Friday, June 8, 2007


Falling down is not defeat. Defeat is when you refuse to get up.

All the best for your all future assignments in your life and happy LINUX.

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Trademarks: Red Hat is a registered trademark of Red Hat, Inc. Linux is a registered trademark of Linus Torvalds. All other trademarks are the property of their respective owners.

UNIX History

First version created in Bell Labs - 1969

AT&T licenses source code for low cost

  • Trademarks UNIX name, "UNIX" name closely held
  • Licensees must create new name for their operating systems
  • Many UNIX "flavors" emerge

GNU Project / FSF

GNU Project started in 1984

r Goal: Create a "free" UNIX clone

r By 1990, nearly all required userspace applications created

r gcc, emacs, etc

l Free Software Foundation

r Non-profit organization that manages the GNU project

Linux Origins

l Linus Torvalds

m Finnish college student in 1991

m Created Linux kernel

l Linux kernel + GNU applications = complete, free, UNIX-like OS

Recommended Hardware Specifications

l Pentium Pro or better with 256 MB RAM or

l 64-bit Intel/AMD with 512 MB RAM

l 2-6 GB disk space

l Bootable CD

l Other processor architectures supported

l Itanium 2, IBM Power, IBM Mainframe

Virtual Consoles

l Multiple non-GUI logins are possible through the use of virtual consoles

l There are by default 6 available virtual consoles

l Available through Ctrl-Alt-F[1-6]

l If X is running, it is available as Ctrl-Alt-F7

The Xorg GUI Framework

l Modern, free implementation of X11

l Highly flexible framework for displaying graphical applications and environments

l Completely network-transparent client/server architecture

l System can be configured to present a graphical login screen on Ctrl-Alt-F7

The Xorg Graphical Environments

l Collections of applications that provide a graphical working environment with a consistent look-and-feel

m GNOME - The default desktop environment

m KDE - Environment based on the Qt toolkit

Starting Xorg

l Nothing needed if system boots to a graphical login. Just authenticate.

l If system boots to a virtual console login, Xorg must be started manually

m Run startx to manually start Xorg

Getting Help

Don't try to memorize everything!

Many levels of help

m whatis

m command --help

m man and info

m /usr/share/doc/

m Red Hat documentation

Extended Documentation

l The /usr/share/doc directory

m Subdirectories for most installed packages

m Location of docs that don't fit elsewhere

r Example configuration files

r Html/pdf/ps documentation

r License details

Some Important Directories

The home directories

m /root, /home/username

l The bin directories

m /bin, /usr/bin, /usr/local/bin

m /sbin, /usr/sbin, /usr/local/sbin

l Foreign filesystem mountpoints

m /media and /mnt

l /etc holds system config files

l /tmp holds temporary files

l /boot holds the kernel and bootloader

l /var and /srv hold server data

l /proc and /sys hold system information

l The lib directories hold shared libraries

m /lib, /usr/lib, /usr/local/lib

Absolute and Relative Pathnames

Absolute pathnames begin with a forward slash

l Complete "road map" to file location

l Can be used anytime you wish to specify a file name

Relative pathnames do not begin with a slash

l Specifies location relative to your current working directory

l Can be used as a shorter way to specify a file name


If OS is not recognizing the SATA HDD

boot: linux all-generic-ide noapic noiapic

After installing the OS go to rescue mode and edit the grub.conf

#vi /etc/grub.conf

Root (hd0,0)

Kernel (vmlinuz-2.6.9-22.EL ro root=LABEL=/ linux all-generic-ide noapic noiapic rhgb quiet

Initrd /initrd-

Basic commands:

#echo $SHELL shows the default SHELL

#cat /etc/shells shows other existing Shells

#vi /etc/login.defs login information file

#chvt to switch to other terminal

#chage -l <username> to see the A/c policies.

#chage <username> to modify A/c policies

#chage -E -1 <username> to make account never expires.

#date --set "-----------"

Present Working Directory


Listing Commands:


#ls -a


#ls -ld <filename>

#ll <filename>

#ls -al

Creating the Directory and Files

#mkdir <dir>

#mkdir <dir1> <dir1> <dir1>

#mkdir -p d1/d2/d3

Changing Directory

#cd <dir>

Creating a File

#cat > <filename>


#touch <filename>

Copy file

#cp <source path> <destination path>

Move and Renaming file

#mv <source path> <destination path>

Deleting file

#rm -rf <filename>

Append an existing file

#cat >> <filename>

Determining File Content

Files can contain many types of data

l Check file type with file before opening to determine appropriate command or application to use

l file [options] <filename>...

