boot-scripts - General description of boot sequence
The boot sequence varies in details among systems but can be roughly divided to the fol-
lowing steps: (i) hardware boot, (ii) OS loader, (iii) kernel startup, (iv) init and init-
tab, (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 (non-volatile 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
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.
Note: We do not cover here booting from network. Those who want to investigate this sub-
ject may want to research: DHCP, TFTP, PXE, Etherboot.
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 it.
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 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.
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.
init and inittab
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.
Note: 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 dis-
tributions (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 direc-
tories 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
To assert order withing the same run-level, the names of the links contains order-numbers.
Also, to make the names clearer, they usually ends with the name of the service they refer
to. Example: the link /etc/rc2.d/S80sendmail is starting the sendmail service on runlevel
2. This is happening after /etc/rc2.d/S12syslog is run but before /etc/rc2.d/S90xfs is
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: chkconfig(8)).
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.d source the configuration files, and then use the variable
/etc/init.d/, /etc/rc[S0-6].d/. /etc/sysconfig/
inittab(5), bootparam(7), init(8), runlevel(8), shutdown(8)