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.\" 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
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
.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)