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7/23/2019 Boot Process of Linux
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Welcome
Boot Process inLINUX
7/23/2019 Boot Process of Linux
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System Boot-up
• USER
PressesSwitch
Switch SMPS
Power Passesto
SMPS( Switch MoePower Supply! is a "in
o# RE$UL%&'R &hrou)h SMPS* Power
is Re)ulate to MainBoar an 'ther %reas
Mothe
rBoar
'ther%reas
P o w e r
P a s s
e s
t o
P o w e r P a s s e s
t o
+ ,PU %N. /%R0 0IS1 2 ,0 R'M
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IN M'&/ER B'%R0 (%1% M%IN B'%R0!
BI'
S
BI'S(Basic Input3OutputSystem! is 4uilt usin) %ssem4ly
Lan)ua)e Its main #unction is toLoa 1ernel into Memory BI'S isa Pro)ram * locate at a R'M,hip in Mother4oar It is also"nown as R'M Pro)ram It1eeps the in#ormation o# all the/arware an is responsi4le #orma5or tas"s in Boot ProcessBI'S $ets Loae in MEM'R6 (Boot Strappin)!
R%M(MEM'R6! + BI'S
.,M'S
P'S&
P'S&(power-on sel# test ! is a Pro)ram thatpro7ies Status o# all the peripherals connecte tothe System an chec"s i# e7ery e7ice is#unctionin) properly or
&he BI'S per#orms the power-on sel# test (P'S&! I# there are any #atal errors* the 4oot processstopsP'S& 4eep coes can 4e #oun in this area o# the &rou4leshootin) E8pert
,M'
S
$et s
Loae
,M'S is a Pro)ram * itchec"s the 4oot e7icespriority an also ienti9es
the +st sector (: ,yliner * : &rac"s! o# /30 *;+. 4ytes in
$ e t s L o
a
e t h r u
B I ' S
,M'S
B%&&ER6
,mos(ComplementaryMetal-O8ie Semiconuctor!4attery )i7es power to
,mos pro)ram * so thatit retains all its,ontains (settin)s!
,PUPIN RESE&S
P r o 7 o " e s
,PU-E8ecute a =ump at :8: ( +st instruction a#ter
power on ! >5ump> instruction tellin) the processor where to)o to 9n the real BI'S startup pro)ram
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At the First Sector of HD (0 Cylinder = 0
Tracks = st Sector!
MBR
HA"DDIS#
st
Sector
R%M(MEM'R6!
+BI'S.,M'S2MBR
$e t s
Loae ( t h r u
B I ' S !Sta)e + 4oot loaer
&he primary 4oot loaer that resies in the MBR is a ;+.-4yte ima)e containin) 4oth pro)ram coe an a smallpartition ta4le (see i)ure .! &he 9rst ??@ 4ytes are theprimary 4oot loaer* which contains 4oth e8ecuta4lecoe an error messa)e te8t &he ne8t si8ty-#our 4ytesare the partition ta4le* which contains a recor #or eacho# #our partitions (si8teen 4ytes each! &he MBR enswith two 4ytes that are e9ne as the ma)ic num4er(:8%%;;! &he ma)ic num4er ser7es as a 7aliationchec" o# the MBR
&he 5o4 o# the primary 4oot loaer is to 9n an loa the
seconary 4oot loaer (sta)e .! It oes this 4y loo"in)throu)h the partition ta4le #or an acti7e partition When it9ns an acti7e partition* it scans the remainin) partitionsin the ta4le to ensure that theyAre all inacti7e When this is7eri9e* the acti7e partitionAs 4oot recor is rea #rom thee7ice into R%M an e8ecuteSta)e . 4oot loaer
&he seconary* or secon-sta)e* 4oot loaer coul 4emore aptly calle the "ernel loaer &he tas" at this sta)eis to loa the Linu8 "ernel an optional initial R%M is"
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Let us Unerstan MBR in0etail
Master Boot Recor (MBR! is a ,ommon Pro)ram in 'S * whene7er you4oot your System with the Boota4le ,0 * &he Installer (%nacona inLinu8! writes the MBR at the irst Sector o# your /30 %s shown in a4o7eia)ram* MBR is i7ie into 2 main parts
$ Boot Sector (%%& Bytes! ' Boot Sector is a %rea in MBR * whichcontains the in#ormation o# Boot LoaerC li"e LIL' D $RUB (o#
Linu8 (PO)"F*+!! an N&L0R (o# Winows (+ess Po,erf-lthan +I+O . /"*B!!
