The  boot  sequence  varies in details among systems but can be roughly
       divided to the following steps: (i) hardware boot, (ii) operating  sys-
       tem  (OS) loader, (iii) kernel startup, (iv) init and inittab, (v) boot
       scripts.  We will describe each of these in more detail below.

       After power-on or hard reset, control is given to a program  stored  on
       read-only  memory  (normally PROM).  In PC we usually call this program
       the BIOS.

       This program normally makes  a  basic  self-test  of  the  machine  and
       accesses nonvolatile memory to read further parameters.  This memory in
       the PC is battery-backed CMOS memory, so most people refer to it as the
       CMOS,  although  outside  of  the  PC world, it is usually called nvram
       (nonvolatile ram).

       The parameters stored in the nvram vary between systems, but as a mini-
       mum,  the hardware boot program should know what is the boot device, or
       which devices to probe as possible boot devices.

       Then the hardware boot stage accesses the boot  device,  loads  the  OS
       loader,  which  is  located on a fixed position on the boot device, and
       transfers control to it.

       Note:  We do not cover here booting from network.  Those  who  want  to
              investigate  this subject may want to research: DHCP, TFTP, PXE,

   OS loader
       In PC, the OS loader is located in the first sector of the boot  device
       - this is the MBR (Master Boot Record).

       In  most  systems,  this  primary loader is very limited due to various
       constraints.  Even on non-PC systems there are some limitations to  the
       size  and  complexity of this loader, but the size limitation of the PC
       MBR (512 bytes including the partition table) makes it almost  impossi-
       ble to squeeze a full OS loader into it.

       Therefore,  most  operating systems make the primary loader call a sec-
       ondary OS loader which may be located on a specified disk partition.

       In Linux the OS loader is normally lilo(8) or grub(8).   Both  of  them
       may  install  either  as secondary loaders (where the DOS installed MBR
       points to them), or as a two part loader where they provide special MBR
       containing  the  bootstrap  code  to load the second part of the loader
       from the root partition.

       The main job of the OS loader is to locate the kernel on the disk, load
       it and run it.  Most OS loaders allow interactive use, to enable speci-
       fication of alternative kernel (maybe a backup in case  the  last  com-
       piled  one  isn't  functioning)  and to pass optional parameters to the
       parameters that weren't handled by the kernel already.

   init and inittab
       When init starts it reads /etc/inittab for further instructions.   This
       file defines what should be run in different run-levels.

       This  gives  the  system administrator an easy management scheme, where
       each run-level is associated with a set of services (e.g, S is  single-
       user,  on 2 most network services start).  The administrator may change
       the current run-level via init(8) and query the current  run-level  via

       However,  since  it  is not convenient to manage individual services by
       editing this file, inittab only bootstraps a set of scripts that  actu-
       ally start/stop the individual services.

   Boot scripts
       Note:  The  following  description  applies to System V release 4-based
              systems, which currently covers  most  commercial  UNIX  systems
              (Solaris, HP-UX, Irix, Tru64) as well as the major Linux distri-
              butions (Red Hat, Debian, Mandriva, SUSE, Ubuntu).  Some systems
              (Slackware  Linux,  FreeBSD,  OpenBSD) have a somewhat different
              scheme of boot scripts.

       For each managed service (mail, nfs server, cron, etc.) there is a sin-
       gle startup script located in a specific directory (/etc/init.d in most
       versions of Linux).  Each of these scripts accepts as a single argument
       the word "start" -- causing it to start the service, or the word "stop"
       -- causing it to stop the service.  The script  may  optionally  accept
       other "convenience" parameters (e.g: "restart", to stop and then start,
       "status" do display the service status).  Running  the  script  without
       parameters displays the possible arguments.

   Sequencing directories
       To  make specific scripts start/stop at specific run-levels and in spe-
       cific order, there are sequencing directories.  These are  normally  in
       /etc/rc[0-6S].d.  In each of these directories there are links (usually
       symbolic) to the scripts in the /etc/init.d directory.

       A primary script (usually /etc/rc) is called from inittab(5) and  calls
       the  services scripts via the links in the sequencing directories.  All
       links with names that begin with 'S' are being called with the argument
       "start"  (thereby  starting  the  service).   All links with names that
       begin with 'K' are being called with the argument "stop" (thereby stop-
       ping the service).

       To define the starting or stopping order within the same run-level, the
       names of the links contain order-numbers.   Also,  to  make  the  names
       clearer,  they  usually end with the name of the service they refer to.
       Example: the link /etc/rc2.d/S80sendmail starts the sendmail service on
       runlevel  2.  This happens after /etc/rc2.d/S12syslog is run but before
       /etc/rc2.d/S90xfs is run.

       To manage the boot order and run-levels, we have to manage these links.
       UX), these files just contain shell variables.   The  boot  scripts  in
       /etc/init.d  source  the configuration files, and then use the variable

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

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

       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

Linux                             2010-09-19                           BOOT(7)
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