The boot sequence varies in details among systems but can be roughly divided to
the following steps: (i) hardware boot, (ii) OS loader, (iii) kernel startup,
(iv) init and inittab, (v) boot scripts. We will describe each of these in
more detail below.
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 BIOS.
This program normally makes a basic self-test of the machine and accesses
non-volatile memory to read further parameters. This memory in the PC is
battery-backed CMOS memory, so most people refer to it as the CMOS,
although outside of the PC world, it is usually called nvram
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
We do not cover here booting from network. Those who want
to investigate this subject may want to research: DHCP, TFTP, PXE,
In PC, the OS Loader is located in the first sector of the boot device - this is
the MBR (Master Boot Record).
In most systems, this primary loader is very limited due to various constraints.
Even on non-PC systems there are some limitations to 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
Therefore, most operating systems make the primary loader call a secondary OS
loader which may be located on a specified disk partition.
In Linux the OS loader is normally lilo(8) or 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
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.
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 file system (/).
Some of the parameters that may be passed to the kernel relate to these
activities (e.g: You can override the default root file system). For further
information on Linux kernel parameters read bootparam(7).
Only then the kernel creates the first (user land) process which is numbered 1.
This process executes the program /sbin/init, passing any parameters
that weren't handled by the kernel already.
When init starts it reads /etc/inittab for further instructions. This
file defines what should be run in different run-levels.
This gives the system administrator an easy management scheme, where each
run-level is associated with a set of services (e.g: S is
single-user, on 2 most network services start, etc.). The
administrator may change the current run-level via init(8) and query
the current run-level via runlevel(8).
However, since it is not convenient to manage individual services by editing
this file, inittab only bootstraps a set of scripts that actually start/stop
the individual services.
The following description applies to SYSV-R4 based system,
which currently covers most commercial Unices (Solaris, HPUX, Irix, Tru64)
as well as the major Linux distributions (RedHat, Debian, Mandrake, Suse,
Caldera). Some systems (Slackware Linux, FreeBSD, OpenBSD) have a somewhat
different scheme of 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 accept other
"convenience" parameters (e.g: 'restart', to stop and then start,
'status' do display the service status). Running the script without parameters
displays the possible arguments.
To make specific scripts start/stop at specific run-levels and in specific
order, there are sequencing directories. These are normally in
/etc/rc[0-6S].d. In each of these directories there are links (usually
symbolic) to the scripts in the init.d directory.
A primary script (usually /etc/rc) is called from inittab(5) and calls
the services scripts via the links in the sequencing directories. All links
with names that begin with 'S' are being called with the argument 'start'
(thereby starting the service). All links with names that begin with 'K' are
being 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 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 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:
Usually the daemons started may optionally receive 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 ( /etc/sysconfig on RedHat systems) and
are used by the boot scripts.
In older Unices, these files contained the actual command line options for the
daemons, but in modern Linux systems (and also in HPUX), these files just
contain shell variables. The boot scripts in /etc/init.dsource
the configuration files, and then use the variable values.