.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.\" 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
.\"
.TH boot 7 2025-09-21 "Linux man-pages (unreleased)"
.SH NAME
boot \- System bootup process based on UNIX System V Release 4
.SH DESCRIPTION
The
.B bootup process
(or
.RB \[dq] boot\~sequence \[dq])
varies in details
among systems, but can be roughly divided into phases controlled by
the following components:
.IP (1) 5
hardware
.IP (2)
operating system (OS) loader
.IP (3)
kernel
.IP (4)
root user-space process
.RB ( init (8)
and
.BR inittab (5))
.IP (5)
boot scripts
.P
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
.BR BIOS \[dq].
.P
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
.BR CMOS \[dq];
outside of the PC world,
it is usually called
"the
.BR NVRAM \[dq]
(nonvolatile RAM).
.P
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
.BR boot\~device \[dq].
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.
.P
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
.BR MBR \[dq]
(Master Boot Record).
.P
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.
.P
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.
.P
In Linux,
the OS loader is often
.BR grub (8)
(an alternative is
.BR lilo (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 considered
"a
.BR driver \[dq]
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 / .
.P
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).
.P
Only then does the kernel create the initial user-space 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).
.P
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 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
.B S
is single-user mode,
and run level
.B 2
entails running most network services).
.P
The administrator may change the current run level via
.BR init (1),
and query the current run level via
.BR runlevel (8).
.P
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.
.P
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
.IR sequencing\~directories ,
normally of the form
.IR /etc/rc[0\-6S].d .
In each of these directories,
there are links
(usually symbolic)
to the scripts in the
.I /etc/init.d
directory.
.P
A primary script
(usually
.IR /etc/rc )
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 \[aq]S\[aq] is called
with the argument "start"
(thereby starting the service).
Each link whose name begins with \[aq]K\[aq] is called
with the argument "stop"
(thereby stopping the service).
.P
To define the starting or stopping order within the same run level,
the name of a link contains an
.BR 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
.I /etc/rc2.d/S80sendmail
starts the
.BR sendmail (8)
service on
run level 2.
This happens after
.I /etc/rc2.d/S12syslog
is run
but before
.I /etc/rc2.d/S90xfs
is run.
.P
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
.RB \[dq] daemon \[dq].
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
.RI ( /etc/sysconfig
on older Red Hat systems).
.P
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
.I /etc/init.d
reads and includes its configuration file
(that is,
it
.RB \[dq] sources \[dq]
its configuration file)
and then uses the variable values.
.SH FILES
.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)