Overview of Linux System

Linux basics

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Linux
Linux is, in simplest terms, an operating system .It belongs to the Unix family of 
operating System. It was developed by Linus Torvalds. 
External Links: tldp


Kernel

External Links IBM

Kernel is a main program of Linux system .It controls hard wares, CPU,memory, hard disk, network card etc. The kernel is the heart of the Linux operating system. It manages the resources of Linux such as:

• File management
• Multitasking
• Memory management
• I/O management
• Process management
• Device management
• Networking support including IPv4 and IPv6

The kernel decides who will use these resources and for how long and when. It runs your programs or sets up to execute binary files. The kernel acts as an intermediary between the computer hardware and various applications.

The kernel is a program that constitutes the central core of a computer operating system. It has complete control over everything that occurs in the system.

A kernel can be contrasted with a shell (such as bash, csh or ksh in Unix-like operating systems), which is the outermost part of an operating system and a program that interacts with user commands. The kernel itself does not interact directly with the user, but rather interacts with the shell and other programs as well as with the hardware devices on the system, including the processor (also called the central processing unit or CPU), memory and disk drives

Shell

Shell is an interface between user and kernel. Shell interprets your input as commands and pass them and pass them to kernel ( C Shell, Sh, K Shell, Bash)

·         A program that interprets commands

·         Allows a user to execute commands by typing them manually at a terminal, or automatically in programs called shell scripts.

·         A shell is not an operating system. It is a way to interface with the operating system and run commands.

What is a Linux Distribution?

A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions (often called distros for short) are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications. These operating systems consist of the Linux kernel and, usually, a set of libraries and utilities from the GNU Project, with graphics support from the X Window System.

The Various Distros

Cent OS, Debian, Red Hat Enterprise Linux, Ubuntu, Fedora, Gentoo, Knoppix, Linux Mint, Mandriva

Features of Linux OS

Multi­user & Multi­ process

Many people can use one machine at the same time.

Files  and Process

Data, directory, process, hard disk etc (almost everything) are expressed as a file.

Process is an running program identified by a unique id (PID).

Normal user and Super user

In Linux system, there is one special user for administrator, which can do anything.  This special user is called root or super user.

Normal User $ 

Super User  #    

               

Case Sensitivity

<Linux like UNIX is case­ sensitive ( MYFILE.doc, Myfile.doc, mYfiLe.Doc are different)

Linux Directory Structure

Linux directory structure starts with / (root), files are put in a directory. All directories are in a hierarchical structure (tree structure).  User can put and remove any directories on the tree.  Top directory is “/”, which is called slash or root.

Important Directories

/bin This contains files that are essential for correct operation of the system. These are available for

use by all users.

/mnt Provides a location for mounting devices,such as remote filesystems and removable media

/home This is where user home directories are stored.

 /var This directory is used to store files which change frequently, and must be available to be written to.

/etc Various system configuration files are stored here.

/dev This contains various devices as files, e.g. hard disk, CD­ROM drive, etc.

 /root This is the root (administrator) user's home directory

 /sbin Binaries which are only expected to be used by the super user.

 /tmp Temporary files.

 /boot Has the bootable Linux kernel and boot loader configuration files(GRUB)

 /usr Contains user documentation, games, graphical files, libraries

Basic Commands

External Link: SS64  oreillynet

man - format and display the on-line manual page

            Usage : man date

help - Display help for a built-in command

            Usage : date --help

info - information about the command 

            info date

ls lists the content of directory

ls

ls -l       list the files with all information

ls –a     list files including hidden files

ls –la    long list with hidden files

cd Change Directory

cd  /home/user/

Absolute bath /home  (absolute path: Relating to file systems, the location of a directory or file that can be accessed regardless of the current working location of a user)          

eg  /home/user/mypicture.jpg

Relative path The relative pathnames are opposite to absolute pathname. A reative pathname does not begin with a slash ( / ). Generally you specifies location relative to your current working directory.

eg  cd home/user/myfile.jpg,

cd ..  goes one directory back

cd - return to the last directory

cd ~ go to the current user's home directory

mkdir  Create one or more directories

mkdir folder1 folder2

mkdir -p work/junk/questions  (mkdir -p Create intervening parent directories if they don't exist)

pwd  Print the full pathname of the current working directory. See also the dirs shell command built intobash.

pwd

cp  Copy files/directories

cp file1 file2

cp –vrf dirA dirB

echo  display a line of text           

echo "hello world"   (displays hello world in command line)

mv move files to another location or rename files

mv  /home/user/myfile /home/Admin/myfile

mv myfile your file

rm remove files/directories

rm myfile

rm -rf mydirectory  (rmdir remove directory)

find  search for files in a directory hierarchy, an extremely useful command for finding particular groups of files (numerous examples follow this description). find descends the directory tree beginning at each pathname and locates files that meet the specified conditions. The default pathname is the current directory. The most useful conditions include -name and -type (for general use), -exec and -size (for advanced use), and -mtime and -user

find /home -name myfile

history shows the history of commands

            -c   Clear the history list

!             Start a history substitution, except when followed by a space

!n            Refer to command line n.

!-n           Run the command n lines back.

