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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 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.
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 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 either lilo(8) or grub(8).
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 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 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.,
chkconfig(8)).
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), shutdown(8)
This page is part of release 4.15 of the Linux 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/.
Linux 2015-03-11 BOOT(7)
Pages that refer to this page: bootup(7)
Copyright and license for this manual page
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