Linux Boot Process

              6 Stages of Linux Boot Process (Startup Sequence)

Press the power button on your system, and after few moments you see the Linux login prompt.

Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?

The following are the 6 high level stages of a typical Linux boot process.

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).
• Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample
grub.conf of CentOS.

#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.el5PAE)
root (hd0,0)
kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
initrd /boot/initrd-2.6.18-194.el5PAE.img

• 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.
There you have it. That is what happens during the Linux boot process.



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Difference between ext2 vs ext3 vs ext4

                       Difference between ext2 vs ext3 vs ext4

Ext2, Ext3 and Ext4 are all filesystems created for Linux. This article explains the following:
• High level difference between these filesystems.
• How to create these filesystems.
• How to convert from one filesystem type to another.

 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”. 



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Use cat Command In Linux / UNIX

 Cat command of the most frequently used commands on Linux OS.

we can use for lots of purposes under Linux:

*  Display text files on screen.
*  Copy text files.
*  Combine text files.
*  Create new text files.

Cat command

cat filename
cat options filename
cat file1 file2
cat file1 file2 > file1&2

Displaying The Contents of Files

To read the contents of files

$ cat /etc/passwd

The above command will display the contents of a file named /etc/passwd . By default cat will send output to the monitor screen. But, you can redirect from the screen to another command or file using redirection operator as follows:
$ cat /etc/passwd > /tmp/test.txt

In the above example, the output from cat command is written to /tmp/text.txt file instead of being displayed on the monitor screen. You can view /tmp/text.txt using cat command itself:
$ cat /tmp/test.txt

Multiple file in single file . The original file or files are not modified or deleted. Example, cat will copies 2 or 3 or more than 3 files of the contents in single file . Let the three files /etc/hosts, /etc/resolv.conf, and /etc/fstab:
$ cat /etc/hosts /etc/resolv.conf /etc/fstabwe can redirect the output as follows using shell standard output redirection:
$ cat /etc/hosts /etc/resolv.conf /etc/fstab > /tmp/outputs.txt
$ cat /tmp/outputs.txtwe can also use a pipe to filter data. In this example send output of cat to the less command using a shell pipe as the file is too large for all of the text to fit on the screen at a time:
$ cat /etc/passwd | less

How Do I Create a File?

we can use cat command for file creation. To create a file called foo.txt, $ cat > foo.txt
Sample outputs:

This is a test.

To save and exit press the CONTROL and d keys (CTRL+D). Please note that if a file named foo.txt already exists, it will be overwritten. You can append the output to the same file using >> operator:
$ cat >> bar.txt
The existing bar.txt file is preserved, and any new text is added to the end of the existing file called bar.txt. To save and exit press the CONTROL and d keys (CTRL+D).

How Do I Copy File?

The cat command can also be used to create a new file and transfer to it the data from an existing file. To make copy of
$ cat oldfile.txt > newfile.txt
To output file1's contents, then standard input, then file2's contents,
$ cat file1 - file2
A hyphen indicates that input is taken from the keyboard. In this example, to create a new file file2 that consists of text typed in from the keyboard followed by the contents of file1,
$ cat - file1 > file2

cat command options

To number non-blank output lines, enter (only works with GNU cat command version):
$ cat -b /etc/passwd
Sample outputs:

     1 root:x:0:0:root:/root:/bin/bash
     2 daemon:x:1:1:daemon:/usr/sbin:/bin/sh
     3 bin:x:2:2:bin:/bin:/bin/sh
     4 sys:x:3:3:sys:/dev:/bin/sh
     5 sync:x:4:65534:sync:/bin:/bin/sync
     6 games:x:5:60:games:/usr/games:/bin/sh
     7 man:x:6:12:man:/var/cache/man:/bin/sh
     8 lp:x:7:7:lp:/var/spool/lpd:/bin/sh
     9 mail:x:8:8:mail:/var/mail:/bin/sh
    10 news:x:9:9:news:/var/spool/news:/bin/sh

