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

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

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

       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.

       After power-on or hard reset, control is given to a program stored in
       read-only memory (normally 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 memory 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 device; this block is known as "the MBR" (Master Boot

       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 impossible 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 persistent storage, such as a disk

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

       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 example, 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 approach 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 services.

   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

       For each managed service (mail, nfs server, cron, etc.), there is a
       single 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" 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 begins 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 contains 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.,

   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/sysconfig 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.

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

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

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Linux                              2015-03-11                            BOOT(7)