!!            Run the previous command.

mount mount a file structure. The file structure on device is mounted ondirectory. If no device is specified, mount looks for an entry in /etc/fstab to find out what device is associated with the given directory. 

mount /dev/sdb1 /media

mount -t   to limit this to all filesystems of a particular type.

umount allos to unmount the mounted filesystems 

umount /dev/sdb1          or umount /media

fdisk  Linux command line disk partitioning tool

Touch  to create empty file or updating the date and time to the current time

Date    shows the date and time

cal       shows the calendar

du         Estimate file space usage

df           shows the disk free 

diff      Display the differences between two files

clear  Clear terminal screen

dd        Convert and copy a file, write disk headers, boot records, create a boot floppy. dd can make an exact clone of an (unmounted) disk, this will include all blank space so the output destination must be at least as large as the input.

dd if=linux_cd_image.iso of=/dev/sdb

tar
tar  was
initially developed to write data to sequential I/O devices for tape backup purposes. It is now commonly used to collect many
files into one larger file for distribution orarchiving, while preserving file system information such as user and group permissions,
dates, and directory structures
tar -cvf mytarball.tar file1 file2 file3
tar -xvf mytarball.tar
System states
shutdown              init 0, poweroff
reboot                  init 6, reboot
Sleep                   echo mem >
/sys/power/state
Hibernation          echo disk > /sys/power/state   

File system types

external Links thegeekstuff

Ext2

§  Ext2 stands for second extended file system.

§  It was introduced in 1993. Developed by Rémy Card.

§  This was developed to overcome the limitation of the original ext file system.

§  Ext2 does not have journaling feature.

§  On flash drives, usb drives, ext2 is recommended, as it doesn’t need to do the over head of journaling.

§  Maximum individual file size can be from 16 GB to 2 TB

§  Overall ext2 file system size can be from 2 TB to 32 TB

Ext3

• Ext3 stands for third extended file system.
• It was introduced in 2001. Developed by Stephen Tweedie.
• Starting from Linux Kernel 2.4.15 ext3 was available.
• The main benefit of ext3 is that it allows journaling.
• Journaling has a dedicated area in the file system, where all the changes are tracked. When the system crashes, the possibility of file system corruption is less because of journaling.
• Maximum individual file size can be from 16 GB to 2 TB
• Overall ext3 file system size can be from 2 TB to 32 TB
• There are three types of journaling available in ext3 file system.
• Journal – Metadata and content are saved in the journal.
• Ordered – Only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default.
• Writeback – Only metadata is saved in the journal. Metadata might be journaled either before or after the content is written to the disk.
• You can convert a ext2 file system to ext3 file system directly (without backup/restore).

§   

Ext4

§  Ext4 stands for fourth extended file system.

§  It was introduced in 2008.

§  Starting from Linux Kernel 2.6.19 ext4 was available.

§  Supports huge individual file size and overall file system size.

§  Maximum individual file size can be from 16 GB to 16 TB

§  Overall maximum ext4 file system size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).

§  Directory can contain a maximum of 64,000 subdirectories (as opposed to 32,000 in ext3)

§  You can also mount an existing ext3 fs as ext4 fs (without having to upgrade it).

§  Several other new features are introduced in ext4: multiblock allocation, delayed allocation, journal checksum. fast fsck, etc. All you need to know is that these new features have improved the performance and reliability of the filesystem when compared to ext3.

§  In ext4, you also have the option of turning the journaling feature “off”.

mke2fs /dev/sda1

mkfs.ext3 /dev/sda1

mkfs.ext4 /dev/sda1

Linux Booting

Linux Booting

External Links   Thegeekstuff  IBM

1. BIOS

BIOS stands for Basic Input/Output System

 Performs some system integrity checks

 Searches, loads, and executes the boot loader program.

 It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.

 Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.

 So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

 MBR stands for Master Boot Record.

 It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda

 MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.

 It contains information about GRUB (or LILO in old systems).

 So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

 GRUB stands for Grand Unified Bootloader.

 If you have multiple kernel images installed on your system, you can choose which one to be executed.

 GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

 GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).

 As you notice from the above info, it contains kernel and initrd image.

 So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

 Mounts the root file system as specified in the “root=” in grub.conf

 Kernel executes the /sbin/init program

 Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.

 initrd stands for Initial RAM Disk.

 initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

 Looks at the /etc/inittab file to decide the Linux run level.

 Following are the available run levels

 0 – halt

 1 – Single user mode

 2 – Multiuser, without NFS

 3 – Full multiuser mode

 4 – unused

 5 – X11

 6 – reboot

 Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

 Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level

 If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.

 Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

 When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.

 Depending on your default init level setting, the system will execute the programs from one of the following directories.

 Run level 0 – /etc/rc.d/rc0.d/

 Run level 1 – /etc/rc.d/rc1.d/

 Run level 2 – /etc/rc.d/rc2.d/

 Run level 3 – /etc/rc.d/rc3.d/

 Run level 4 – /etc/rc.d/rc4.d/

 Run level 5 – /etc/rc.d/rc5.d/

 Run level 6 – /etc/rc.d/rc6.d/

 Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.

 Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.

 Programs starts with S are used during startup. S for startup.

 Programs starts with K are used during shutdown. K for kill.

 There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.

 For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

Most Popular Text Processing Commands

cat

cat Read one or more files and print them on standard output.

cat  filename.txt  (display the content of filename.txt)

cat  -n myfiles.txt   number all output lines, starting with 1

cat > newfile to exit, enter EOF (Ctrl-D)

cat >> newfile to append the file

vi editor

vi editor  External Link 1 2 A screen-oriented text editor. Vi has two modes insert mode and command mode. The editor begins in command mode, where cursor movement and copy/paste commands may be issued

vi filename (creating or opening a filename)

esc + i  insert the text

:wq  Save and exit

:q  exit without save

:wq! save and exit forcefully (! forcefully)