To number all output lines, enter (GNU cat version only):
$ cat -n /etc/passwd
To squeeze multiple adjacent blank lines, enter (GNU cat version only):
$ cat -s /etc/passwd
To display all nonprinting characters as if they were visible, except for tabs and the end of line character, enter (GNU cat version only):
$ cat -v filename

cat Command Abuse

The main purpose of cat is to catenate files. If it's only one file, concatenating it with nothing at all is a waste of time, and costs you a process. For example,
$ cat /proc/cpuinfo | grep model
Can be used as follows:
$ grep model /proc/cpuinfo
Another example,
cat filename | sed -e 'commands' -e 'commands2'
Can be used as follows which is cheaper:
sed sed -e 'commands' -e 'commands2' filename

    
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To Break Password of root on Linux ( Centos, Redhat, Fedora, etc ) .

To enter single-user mode, reboot your computer. If you use the default boot loader, GRUB, you can enter single user mode by performing the following:

1. At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.
2. You are presented with a prompt that looks similar to the following:

   grub append> ro root=LABEL=/

3. Press the Spacebar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:

   ro root=LABEL=/ single

4. Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:

   sh-2.05b#

5. You can now change the root password by typing

   sh-2.05b# passwd root

   You will be asked to re-type the password for verification. Once you are finished, the password will be changed. You can then reboot by typing reboot at the prompt; then you can log in to root as you normally would.
   
   
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Take Backup through script on linux

Through this script , we can take backup of file , folder , any drive etc 

Create a folder to take backup . For Example :-

$mkdir ~/backup/
$cd  /usr/local/bin/
$vi backup.sh
***********************************************************************************
***********************************************************************************
         #!/bin/bash
         # A script for backing up any file & folder
         # First argument : The directory to be backed up
         # Second argument : The location to back-up to.
                        ORIG=$1
                        BACK=~/backups/

          if [ -e $BACK ]
          then
                        echo "WARNING:$BACK exists"
                        read -p "Press Ctrl+c to exit or Enter to Proceed:"
          fi
          cp -av $ORIG $BACK
          echo "Backup of $ORIG to $BACK completed at:$(date)"


       :wq
************************************************************************************
************************************************************************************
$ chmod 755 backup.sh
$ backup  (Path of file & folder Only )
$ cd  ~/backup/
$ ls -l                                             ( Your all backup in this folder )


*********************************************************************************
**************************************Enjoy *************************************
*********************************************************************************

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How to install teamviewer on Ubuntu 12.04 step by step ?

How to install teamviewer on Ubuntu 12.04 step by step ?

It is used to remote access one system to another system through internet anywhere in the world .

If you want to take remote of your friend system , than you need to install TeamViewer both the system . If you install teamviewer in your system or friend system , than you see that automatically gerate user ID or Password 

Open the terminal and type the following command

# sudo wget http://www.teamviewer.com/download/teamviewer_linux.rpm

                                                       OR 

we can visit to the site for manually downloading the .deb.

http://www.teamviewer.com/en/download/linux.aspx

Change the permission

# chmod 777  teamviewer_linux_x64.deb

Than Double click on "teamviewer_linux_x64.deb" , if you click on deb package . It is

automatically open "Ubuntu Software Center"  .

Click on install put the password , it will take few minutes to install all packages .

when your teamviewer is install follow some steps Application>internet>teamviewer

Take Teamviewer user ID or Password of your friend system . Put your friend user ID 

in "Partener ID" when you connect than it ask password "Put your friend password "

Now you can see that friend desktop . You can do any thing on your friend desktop .

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Convert gsm to mp3 on ubuntu

Convert gsm to mp3 on ubuntu

Visit Our video Link on youtube.com

https://www.youtube.com/watch?v=Tq7QwlT_FOs
 https://www.youtube.com/watch?v=Tq7QwlT_FOs

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How to find port number on ubuntu , centos , redhat ?

How to find port number on ubuntu , centos , redhat  ? 

How to see port no of  FTP, HTTP, SSH etc

Open terminal type a command , 

#cat /etc/services |grep ftp

 #cat /etc/services | grep http

 #cat /etc/services | grep ssh

We can see that all port number , type the command

#cat /etc/services

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