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What is GRUB

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  2. 2. 6 Stages of Linux Boot Process
  3. 3. BIOS stands for Basic Input/Output System • Perf orms some syst em int egrit y checks • Searches, loads, and execut es t he boot loader program. • I t looks f or boot loader in f loppy, CD-ROM, or hard drive. You can press a key (t ypically F12 of F2, but it depends on your syst em) during t he BI OS st art up t o change t he boot sequence. • So, in simple t erms BI OS loads and execut es t he MBR boot loader.
  4. 4. MBR stands for Master Boot Record. • I t is locat ed in t he 1st sect or of t he boot able disk. Typically / dev/ hda, or / dev/ sda • MBR is less t han 512 byt es in size. This has t hree component s 1) primary boot loader inf o in 1st 446 byt es 2) part it ion t able inf o in next 64 byt es 3) mbr validat ion check in last 2 byt es. • I t cont ains inf ormat ion about GRUB (or LI LO in old syst ems)
  5. 5. MBR stands for Master Boot Record.
  6. 6. • Furt hermore, GRUB works in st ages. • St age 1 is locat ed in t he MBR and mainly point s t o St age 2, since t he MBR is t oo small t o cont ain all of t he needed dat a. • St age 2 point s t o it s conf igurat ion f ile, which cont ains all of t he complex user int erf ace and opt ions we are normally f amiliar wit h when t alking about GRUB. St age 2 can be locat ed anywhere on t he disk. I f St age 2 cannot f ind it s conf igurat ion GRUB “Grand Unified Bootloader”
  7. 7. GRUB “Grand Unified Bootloader” • I f you have mult iple kernel images inst alled on your syst em, you can choose which one t o be execut ed. • GRUB displays a splash screen, wait s f or f ew seconds, if you don’t ent er anyt hing, it loads t he def ault kernel image as specif ied in t he grub conf igurat ion f ile. • GRUB has t he knowledge of t he f ilesyst em • Grub conf igurat ion f ile is / boot / grub/ grub.conf
  8. 8. GRUB “Grand Unified Bootloader”
  9. 9. GRUB “Grand Unified Bootloader”
  10. 10. GRUB “Grand Unified Bootloader” • This is t he f irst operat ing syst em ent ry in t he menu. • t it le is as simple as it sounds. I t ' s merely a st ring t hat ' s meant t o help t he user read t he menu in human t erminology. • root (hd0,2) t ells GRUB where it s conf igurat ion f iles are locat ed. I n t his inst ance, t hey can be f ound under (hd0,2)/ boot / grub. • kernel / boot / vmlinuz-... boot s t he act ual kernel image. There can be many such images available. The f act t here is no device specif ied bef ore t he / boot / vmlinuz indicat es t he image is locat ed on t he same part it ion as t he GRUB it self . This is
  11. 11. Linux and Unix shutdown command • The shut down command br ings t he syst em down in a secure way. All logged-in user s ar e not if ied t hat t he syst em is going down, and login operat ions ar e blocked. I t is possible t o shut t he syst em down immediat ely, or af t er a specif ied delay. • All processes are f irst not if ied t hat t he syst em is going down by t he signal SI GTERM. This gives programs like vi t ime t o save t he f ile being edit ed, mail and news pr ocessing
  12. 12. GOOD LUCK