system sports a number of features:

• ISO images so that you can make your own CDs
• Supports on-campus NFS installs
• Supports HTTP installs
• Supports on-campus NFS updates
• Automatic e-mail notification of updates
• Provides DOC CDs
• Everything is web browsable
• Provides installation and updating instructions
• Includes several Redhat versions

Linux Documentation

The Linux Documentation Project has been around a long time and is one of the most valuable resources around. This project includes many different documents that will ease the use of GNU/Linux. Here are a few examples:

• The HOWTOs - These are wonderful documents that explain how to do just about everything you could want.
• The Mini HOWTOs - These are smaller documents that often cover new topics or very simple concepts.
• The Unmaintained HOWTOs - These are older HowTos that no one is keeping current but they often have useful information.
• Redhat - Along with selling the dominant U. S. GNU/Linux distribution, they provide numerous guides, patches, and forums.

Other Important Resources

• Freshmeat - daily list of updated software and software repository
• rpmfind - source for rpms
• Security Focus
• SlashDot - news for geeks

Return to top of page

---

The statements and opinions included in these pages are those of Isaac W. Traxler, Allen B. Gordon and Brian D. Ropers-Huilman only. Any statements and opinions included in these pages are not those of Louisiana State University or the LSU Board of Supervisors.
© 1998-2003 Brian D. Ropers-Huilman, Allen B. Gordon, Isaac W. Traxler, Greg Brignac

---