Boot Loaer will 4e responsi4le #or Loain) the 1ernelC (mlinu<in our case! * a#ter BI'S assi)ns it the &%S1 to o so
$ Partition Ta1le (&% Bytes! ' Partition &a4le is a)ain a su4-i7iepart o# MBR It
has ? pro)rams o# +@ Bytes Each (? X +@ 4 F @? Bytes! EachPro)ram is Responsi4le #or each Partition in /30 /ence you cannotcreate more than ? partitions in a /30 'ut o# this ? partition * youcan create one partition as E8teneC an create Su4 partitions orLo)ical Partitions in it2$ Ma3ic 4-51er ( Bytes! ' Ma)ic Num4er 4asically shows thestatus o# other two
i7isons o# MBR I# Boot Sector an Partition &a4le are writtenSucess#ully * Ma)ic Num4er will 4e 6esC * 'therwise it will 4e NoC
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BI'S
irst
,/E,1S
Ma)icNum4er
I#6esC
I#
NoC
ReportsError
Partition &a4le
,hec"s #or %cti7e Partition inP&
Boot Sector
ins the Boot Loaerin BS
BI'S now Loas the BootLoaer (LIL' or $RUB in our,ase! in Memory * a"a(irst sta)e o# LIL'! an
/ans o7er the 1ernelLoain C tas" to it
R%M(MEM'R6!
+BI'S
.,M'S2MBR?LIL' G$RUB
L o a e
LIL' G$RUB
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+I+O I4FO
34oot34oot4 is a4inary 9le
,/S Num4ers is
,onsiere to 4e theM'&/ER &'N$UE o#BI'S ( Int +2 n
H!
(Int +2 n .!
( Int +2 n
.!
BIOS
unctions to ,%LL BI'S+ Int +2 #n H -- >$et ri7eparameters>. Int +2 #n . -- >Rea sectors#rom ri7e>
34oot34oot4
6 61oot6Ma7 6
&o
loa
&o
, / S
N '
(
, / S
N '
(
,/S N'
BIOS
(Int +2 n .!
BIOS
, / S N ' (
+I+O /"*B
+I+O has no interacti8eco55and interface
$RUB has interacti7e commaninter#ace
+I+O does not s-77ort
1ootin3 fro5 a net,ork
$RUB oes support 4ootin) #rom a
networ"If yo- chan3e yo-r +I+O
con:3 :le; yo- ha8e to
re,rite the +I+O sta3e
one 1oot loader to the
MB"
$RUB automatically etects any
chan)e in con9) 9le an auto
loas the 'S
+I+O s-77orts only lin-<
o7eratin3 syste5
$RUB supports lar)e num4er o#
'S
Below are )i7en some #ew important iKerences a4outLIL' an $RUB
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Installer (%nacona!* pro7ies the ,ylinrical /ea Sector (,/S! num4er o#34oot34oot4 * 34oot3Messa)e D 34oot3Map 9le to LIL' LIL' can Loa allthese 9le with the help o# ,/S Num4er * howe7er it oesnCt unerstan,/S num4er * So it ,alls BI'S to help it Loa all these 9les LIL' uses a#unction (Int +2 n H! #or 34oot34oot4 D #unction (Int +2 n .! #or other9les * to ,all BI'S
%#ter unction (Int +2 n H! is e8ecute *BI'S Loas 4oot4 9le intoMemory * it is "nown as Secon sta)e o# LIL' %n When unction (Int +2
n .! is e8ecute one 4y one *BI'S Loas 4oth Messa)e an Map 9le intoMemory R%M(MEM'R6!
+ BI'S. ,M'S2 MBR? LIL' GG $RUB
; Boot4@ Messa)e Map
Important 9les in 34ootirectory
+ Boot4. Messa)e2 Map
? mlinu<; Initrim)
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$ 6Messa3e has CHS 4-51er of S7lash Screen (also kno,n as#ernel +istin3s!> and it calls BIOS to +oad this ; for the sa5ereason ;as it doesn>t -nderstand CHS 4-51er$
$ 6Ma7 has CHS 4-51er of 85lin-9> and it calls BIOS to +oad this ;as it doesn>t -nderstand CHS 4-51er$6Messa3e
6Ma7
( Int +2 n.!
BIOS
( Int +2 n
.!
BIOS
/ope this is Sel#E8planatory
mlinu<
(1ernel!
,/SNo
,/SNo+ BI'S
. ,M'S
2 MBR? LIL' GG $RUB; Boot4@ Messa)e MapH mlinu<
R%M(MEM'R6!
