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Blog Philippe Schweitzer Boot sequence for all Linux-based operating systems

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Boot sequence for all Linux-based operating systems

In this bog posting, I will present a boot sequence that works for all Linux-based operating systems - from switching on the power to the login screen.

 

7 steps

There are 7 steps for Linux startup:

 

1

Power on

Obviously, first of all, you have to switch on your computer, then:

  • Each sub process will execute the following one
  • It is the same process for all Linux distributions
  • Close to BSD and Unix style from which it derives
  • Same as Microsoft's procedure until MBR stage



2

BIOS (Basic Inpout Output System)

  • First firmware executed at startup
  • Inits motherboard hardware components
  • Performs some system integrity checks (i. e. memory)
  • Abstraction layer between hardware and software
  • Searches and executes the boot loader program from MBR disk



3

MBR (Master Boot Record), bootlader stage 1

  • Located on the first sector of booted disk (512 bytes)
  • Contains location and data about 2nd bootloader stage, inside bootloader code section

MBR

  • Partition table (MS DOS limited to 4 primary partitions)
  • Signature helps BIOS to identify from which disk it executes the bootloader code



4

Bootloader, stage 2

  • Located on a disk partition
  • Loads operating system kernel and options

 You can choose several bootloader software:

  • Grub, GRand Universal Bootloader
  • Lilo, LInux LOader
  • ...

Grub configuration (< v1.0) :

Grub version < v1.0 uses a singe configuration file, in which all operating systems and kernel options have to be written by hand.

[root@oel-test ~]# cat /boot/grub/menu.lst

default=0

timeout=5Grub1

splashimage=(hd0,0)/grub/splash.xpm.gz

hiddenmenu

title Oracle Linux Server-uek (2.6.39-200.24.1.el6uek.i686)

root (hd0,0)

kernel /vmlinuz-2.6.39-200.24.1.el6uek.i686

...

...

Grub 2 configuration (> v1.0) :

Grub version > v1.0 (Grub 2) allows to use automatic search and configuration mechanism that updates the menu.lst file:

[root@oel-test ~]# grub-mkconfig

[root@oel-test ~]# upgrade-grub2


Moreover, if needed, we can add customization during these processes:

[root@oel-test ~]# vim /etc/default/grub # common configuration file for all Operating systems

[root@oel-test ~]# ls /etc/grub.d/* # search and configuration scripts location

[root@oel-test ~]# upgrade-grub2



warning #> upgrade-grub2
will overvrite /boot/grub/menu.lst

 

5

Kernel

Kernel

  • Located on a disk partition
  • Contains drivers for hardware support
  • Lowest Operating System software layer
  • Enables multi-task support (scheduler)
  • Mounts root file system
  • Executes init program
    • Usually, an “initrd”, INITial RamDisk (filesystem mounted temporarely in RAM memory), is needed



6

Init processinit_debian

The Init process comes first in the operating system startup and defines the running state (Runlevel):

0 – Halt
1 – Single user mode
2 – Multiuser, without NFS
3 – Full multiuser mode
4 – Unused
5 – Full multiuser and graphical mode
6 – Reboot

Default configuration file location:

[root@oel-test ~]# cat /etc/inittab

id:5:initdefault:



7

Runlevel service script organization:

Runlevel scripts are organized acoording to a strict naming convention, allowing their execution order and purpose:

runlevel_script





It is possible to invoke a manual action related to a service:

[root@oel-test ~]# /etc/init.d/service_name start|stop|restart|status|reload

Starting service : [ OK ]

[root@oel-test ~]# service service_name start|stop|restart|status|reload




runlevel_config_debian

Runlevel service script configuration

It is possible to manage service execution scripts using more high-level commands:

chkconfig [--level levels ] service_nqme on|off|reset

[root@oel-test ~]# chkconfig --level 2345 ntpd on

[root@oel-test ~]# chkconfig --list ntpd

ntpd 0:off 1:off 2:on 3:on 4:on 5:on 6:off





It is also possible to use a graphical tool:ntsysv

[root@oel-test ~]# ntsysv --level 5









8

That's it!

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Philippe Schweitzer is Consultant at dbi services. He has more than three years of experience in Linux systems and networking components. He has a solid expertise in the administration and deployment of open source technologies (Debian/Ubuntu, Xen virtualization, Apache, OpenLDAP, Bind9, DHCP server, etc.) as well as in the analysis of business requirements and architectures. He is also skilled in Java and OpenERP development (Python). Philippe Schweitzer is Oracle Certified Associate (OCA) Java SE 7 Programmer, OCA Weblogic Server 11g System Administrator, and ITIL Foundation V3 certified. Prior to joining dbi services, Philippe Schweitzer was head of its own IT service company, based in France. Philippe Schweitzer holds an Engineer's Degree in Computer Science from the University of Technology of Belfort-Montbéliard (F). His branch-related experience covers Automotive, Food, IT, Public Sector, etc.

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