User & Group Admin

Authentication information is stored in plain text files:

o /etc/passwd

o /etc/shadow

o /etc/group

o /etc/gshadow

Changing Your Identity

To change your password, run passwd

m Insecure passwords are rejected

To start a new shell as a different user:

o su

o su -

o su username

o su - username

User Information Commands

Find out who you are

m whoami

Find out what groups you belong to

m groups, id

Find out who is logged in

m users, who, w

Login/reboot history

m last


#useradd <username> to create user

#groupadd <groupname> to create group

#useradd -u <uid> <username>

#groupadd -g <gid> <groupname>

#userdel <username> to delete user

#userdel -r <username> to delete user including home dir.

#passwd <username> to assign passwd

#passwd -S <username> shows passwd set to user or not

#passwd -d <username> to remove passwd

#id <username>

#finger <username>

Syntax: useradd <option> <username>

#usermod -G <groupname> <username>

#usermod -u <uid> <username>

#usermod -d <dir> <username> to change home directory

#usermod -s /bin/ksh <username> to change default Shell

#usermod -c "admin" <username> to put comments

#usermod -u <uid> -o <username> to change UID

#usermod -g <gid> <groupname> to change GID

#usermod -l <newusername> <oldusername> to rename user

#groupmod -n <newgroup name> <oldgroup name> to rename group

#gpasswd -a <username> <groupname> to add members of group

#gpasswd -M <username>,<username> <groupname>

#chown <new owner>.<new group> <file/dir name>

#chgrp <new group name> <file name>

#vi /etc/passwd (backup file: /etc/passwd-)

#vi /etc/group (backup file: /etc/group-)

#vi /etc/shadow (backup file: /etc/shadow-)

#tail <filename>

#head <filename>


#ln <source file path> <destination file path> Hard Link

#ln -s <source file path> <destination file path> Soft Link

Permission of a File

Symbolic Notation: Read=r, Write=w and execute=x

Numeric Notation: Read =4, Write=2 and Execute=1

r w x (Where 1 = on and 0 = off)

1 0 0 =4

0 1 0 =2

0 0 1 =1

UMASK: Universal Mask is a value which is subtracted from the full permissions to generate the default permissions.

Root User Normal User

The full permission of a File is 666 The full permission of a File is 666

-rw- rw- rw- 666 -rw- rw- rw- 666

-rw- r- - r- - 644 -rw- r- - r- - 664

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

-w- -w- 022 (umask) -w- -w- 002 (umask)

The full permission of a Directory is 777 The full permission of a Directory is 777

-rwx rwx rwx 777 -rwx rwx rwx 777

-rwx r- x r- x 755 -rwx r- x r- x 775

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

-w- -w- 022 (umask) -w- -w- 002 (umask)


#umask to see the umask value

#vi /etc/bashrc to change umask value permanently

#chmod <numeric permission> <file/dir>

e.g. #chmod 777 <file/dir>

Special Permissions
SUID Value is 4
Value is 2

  • Normally, files created in a directory belong to the default group of the user
  • When a file is created in a directory with the setgid bit set, it belongs to the same group as the directory

Sticky Bit Value is 1
Normally, users with write permissions to a directory can delete any file in that

directory regardless of that file's permissions or ownership

With the sticky bit set on a directory, only the owner of a file can delete the file

Example: /tmp

drwxrwxrwt 12 root root 4096 Nov 2 15:44 tmp

#mount -o remount,acl <mount point> enable the partition with ACL properties

#setfacl -m u:<uname>:<permission> <filename> applying acl to the user level

#setfacl -x u:<uname>:<permission> <filename> to remove the acl

#setfacl -m g:<gname>:<permission> <filename> applying acl to the group level

#getfacl <filename> to check the acl permission

Note: when applying ACL present working directorie must be acl directorie and to make it permanent write inside /etc/fstab


#chattr +i <filename>

#chattr -i <filename>

#chattr +a <filename>

#chattr -a <filename>

#lsattr <filename>

Special Users

#useradd admin
#passwd admin
#vi /etc/sudoers or #visudo put the admin user in sudo file.
#sudo su - to get the root privileges as a sudo user.

