1BOOT(7)                    Linux Programmer's Manual                   BOOT(7)
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NAME

6       boot-scripts - General description of boot sequence
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DESCRIPTION

9       The  boot  sequence  varies in details among systems but can be roughly
10       divided to the following steps: (i)  hardware  boot,  (ii)  OS  loader,
11       (iii) kernel startup, (iv) init and inittab, (v) boot scripts.  We will
12       describe each of these in more detail below.
13
14   Hardware-boot
15       After power-on or hard reset, control is given to a program  stored  on
16       read-only  memory  (normally PROM).  In PC we usually call this program
17       the BIOS.
18
19       This program normally makes  a  basic  self-test  of  the  machine  and
20       accesses  non-volatile  memory to read further parameters.  This memory
21       in the PC is battery-backed CMOS memory, so most people refer to it  as
22       the  CMOS, although outside of the PC world, it is usually called nvram
23       (non-volatile ram).
24
25       The parameters stored in the nvram vary between systems, but as a mini‐
26       mum,  the hardware boot program should know what is the boot device, or
27       which devices to probe as possible boot devices.
28
29       Then the hardware boot stage accesses the boot  device,  loads  the  OS
30       Loader,  which  is  located on a fixed position on the boot device, and
31       transfers control to it.
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33       Note:  We do not cover here booting from network.  Those  who  want  to
34              investigate  this subject may want to research: DHCP, TFTP, PXE,
35              Etherboot.
36
37   OS Loader
38       In PC, the OS Loader is located in the first sector of the boot  device
39       - this is the MBR (Master Boot Record).
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41       In  most  systems,  this  primary loader is very limited due to various
42       constraints.  Even on non-PC systems there are some limitations to  the
43       size  and  complexity of this loader, but the size limitation of the PC
44       MBR (512 bytes including the partition table) makes it almost  impossi‐
45       ble to squeeze a full OS Loader into it.
46
47       Therefore,  most  operating systems make the primary loader call a sec‐
48       ondary OS loader which may be located on a specified disk partition.
49
50       In Linux the OS loader is normally lilo(8) or grub(8).   Both  of  them
51       may  install  either  as secondary loaders (where the DOS installed MBR
52       points to them), or as a two part loader where they provide special MBR
53       containing  the  bootstrap  code  to load the second part of the loader
54       from the root partition.
55
56       The main job of the OS Loader is to locate the kernel on the disk, load
57       it and run it.  Most OS loaders allow interactive use, to enable speci‐
58       fication of alternative kernel (maybe a backup in case  the  last  com‐
59       piled  one  isn't  functioning)  and to pass optional parameters to the
60       kernel.
61
62   Kernel Startup
63       When the kernel is loaded, it initializes the devices (via their  driv‐
64       ers),  starts  the  swapper (it is a "kernel process", called kswapd in
65       modern Linux kernels), and mounts the root file system (/).
66
67       Some of the parameters that may be passed to the kernel relate to these
68       activities  (e.g:  You can override the default root file system).  For
69       further information on Linux kernel parameters read bootparam(7).
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71       Only then the kernel creates the first (user  land)  process  which  is
72       numbered  1.  This process executes the program /sbin/init, passing any
73       parameters that weren't handled by the kernel already.
74
75   init and inittab
76       When init starts it reads /etc/inittab for further instructions.   This
77       file defines what should be run in different run-levels.
78
79       This  gives  the  system administrator an easy management scheme, where
80       each run-level is associated with a set of services (e.g: S is  single-
81       user,  on  2 most network services start, etc.).  The administrator may
82       change the current run-level via init(8) and  query  the  current  run-
83       level via runlevel(8).
84
85       However,  since  it  is not convenient to manage individual services by
86       editing this file, inittab only bootstraps a set of scripts that  actu‐
87       ally start/stop the individual services.
88
89   Boot Scripts
90       Note:  The  following  description  applies to System V release 4 based
91              system, which currently  covers  most  commercial  Unix  systems
92              (Solaris, HP-UX, Irix, Tru64) as well as the major Linux distri‐
93              butions (RedHat, Debian, Mandrake, Suse, Caldera).  Some systems
94              (Slackware  Linux,  FreeBSD,  OpenBSD) have a somewhat different
95              scheme of boot scripts.
96
97       For each managed service (mail, nfs server, cron, etc.) there is a sin‐
98       gle startup script located in a specific directory (/etc/init.d in most
99       versions of Linux).  Each of these scripts accepts as a single argument
100       the word "start" -- causing it to start the service, or the word "stop"
101       -- causing it to stop the service.  The script  may  optionally  accept
102       other "convenience" parameters (e.g: "restart", to stop and then start,
103       "status" do display the service status).  Running  the  script  without
104       parameters displays the possible arguments.
105
106   Sequencing Directories
107       To  make specific scripts start/stop at specific run-levels and in spe‐
108       cific order, there are sequencing directories.  These are  normally  in
109       /etc/rc[0-6S].d.  In each of these directories there are links (usually
110       symbolic) to the scripts in the /etc/init.d directory.
111
112       A primary script (usually /etc/rc) is called from inittab(5) and  calls
113       the  services scripts via the links in the sequencing directories.  All
114       links with names that begin with 'S' are being called with the argument
115       "start"  (thereby  starting  the  service).   All links with names that
116       begin with 'K' are being called with the argument "stop" (thereby stop‐
117       ping the service).
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119       To define the starting or stopping order within the same run-level, the
120       names of the links contain order-numbers.   Also,  to  make  the  names
121       clearer,  they  usually end with the name of the service they refer to.
122       Example: the link /etc/rc2.d/S80sendmail starts the sendmail service on
123       runlevel  2.  This happens after /etc/rc2.d/S12syslog is run but before
124       /etc/rc2.d/S90xfs is run.
125
126       To manage the boot order and run-levels, we have to manage these links.
127       However,  on  many versions of Linux, there are tools to help with this
128       task (e.g: chkconfig(8)).
129
130   Boot Configuration
131       Usually the daemons started may optionally receive command-line options
132       and parameters.  To allow system administrators to change these parame‐
133       ters without editing the boot scripts themselves,  configuration  files
134       are used.  These are located in a specific directory (/etc/sysconfig on
135       RedHat systems) and are used by the boot scripts.
136
137       In older Unix systems, these files contained the  actual  command  line
138       options  for  the daemons, but in modern Linux systems (and also in HP-
139       UX), these files just contain shell variables.   The  boot  scripts  in
140       /etc/init.d  source  the configuration files, and then use the variable
141       values.
142

FILES

144       /etc/init.d/, /etc/rc[S0-6].d/, /etc/sysconfig/
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SEE ALSO

147       inittab(5), bootparam(7), init(8), runlevel(8), shutdown(8)
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COLOPHON

150       This page is part of release 3.22 of the Linux  man-pages  project.   A
151       description  of  the project, and information about reporting bugs, can
152       be found at http://www.kernel.org/doc/man-pages/.
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156Linux                             2007-06-03                           BOOT(7)
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