Splash
Screen
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&he "ernel is the central part o# an o7eratin3 syste5* that irectlycontrols the co57-ter hard,are Usually* the "ernel is the 9rst o# theuser-installe soft,are on a computer* 4ootin) irectly a#ter the BIOS1ernel is in 4<ip #ormat1ernel has a ,/S Num4er o# initrim) (INI&I%L R%M 0IS1! * it ,alls BI'S 4y
(Int +2 #n .! #unction an it loas initrim) into Memory Initrim) islocate at 34oot an is in )<ip #ormat Initrim) has ? main 9les
+ nash - Nash is a Minimal shell * it ta"es low memory It is useto run linu8rc script . e8t2 : - "ernel nees a ri7er 9le calle e8t2 :C #or 9lesystems
without which * youyou cannot mount any partition
2 =B0 : O ItCs a /30 is" ri7er 9le ? Linu8rc O ItCs a Script 9le * ha7in) #unctions o# e8t 2 : an =B0 :an also #unction to
mount 3C (slash! partition
Initrim) is e8tracte 3 ecompresse an its ,ontents (a4o7e ? 9les! areLoae into R%M 0IS1C
W/%& IS R%M 0IS1
% R%M is" is a portion o# R%M which is 4ein) use as i# it were a is" ri7eR%M is"s ha7e 98e si<es an act li"e re ular is" artitions %ccess time
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(int +2 #n.!
BIOS
initrim)
$ e t s
E 8 t r a
c t e i n t
o R 0
a n
$ e t s
L o a
e i n t o m
e m o r y
R%M(MEM'R6!
+ BI'S
. ,M'S2 MBR? LIL'; Boot4@ Messa)e Map
H imlinu<Q e8t2:+:54:
R%M
0IS1
+ Nash. E8t2 :2 =B0 :? Linu8rc
%t R%M 0IS1 * 9rst NashC will 4eacti7ate to run the script 9le *linu8rc
&his Script 9le will now run #unctions o#e8t2 : * =B0 : an mountin) #unction o#
3C (slash! * this mountin) is also calleSysrootC
In linu8rc script Mountin) o# 3C(slash! *loo"s somethin) li"e thismount 3e73root 3 ro ------ (rea
only!
, / S N o (
XXXXXXXXXXXXXX XX EN0 ' 1ERNEL L%N0
61oot68i5lin-9
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N%S/ ItCs a minimal shell Uses 7ery lowmemory Nash will 4e acti7ate9rst
LINUXR,
RUNS
ItCs a Script 9le * ha7in)
instructions to loa e8t2: an 54: an also #unction tomount 3C (slash! partition inro (rea only !
Mounts3C
partition
XXXXXXXXXXXXXX XX EN0 ' 1ERNEL L%N0
mount3e73root 3 ro
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S&%R& ' USERL%N0
%s the 3C (slash! Partition is now mounte * 1ernel now opens the irst 9lewith the Inoe Num4erC &he ile it opens is locate at 3s4in3initC Init is the
7ery 9rst process to start an hence it has the pi (process I! Num4er as +Init 9le is a Binary 9le (e8ecuta4le! an it has 4een )i7en an Instructions toLoa a 9le calle3etc3initta41ernel
3s4in3init
I n o e n u m 4 e r
3etc3initta4
I n s t r u
c t i o n s t o L o a
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3s4in3init 3etc3initta4
/IER%R,/6
,hec"s 0e#ault RUNLEELS
System Initialisation (System !( 3etc3rc3rcSysinit!
