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:
- operating system (OS) loader
- root user-space process (init and
- boot scripts
Each of these is described below in more detail.
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
"; outside of the PC world, it is usually called "the
" (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
". The hardware boot stage loads the OS loader from a fixed
position on the boot device, and then transfers control to it.
- 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.
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
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
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)
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.
- 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
starts, it reads /etc/inittab
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
mode, and run-level 2
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.
- 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
For each managed service (mail, nfs server, cron, etc.), there is a single
startup script located in a specific directory (/etc/init.d
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.
To make specific scripts start/stop at specific run-levels and in a specific
order, there are sequencing directories
, normally of the form
. In each of these directories, there are links (usually
symbolic) to the scripts in the /etc/init.d
A primary script (usually /etc/rc
) is called from inittab(5)
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
starts the sendmail service on runlevel 2.
This happens after /etc/rc2.d/S12syslog
is run but before
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)
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 (
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.
This page is part of release 4.16 of the Linux man-pages
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