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boot.7: Copy edit 

While a lot of the changes are issues of presentation,
there are also issues of grammar and punctuation.

Signed-off-by: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Witten 2015-03-11 19:54:44 +00:00 committed by Michael Kerrisk
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.\"
.\" Modified 2004-11-03 patch from Martin Schulze <joey@infodrom.org>
.\"
.TH BOOT 7 2010-09-19 "Linux" "Linux Programmer's Manual"
.TH BOOT 7 2015-03-11 "Linux" "Linux Programmer's Manual"
.SH NAME
boot-scripts \- general description of boot sequence
boot \- System bootup process based on UNIX System V Release 4
.SH DESCRIPTION
.LP
The boot sequence varies in details among systems
but can be roughly divided to the following steps:
(i) hardware boot, (ii) operating system (OS) loader,
(iii) kernel startup, (iv) init and inittab,
(v) boot scripts.
We will describe each of these in more detail below.
.SS Hardware-boot
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 on read-only memory (normally
PROM).
In PC we usually call this program the \fBBIOS\fR.
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".
This program normally makes a basic self-test of the
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, although outside
of the PC world, it is usually called \fBnvram\fR
(nonvolatile ram).
refer to it as "the \fBCMOS\fR"; outside
of the PC world, it is usually called "the \fBNVRAM\fR"
(nonvolatile RAM).
The parameters stored in the nvram vary between
systems, but as a minimum, the hardware boot program
should know what is the boot device, or which devices
to probe as possible boot devices.
Then the hardware boot stage accesses the boot device,
loads the OS loader, which is located on a fixed position
on the boot device, and transfers control to it.
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:
We do not cover here booting from network.
Those who want
to investigate this subject may want to research:
DHCP, TFTP, PXE, Etherboot.
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
In PC, the OS loader is located in the first sector
of the boot device \- this is the \fBMBR\fR
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 \fBMBR\fR"
(Master Boot Record).
In most systems, this primary loader is very
In most systems, the OS loader is very
limited due to various constraints.
Even on non-PC systems
there are some limitations to the size and complexity
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 a full OS loader into it.
(512 bytes, including the partition table) makes it
almost impossible to squeeze much functionality into it.
Therefore, most operating systems make the primary loader
call a secondary OS loader which may be located on
a specified disk partition.
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 normally
In Linux, the OS loader is often either
.BR lilo (8)
or
.BR grub (8).
Both of them may install either as secondary loaders
(where the DOS installed MBR points to them), or
as a two part loader where they provide special MBR
containing the bootstrap code to load the second part
of the loader from the root partition.
The main job of the OS loader is to locate the kernel
on the disk, load it and run it.
Most OS loaders allow
interactive use, to enable specification of alternative
kernel (maybe a backup in case the last compiled one
isn't functioning) and to pass optional parameters
to the kernel.
.SS Kernel startup
When the kernel is loaded, it initializes the devices (via
their drivers), starts the swapper (it is a "kernel process",
called kswapd in modern Linux kernels), and mounts the root
filesystem (/).
.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 / .
Some of the parameters that may be passed to the kernel
relate to these activities (e.g: You can override the
default root filesystem).
For further information
on Linux kernel parameters read
relate to these activities (for example, the default root filesystem
can be overriden); for further information
on Linux kernel parameters, read
.BR bootparam (7).
Only then the kernel creates the first (user land)
process which is numbered 1.
This process executes the
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 ,
passing any parameters that weren't handled by the kernel already.
.SS init and inittab
When init starts it reads
to which are passed the parameters that haven't already been
handled by the kernel.
.SS Root user-space process
When
.I /sbin/init
starts, it reads
.I /etc/inittab
for further instructions.
This file defines what should be run in different \fIrun-levels\fR.
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).
This gives the system administrator an easy management scheme, where
each run-level is associated with a set of services (e.g,
\fBS\fR is \fIsingle-user\fR, on \fB2\fR most network
services start).
The administrator may change the current
run-level via
.BR init (1)
.BR init (1),
and query the current run-level via
.BR runlevel (8).
However, since it is not convenient to manage individual services
by editing this file, inittab only bootstraps a set of scripts
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 System V release 4-based systems, which
currently covers most commercial UNIX systems (Solaris, HP-UX, Irix, Tru64)
as well as the major Linux distributions (Red Hat, Debian, Mandriva,
The following description applies to an OS based on UNIX System V Release 4,
which currently covers most commercial UNIX systems (Solaris, HP-UX, Irix,
Tru64) as well as the major Linux distributions (Red Hat, Debian, Mandriva,
SUSE, Ubuntu).
Some systems (Slackware Linux, FreeBSD, OpenBSD)
have a somewhat different scheme of boot scripts.
have a somewhat different scheme for boot scripts.
.LP
For each managed service (mail, nfs server, cron, etc.) there is
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 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).
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
specific order, there are \fIsequencing directories\fR.
These
are normally in \fI/etc/rc[0\-6S].d\fR.
In each of these 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.
A primary script (usually \fI/etc/rc\fR) is called from
.BR inittab (5)
and calls the services scripts via the links in the sequencing directories.
All links with names that begin with \(aqS\(aq are being called with
.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).
All links with
names that begin with \(aqK\(aq are being called with the argument "stop"
(thereby stopping the service).
Each link whose name begins with \(aqK\(aq is called with
the argument "stop" (thereby stopping the service).
To define the starting or stopping order within the same run-level,
the names of the links contain order-numbers.
Also, to make the names clearer, they usually
end with the name of the service they refer to.
Example:
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.
To manage the boot order and run-levels, we have to manage these links.
However, on many versions of Linux, there are tools to help with this task
(e.g:
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
Usually the daemons started may optionally receive command-line options
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 system administrators to change these
parameters without editing the boot scripts themselves,
configuration files are used.
These are located in a specific
directory (\fI/etc/sysconfig\fR on Red Hat systems) and are
used by the boot scripts.
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 Red Hat systems).
In older UNIX systems, these files contained the actual command line
options for the daemons, but in modern Linux systems (and also
in HP-UX), these files just contain shell variables.
The boot scripts in \fI/etc/init.d\fR
\fBsource\fR the configuration
files, and then use the variable values.
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
.LP
.IR /etc/init.d/ ,