#grep sudo /var/log/secure

Network User
Usr Profiles
#vi /etc/default/useradd
# useradd defaults file
SKEL=/etc/skel (user profiles)

System Initialization

Boot Sequence Overview

  • BIOS initialization
  • Boot loader
  • Kernel initialization
  • init starts and enters the desired runlevel by executing:
    • /etc/rc.d/rc.sysinit
    • /etc/rc.d/rc and /etc/rc.d/rc?.d/
    • /etc/rc.d/rc.local
    • X Display Manager (if appropriate)

BIOS Initialization

§ Peripherals detected

§ Boot device selected

§ First sector of boot device read and executed

Boot Loader Components

  • Boot loader
    • First stage - small, resides in the MBR or a boot sector
    • Second stage - loaded from a boot partition
  • Minimum specifications for Linux:
    • Label, kernel location, OS root filesystem and location of the initial RAM disk (initrd)
  • Minimum specifications for other operating systems:
    • Boot device, label

GRUB and grub.conf

  • GRUB "the GRand Unified Bootloader"
    • Command-line interface available at boot prompt
    • Boot from ext2/ext3, ReiserFS, JFS, FAT, minix, or FFS file systems
    • Supports MD5 password protection
  • /boot/grub/grub.conf
  • Changes to grub.conf take effect immediately
  • If MBR on /dev/hda is corrupted, reinstall the first stage bootloader with:
    • /sbin/grub-install /dev/hda

Starting the Boot Process: GRUB

  • Image selection
    • Select with space followed by up/down arrows on the boot splash screen
  • Argument passing
    • Change an existing stanza in menu editing mode
    • Issue boot commands interactively on the GRUB command line

The Chicken/Egg Module Problem and the Initial RAM Disk

To mount the root filesystem, the kernel typically needs to load modules

o Examples: ext3, jbd, raid1, scsi_mod

An initial RAM disk provides modules

o Compressed cpio archive containing modules, other material

o Created at install time

o Specific to a particular hardware and software platform

Made available to the kernel by GRUB

Use mkinitrd to rebuild

o Example:

o mkinitrd /boot/initrd-$(uname -r).img $(uname -r)

Kernel Initialization

  • Kernel boot time functions
    • Device detection
    • Device driver initialization
    • Mounts root filesystem read only
    • Loads initial process (init)

init Initialization

  • init reads its config: /etc/inittab
    • initial run level
    • system initialization scripts
    • run level specific script directories
    • trap certain key sequences
    • define UPS power fail / restore scripts
    • spawn gettys on virtual consoles
    • initialize X in run level 5

Run Levels

§ init defines run levels 0-6, S, emergency

§ The run level is selected by either

· the default in /etc/inittab at boot

· passing an argument from the boot loader

· using the command init new_runlevel

§ Show current and previous run levels

        • /sbin/runlevel


§ Important tasks include:

§ Activate udev and selinux

§ Sets kernel parameters in /etc/sysctl.conf

§ Sets the system clock

§ Loads keymaps

§ Enables swap partitions

§ Sets hostname

§ Root filesystem check and remount

§ Activate RAID and LVM devices

§ Enable disk quotas

§ Check and mount other filesystems

§ Cleans up stale locks and PID files


  • Initializes the default run level per the /etc/inittab file's initdefault line such that:
  • id:3:initdefault:
      • l0:0:wait:/etc/rc.d/rc 0
      • l1:1:wait:/etc/rc.d/rc 1
      • l2:2:wait:/etc/rc.d/rc 2
      • l3:3:wait:/etc/rc.d/rc 3 (default)
      • l4:4:wait:/etc/rc.d/rc 4
      • l5:5:wait:/etc/rc.d/rc 5
      • l6:6:wait:/etc/rc.d/rc 6

Daemon Processes

·A daemon process is a program that is run in the background, providing some system service

·Two types of daemons:

o standalone

o Transient - controlled by the "super-daemon" xinetd

System V run levels

· Run level defines which services to start

o Each run level has a corresponding directory:

1 comment:

Jayyy said...

Excellent post....
Nice flow of complete overview about LINUX.