'pens a Su4-Shell
$ 6etc6syscon:364et,ork' &his 9le is relate to Networ"
6ou can Ena4le or 0isa4leNetwor" #rom this 9le
$ Hostna5e'
6ou can also set /'S&N%ME inthis 9le e)Ser7erE8amplecom &he0e#ault /ostname islocalhostC
2$ Mo-nt ?n ?t 7roc 6 7roc
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%$ 6etc6init$d6f-nctions'@3lo1al
-5ask 3lo1al PATH; de:nes shell f-nctions @ s-ccess;fail-re;7assed;,arnin3echos-ccess echofail-re echo7assed; echo,arnin3 ;kill7roc;7idof7roc;7id:leof7roc action;check7id;con:r5;stat-s;strstr;dae5on
$ 6etc6redhatErelease' 6ou can chan)e the Release
name withthis 9le
&$ Press i> to enter interacti8eset-7'
6ou can enter intoInteracti7eC moe tocustomise your 4ootin)
$ 6etc6syscon:36clock' &his 9le contains U&, &ime
Tone&his9le upates the 9le at
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G$ 6etc6syscon:36init ' 6ou can the set this 9leCs
$raphicalC
7aria4le to 6esC or NoC &his4asically
)i7es 4ootin) in#ormation in$raphics or console
0$ 6etc6sysctl$conf'
&his 9le is #or 1ERNEL tunin)
$ 6etc6syscon:36key1oard' 6ou can Unerstan ,ontrol
"eysC with this 9le
$6fast1oot' &his 9le * i# create in 3C
(slash! partition * ensures thatthe #sc"C (ile system chec"!operation is s"ippe &his 9lewill e7en s"ip the #sc"C
operation e7en i# its mentione C
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2$ 6forcefsck ' &his 9le will ma"e sure * i#
create at 3C Slash Partition*
the #sc"C operation isper#orme E7en i# its notmentione in 3etc3#sta4C 9le
%$ 6etc6syscon:36readonlyEroot'
6ou can set the Entireilesystems to Reaonly F 6esCor NoC
$ 6etc6r,ta1' &his can 4e consiere as an
e8ception to3etc3syscon9)3reaonly-rootCwhich means i# you ma"e theentire 9lesystem to rea onlyan wante to )i7e e8ceptionsto certain 9les3ir then you can
ma"e use o# this 9le to )i7e - C
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$ 6etc65ta1' mta4 stans #or Mount &a4C
When you run mountC
comman * it re#ers to this 9leIt "eeps the in#ormation o# allmounte partition only
$ 6s1in6-otaon'
G$)na1lin3 6etc6fsta1 s,a7s'
0$ 68ar6lo36d5es3'
3etc3rc3rc &his 9le is responsi4le #orstartin) 3 stoppin)ser7ices when runle7elchan)es
(R, F RUNLEEL,/%N$E!
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3etc3rc3rc
+ ,hec"s RUNLEEL . ,hec"s RUNLEEL ,'MM%N0
#rom3etc3initta4 9le
2 3etc3rc31
3etc3rc3S
inicates RUNLEEL Num4erwhereas 1C shows that ser7ice
name with 9rst letter 1C will 4e1ILLE0 an ser7ice namestartin) with SC will 4eS&%R&E0C %ll 9les at thislocation are sym4olic lin" o# itsparent 9le at
3etc3init3ser7icename
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.:
Runle7els
• % runle7el is a so#tware con9)urationo# the system which allows only aselecte )roup o# processes to e8ist
• &he processes spawne 4y init #oreach o# these runle7els are e9ne inthe 3etc3initta4 9le
•
Init can 4e in one o# se7en runle7els:-@
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6s1in65in3etty ( "-nle8elE;;2! tty+ *tty.*tty2*tty?*tty;*tty@(,an 4e increase upto tty+.!
'R
6etc6J67refd5 ("-nle8el E!
0epensUpon theRunle7elset in3etc3initta
4
6etc6iss-e
&he 9le 6etc6iss-e is a te8t 9lewhich contains a messa)e orsystem ienti9cation to 4e printe4e#ore the lo)in prompt It maycontain7arious Kchar an Lchar seuenc
es
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61in6lo3in+o3in co55and is use when si)nin) onto asystem It can also 4e use to switch #rom oneuser to another at any time (most moern shells
ha7e support #or this #eature 4uilt into them*howe7er!
61in67ass,d ' Will Prompt you #orPasswor
PAM
6etc67ass,d
6etc6shado,
6etc63ro-7
6etc63shado,
6$h-shlo3in
lastlo3
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6etc65otd NMessa3e of theday
68ar6s7ool65ail
61in61ash
6etc67ro:le
De:nition of PATH>
Hostna5e aria1le
HISTSIQ) (C5d historyli5it!
6etc6in7-trc
6etc67ro:le$d6R$shAfter +o3in ; yo- canr-n any scri7ts fro5this locationa-to5atically$
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6etc61ashrc
-5ask
PS aria4le #orPrompt
$lo4al User settin)s can 4e one at3etc3pro9le an 3etc34ashrc
3root3$ 1ashrc
3root3 1ashrc7ro:le
3root3 1ashlo3o-t
&his 9le is #orrespecti7euser 6ou canset umas" atini7iual
le7el
In this caserootC isconsiere to4e the user
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EN0 ' B''& PR',ESS
&/%N1 6'U