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NAME | DESCRIPTION | FILES | SEE ALSO | COLOPHON |
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boot(7) Miscellaneous Information 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 grub(8) (an alternative is
lilo(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(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.,
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 boot(7)
Pages that refer to this page: bootup(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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