.\" Written by Oron Peled <oron@actcom.co.il>.

.\"

.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)

.\" May be distributed subject to the GPL.

.\" %%%LICENSE_END

.\"

.\" I tried to be as much generic in the description as possible:

.\" - General boot sequence is applicable to almost any

.\" OS/Machine (DOS/PC, Linux/PC, Solaris/SPARC, CMS/S390)

.\" - kernel and init(1) is applicable to almost any UNIX/Linux

.\" - boot scripts are applicable to SYSV-R4 based UNIX/Linux

.\"

.\" Modified 2004-11-03 patch from Martin Schulze <joey@infodrom.org>

.\"

.TH BOOT 7 2015-03-11 "Linux" "Linux Programmer's Manual"

.SH NAME

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

.SH DESCRIPTION

.PP

The \fBbootup process\fR (or "\fBboot sequence\fR") varies in details

among systems, but can be roughly divided into phases controlled by

the following components:

.IP 1. 4

hardware

.IP 2. 4

operating system (OS) loader

.IP 3. 4

kernel

.IP 4. 4

root user-space process (\fIinit\fR and \fIinittab\fR)

.IP 5. 4

boot scripts

.PP

Each of these is described below in more detail.

.SS Hardware

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 \fBBIOS\fR".

.PP

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 \fBCMOS\fR"; outside

of the PC world, it is usually called "the \fBNVRAM\fR"

(nonvolatile RAM).

.PP

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

\fBboot device\fR".

The hardware boot stage loads the OS loader from a fixed position on

the boot device, and then transfers control to it.

.TP

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.

.SS 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.

.PP

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 \fBMBR\fR"

(Master Boot Record).

.PP

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.

.PP

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.

.PP

In Linux, the OS loader is often either

.BR lilo (8)

or

.BR grub (8).

.SS 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 \fBdriver\fR" 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,

.IR / .

.PP

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

.BR bootparam (7).

.PP

Only then does the kernel create the initial userland

process, which is given the number 1 as its

.B PID

(process ID).

Traditionally, this process executes the

program

.IR /sbin/init ,

to which are passed the parameters that haven't already been

handled by the kernel.

.SS Root user-space process

.TP

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

.BR systemd (1),

for which the bootup process is detailed in its associated

.BR bootup (7).

.PP

When

.I /sbin/init

starts, it reads

.I /etc/inittab

for further instructions.

This file defines what should be run when the

.I /sbin/init

program is instructed to enter a particular \fIrun-level\fR, 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 \fBS\fR is \fIsingle-user\fR mode,

and run-level \fB2\fR entails running most network services).

.PP

The administrator may change the current

run-level via

.BR init (1),

and query the current run-level via

.BR runlevel (8).

.PP

However, since it is not convenient to manage individual services

by editing this file,

.I /etc/inittab

only bootstraps a set of scripts

that actually start/stop the individual services.

.SS Boot scripts

.TP

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.

.PP

For each managed service (mail, nfs server, cron, etc.), there is

a single startup script located in a specific directory

.RI ( /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.

.SS Sequencing directories

To make specific scripts start/stop at specific run-levels and in a

specific order, there are \fIsequencing directories\fR, normally

of the form  \fI/etc/rc[0\-6S].d\fR.

In each of these directories,

there are links (usually symbolic) to the scripts in the \fI/etc/init.d\fR

directory.

.PP

A primary script (usually \fI/etc/rc\fR) is called from

.BR inittab (5);

this primary script calls each service's script via a link in the

relevant sequencing directory.

Each link whose name begins with \(aqS\(aq is called with

the argument "start" (thereby starting the service).

Each link whose name begins with \(aqK\(aq is called with

the argument "stop" (thereby stopping the service).

.PP

To define the starting or stopping order within the same run-level,

the name of a link contains an \fBorder-number\fR.

Also, for clarity, the name of a link usually

ends with the name of the service to which it refers.

For example,

the link \fI/etc/rc2.d/S80sendmail\fR starts the sendmail service on

runlevel 2.

This happens after \fI/etc/rc2.d/S12syslog\fR is run

but before \fI/etc/rc2.d/S90xfs\fR is run.

.PP

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.,

.BR chkconfig (8)).

.SS Boot configuration

A program that provides a service is often called a "\fBdaemon\fR".

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

(\fI/etc/sysconfig\fR on older Red Hat systems).

.PP

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 \fI/etc/init.d\fR reads and includes its configuration

file (that is, it "\fBsources\fR" its configuration file) and then uses

the variable values.

.SH FILES

.PP

.IR /etc/init.d/ ,

.IR /etc/rc[S0\-6].d/ ,

.I /etc/sysconfig/

.SH SEE ALSO

.BR init (1),

.BR systemd (1),

.BR inittab (5),

.BR bootparam (7),

.BR bootup (7),

.BR runlevel (8),

.BR shutdown (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.