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



NAME
       boot-scripts - General description of boot sequence

DESCRIPTION
       The  boot  sequence  varies in(1,8) details among systems but can be roughly
       divided to the following steps: (i)  hardware  boot,  (ii)  OS  loader,
       (iii) kernel startup, (iv) init and inittab, (v) boot scripts.  We will
       describe each of these in(1,8) more detail below.


   Hardware-boot
       After power-on or hard reset(1,7,1 tput), control is given to a program  stored  on
       read(2,n,1 builtins)  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 non-volatile memory to read(2,n,1 builtins) further parameters. This memory in(1,8)
       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
       (non-volatile ram).

       The parameters stored in(1,8) 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,
              Etherboot.


   OS Loader
       In PC, the OS Loader is located in(1,8) 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(5,8)(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(7,n) 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(7,n)
       it  and run it. Most OS loaders allow interactive use, to enable speci-
       fication of alternative kernel (maybe a backup in(1,8) case  the  last  com-
       piled  one  isn't  functioning)  and to pass optional parameters to the
       kernel.


   Kernel Startup
       When the kernel is loaded, it initializes the devices (via their  driv-
       ers),  starts  the  swapper (it is a "kernel process", called kswapd in(1,8)
       modern Linux kernels), and mounts the root file(1,n) system (/).

       Some of the parameters that may be passed to the kernel relate to these
       activities  (e.g:  You  can override the default root file(1,n) system). For
       further information on Linux kernel parameters read(2,n,1 builtins) bootparam(7).

       Only then the kernel creates the first (user  land)  process  which  is
       numbered  1.  This process executes the program /sbin/init, passing any
       parameters that weren't handled by the kernel already.


   init and inittab
       When init starts it reads /etc/inittab for further instructions.   This
       file(1,n) defines what should be run in(1,8) different run-levels.

       This  gives  the  system administrator an easy management scheme, where
       each run-level is associated with a set(7,n,1 builtins) of services  (e.g:  S  is  sin-
       gle-user,  on  2  most network services start, etc.). The administrator
       may change the current run-level via init(8) and query the current run-
       level via runlevel(8).

       However,  since  it  is not convenient to manage individual services by
       editing this file(1,n), inittab only bootstraps a set(7,n,1 builtins) of scripts that  actu-
       ally start/stop the individual services.


   Boot Scripts
       Note:  The following description applies to SYSV-R4 based system, which
              currently covers most commercial Unices  (Solaris,  HPUX,  Irix,
              Tru64) as well as the major Linux distributions (RedHat, Debian,
              Mandrake,  Suse,  Caldera).  Some  systems   (Slackware   Linux,
              FreeBSD,  OpenBSD)  have  a  somewhat  different  scheme of boot
              scripts.


       For each managed service (mail(1,8), nfs server, cron, etc.) there is a sin-
       gle startup script located in(1,8) a specific directory (/etc/init.d in(1,8) 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 accept(2,8)
       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(1,8)  spe-
       cific  order,  there  are sequencing directories. These are normally in(1,8)
       /etc/rc[0-6S].d. In each of these directories there are links  (usually
       symbolic) to the scripts in(1,8) the init.d directory.

       A  primary script (usually /etc/rc) is called from inittab(5) and calls
       the services scripts via the links in(1,8) 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 stopping
       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(1,2) /etc/rc2.d/S80sendmail starts the sendmail(1,8) 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.
       However, on many versions of Linux, there are tools to help  with  this
       task (e.g: chkconfig(8)).


   Boot Configuration
       Usually the daemons started may optionally receive command line options
       and parameters. To allow system administrators to change these  parame-
       ters  without  editing the boot scripts themselves, configuration files
       are used. These are located in(1,8) a specific directory (/etc/sysconfig  on
       RedHat systems) and are used by the boot scripts.

       In  older Unices, these files contained the actual command line options
       for the daemons, but in(1,8) modern Linux systems (and also in(1,8) HPUX),  these
       files  just  contain  shell  variables. The boot scripts in(1,8) /etc/init.d
       source the configuration files, and then use the variable values.

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


SEE ALSO
       inittab(5), bootparam(7), init(8), runlevel(8), shutdown(2,8)(8)



                                  2002-06-07                           BOOT(7)

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