boot(7) — Linux manual page


boot(7)             Miscellaneous Information Manual             boot(7)

NAME         top

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

DESCRIPTION         top

       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.

       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 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 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 partition.

       In Linux, the OS loader is often grub(8) (an alternative is

       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 scripts.

       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

       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(8) service on run
       level 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

FILES         top

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

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                          boot(7)

Pages that refer to this page: bootup(7)