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
o 0 – halt
o 1 – Single user mode
o 2 – Multiuser, without NFS
o 3 – Full multiuser mode
o 4 – unused
o 5 – X11
o 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.
o Run level 0 – /etc/rc.d/rc0.d/
o Run level 1 – /etc/rc.d/rc1.d/
o Run level 2 – /etc/rc.d/rc2.d/
o Run level 3 – /etc/rc.d/rc3.d/
o Run level 4 – /etc/rc.d/rc4.d/
o Run level 5 – /etc/rc.d/rc5.d/
o 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/ direcotiries, 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.

6 Examples to Backup Linux Using dd Command (Including Disk to Disk)

Data loss will be costly. At the very least, critical data loss will have a financial impact on companies of all sizes. In some cases, it can cost your job. I’ve seen cases where sysadmins learned this in the hard way.


This article provides 6 practical examples on using dd command to backup the Linux system. dd is a powerful UNIX utility, which is used by the Linux kernel makefiles to make boot images. It can also be used to copy data. Only superuser can execute dd command.

Warning: While using dd command, if you are not careful, and if you don’t know what you are doing, you will lose your data!

Example 1. Backup Entire Harddisk

To backup an entire copy of a hard disk to another hard disk connected to the same system, execute the dd command as shown below. In this dd command example, the UNIX device name of the source hard disk is /dev/hda, and device name of the target hard disk is /dev/hdb.

# dd if=/dev/sda of=/dev/sdb



* “if” represents inputfile, and “of” represents output file. So the exact copy of /dev/sda will be available in /dev/sdb.
* If there are any errors, the above command will fail. If you give the parameter “conv=noerror” then it will continue to copy if there are read errors.
* Input file and output file should be mentioned very carefully, if you mention source device in the target and vice versa, you might loss all your data.

In the copy of hard drive to hard drive using dd command given below, sync option allows you to copy everything using synchronized I/O.

# dd if=/dev/sda of=/dev/sdb conv=noerror,sync



Example 2. Create an Image of a Hard Disk

Instead of taking a backup of the hard disk, you can create an image file of the hard disk and save it in other storage devices.There are many advantages to backing up your data to a disk image, one being the ease of use. This method is typically faster than other types of backups, enabling you to quickly restore data following an unexpected catastrophe.

# dd if=/dev/hda of=~/hdadisk.img



The above creates the image of a harddisk /dev/hda. Refer our earlier article How to view initrd.image for more details.

Example 3. Restore using Hard Disk Image

To restore a hard disk with the image file of an another hard disk, use the following dd command example.

# dd if=hdadisk.img of=/dev/hdb



The image file hdadisk.img file, is the image of a /dev/hda, so the above command will restore the image of /dev/hda to /dev/hdb.

Example 4. Creating a Floppy Image
Using dd command, you can create a copy of the floppy image very quickly. In input file, give the floppy device location, and in the output file, give the name of your floppy image file as shown below.

# dd if=/dev/fd0 of=myfloppy.img



Example 5. Backup a Partition

You can use the device name of a partition in the input file, and in the output either you can specify your target path or image file as shown in the dd command example below.

# dd if=/dev/hda1 of=~/partition1.img



Example 6. CDROM Backup

dd command allows you to create an iso file from a source file. So we can insert the CD and enter dd command to create an iso file of a CD content.

# dd if=/dev/cdrom of=tgsservice.iso bs=2048



dd command reads one block of input and process it and writes it into an output file. You can specify the block size for input and output file. In the above dd command example, the parameter “bs” specifies the block size for the both the input and output file. So dd uses 2048bytes as a block size in the above command.

Note: If CD is auto mounted, before creating an iso image using dd command, its always good if you unmount the CD device to avoid any unnecessary access to the CD ROM.