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



NAME
       boot - System bootup process based on UNIX System V Release 4

DESCRIPTION
       The  bootup  process (or "boot sequence") varies in details among systems, but can be roughly
       divided into phases controlled by the following components:

       1.  hardware

       2.  operating system (OS) loader

       3.  kernel

       4.  root user-space process (init and inittab)

       5.  boot scripts

       Each of these is described below in more detail.

   Hardware
       After power-on or hard reset, control is given to a program stored in read-only memory  (nor‐
       mally  PROM);  for  historical reasons involving the personal computer, this program is often
       called "the BIOS".

       This program normally performs a basic self-test of the machine and accesses nonvolatile mem‐
       ory to read further parameters.  This memory in the PC is battery-backed CMOS memory, so most
       people refer to it as "the CMOS"; outside of the PC world, it is usually called  "the  NVRAM"
       (nonvolatile RAM).

       The  parameters stored in the NVRAM vary among systems, but as a minimum, they should specify
       which device can supply an OS loader, or at least which devices may be probed for one; such a
       device  is  known  as  "the boot device".  The hardware boot stage loads the OS loader from a
       fixed position on the boot device, and then transfers control to it.

       Note:  The device from which the OS loader is read may be attached via a  network,  in  which
              case  the  details  of  booting are further specified by protocols such as DHCP, TFTP,
              PXE, Etherboot, etc.

   OS loader
       The main job of the OS loader is to locate the kernel on some device, load it,  and  run  it.
       Most  OS  loaders  allow  interactive use, in order to enable specification of an alternative
       kernel (maybe a backup in case the one last compiled isn't functioning) and to pass  optional
       parameters to the kernel.

       In  a  traditional PC, the OS loader is located in the initial 512-byte block of the boot de‐
       vice; this block is known as "the MBR" (Master Boot Record).

       In most systems, the OS loader is very limited due to various constraints.   Even  on  non-PC
       systems,  there  are some limitations on 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 much functionality into it.

       Therefore,  most  systems  split the role of loading the OS between a primary OS loader and a
       secondary OS loader; this secondary OS loader may be located within a larger portion of  per‐
       sistent storage, such as a disk partition.

       In Linux, the OS loader is often either lilo(8) or grub(8).

   Kernel
       When  the  kernel  is loaded, it initializes various components of the computer and operating
       system; each portion of software responsible for such a task is usually consider  "a  driver"
       for  the  applicable component.  The kernel starts the virtual memory swapper (it is a kernel
       process, called "kswapd" in a modern Linux kernel), and mounts some filesystem  at  the  root
       path, /.

       Some of the parameters that may be passed to the kernel relate to these activities (for exam‐
       ple, the default root filesystem can be overridden); for further information on Linux  kernel
       parameters, read bootparam(7).

       Only then does the kernel create the initial userland process, which is given the number 1 as
       its PID (process ID).  Traditionally, this process executes the program /sbin/init, to  which
       are passed the parameters that haven't already been handled by the kernel.

   Root user-space process
       Note:  The following description applies to an OS based on UNIX System V Release 4.  However,
              a number of widely used systems have adopted a related but fundamentally different ap‐
              proach known as systemd(1), for which the bootup process is detailed in its associated
              bootup(7).

       When /sbin/init starts, it reads /etc/inittab for further instructions.   This  file  defines
       what should be run when the /sbin/init program is instructed to enter a particular run-level,
       giving the administrator an easy way to establish an environment for some  usage;  each  run-
       level is associated with a set of services (for example, run-level S is single-user mode, and
       run-level 2 entails running most network services).

       The administrator may change the current run-level via init(1), and query  the  current  run-
       level via runlevel(8).

       However,  since  it  is  not  convenient  to manage individual services by editing this file,
       /etc/inittab only bootstraps a set of scripts that actually start/stop  the  individual  ser‐
       vices.

   Boot scripts
       Note:  The following description applies to an OS based on UNIX System V Release 4.  However,
              a number of widely used systems (Slackware Linux, FreeBSD, OpenBSD)  have  a  somewhat
              different scheme for boot scripts.

       For each managed service (mail, nfs server, cron, etc.), there is a single startup script lo‐
       cated 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  "conve‐
       nience"  parameters  (e.g., "restart" to stop and then start, "status" to display the service
       status, etc.).  Running the script without parameters displays the possible arguments.

   Sequencing directories
       To make specific scripts start/stop at specific run-levels and in a specific order, there are
       sequencing directories, normally of the form  /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); this primary script calls  each
       service's  script  via a link in the relevant sequencing directory.  Each link whose name be‐
       gins with 'S' is called with the argument "start" (thereby starting the service).  Each  link
       whose name begins with 'K' is called with the argument "stop" (thereby stopping the service).

       To  define  the starting or stopping order within the same run-level, the name of a link con‐
       tains an order-number.  Also, for clarity, the name of a link usually ends with the  name  of
       the  service  to  which  it  refers.  For 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  these links is to manage the boot order and run-levels; under many systems, there
       are tools to help with this task (e.g., chkconfig(8)).

   Boot configuration
       A program that provides a service is often called a "daemon".  Usually, a daemon may  receive
       various command-line options and parameters.  To allow a system administrator to change these
       inputs without editing an entire boot script, some separate configuration file is  used,  and
       is  located in a specific directory where an associated boot script may find it (/etc/syscon‐
       fig on older Red Hat systems).

       In older UNIX systems, such a file contained the actual command line options  for  a  daemon,
       but  in  modern  Linux systems (and also in HP-UX), it just contains shell variables.  A boot
       script in /etc/init.d reads and includes its configuration file (that is,  it  "sources"  its
       configuration file) and then uses the variable values.

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

SEE ALSO
       init(1), systemd(1), inittab(5), bootparam(7), bootup(7), runlevel(8), shutdown(8)

COLOPHON
       This  page  is  part  of  release  5.10 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be  found
       at https://www.kernel.org/doc/man-pages/.



Linux                                        2015-03-11                                      BOOT(7)