by Prasad P M
Since this article was written, many distros have moved away from inittab. See Unix and Linux startup scripts, Part 1.
"Booting the computer", this is a common word associated with starting the computer.
Though we use it casually in our daily life, have you ever thought of
what exactly it is ? or how the system brings itself to a workable
environment ? Well, my attempt in this article is to explain all the stages involved in booting your linux machine. In simple words, "bootstrapping" means starting up your computer. It involves all those stages, from the moment you power on your machine
till the system is ready to log you in.
2. The Boot Process
The Boot process involves several different stages that the system undergoes while it is being booted. If any of these stages fail, then the system cannot start
* The BIOS
* Kernel Initialization
* Hardware Configuration
* System Processes
* Startup Scripts
* Multiuser Mode
Lets look at each of the above stages in detail.
2.1 The BIOS
This is the initial stage of the boot process. The moment you power your system, the microprocesser is not aware of your computer environment or even your operating system. It is the BIOS, that provides the necessary instructions to microprocessor and helps to initialize the computer environment. That is why it is called Basic Input/Output System.
These are the main tasks of BIOS.
* POST (Power On Self Test): The BIOS performs a power
on self test on all hardware components attached to the computer
during booting. You might have noticed the LEDs on your keyboard flashing
during booting. That is an example of POST. If anything fails, it will be
reported to you on your screen.
* The BIOS provides a set of low level routines for the
hardware components to interface with the Operating System.
These routines act like drivers for your Screen, Keyboard, Ports etc.
* The BIOS helps to manage the settings of hard disks,
your first boot device, system time and more.
The BIOS also initiates the bootstrapping sequence by loading the Initial
Program Loader (The boot loader program) into the computer memory. The
software is usually stored in the ROM. This stage is actually outside
of the domain of the Operating System and it is more vendor specific.
2.2 Kernel Initialization
Linux actually implements a two stage boot process.
In the first stage, the BIOS loads the boot program (Initial Program Loader) from
the hard disk to the memory. In the second stage, the boot program loads the
Operating System kernel vmlinuz into memory. Though we can name the
kernel anything of your like, we'll call it vmlinuz. Well, it's just a tradition,
where 'vm' stands for the Virtual Memory support and last 'z' denotes
that it is a compressed image, ie vmlinux.z => vmlinuz.
When the kernel loads into memory, it performs a memory test.
Most of the kernel data structures are initialized statically. So it
sets aside a part of memory for kernel use. This part of the memory cannot be used by any other processes. It also reports the total amount of physical memory
available and sets aside those for the user processes.
2.3 Hardware Configuration
If you have configured a linux kernel, you would have configured the hardware sections as well. This is how the kernel knows what hardware to find. Based on
the configuration, when the kernel boots, it tries to locate or
configure those devices. It also prints the information of the devices
it found during the bootup. It will probe the the bus for devices or
asks the driver for information of the devices. Devices that are not
present in the system or not responding to the probing will be
It is possible to add more devices using the util 'kudzu'.
2.4 System Processes
Once the hardware initialization is complete, the kernel will create
several spontaneous processes in the user space. The following are those
These are called spontaneous processes because they are not created by
the usual fork mechanism. Of these, only 'init' is actually in the user
space(only processes in the user space can be controlled by us) , we have no control over others. The rest of the boot up procedure is controlled by init.
2.5 Startup Scripts
Before explaining how startup scripts work, let's have a look at the
tasks performed by them. The following are the important tasks
performed by startup scripts.
* Set the name of the computer
* Set the time zone
* Check the hard disk with fsck
* Mount system disk
* Remove old files from /tmp partition
* Configure network interfaces with correct IP
* Startup deamons and other network services
The startup scripts are found in /etc/rc.d/init.d folder in your
2.5.1 Init and runlevels
You can boot your linux machine to different runlevels. A
runlevel is a software defined configuration of your system where the
system behaviour will vary in different runlevels. Though, linux can
have 10 different runlevels, only 7 of them are used. I have
mentioned them below.
| 1 or S
|single user mode
||multiuser mode without nfs
|full multiuser mode
You can specify the runlevel in the init configuration file /etc/inittab.
2.5.2 Startup Scripts and runlevels
You may see folders (rc[0-7].d) corresponding to each runlevel in
the /etc folder. These folders contain files symbolically linked (in
linux everything is a file) to the startup scripts in folder
/etc/rc.d/init.d. If you look at these folders, you may see that the
name of the symbolic links starts with the letter "S" or "K" followed by a
number and the name of the startup script /service to which it is linked
For example, the following are the files in runlevel 2 and 3.
/etc/rc2.d/K20nfs -> ../init.d/nfs
The name of those files are important. Because when you switch between
runlevels, the services are started/stopped based on these names. Consider these two cases here.
/etc/rc2.d/S55named -> ../init.d/named
* switching to higher runlevels - init will run
scripts that start with letter S, in ascending order of the number with
The runlevels init checks to switch between them, depends on the
configuration of your system. The following commands will help. For more
details of the commands, refer to the manual pages.
* switching to lower runlevels - init will run
scripts that start with letter K, in decending order of the number with
The commands that deal with runlevels are:
/sbin/runlevel - shows the previous and current
/sbin/init and /sbin/telinit - to switch
/sbin/chkconfig - to enable/disable services in
2.5.3 Startup Scripts and /etc/sysconfig files
The files in the /etc/sysconfig folder are read by the startup scripts. So it's
worth mentioning them here.
* network - contains information of your hostname,
nisdomain name etc.
* clock - timezone information
* autofsck - automatic filesystem check during boot
* network-scripts - folder contains interface
configuration files ifcfg-lo, ifcfg-eth0 etc.
* hwconf - hardware information
* sendmail, spamassassin, syslog, yppasswdd -
information about the corresponding daemons.
Edit the files in /etc/sysconfig folder to make changes to your system.
2.5.4 Init and single user mode
This runlevel is used by sysadmins to perform routine maintenance.
Its most commonly used for checking errors in file system with command 'fsck'. Only the root file system will be mounted in this
runlevel and the system administrator is provided with a shell. If
necessary, other partitions needs to be mounted manually. Also none
of the deamons will be running in this runlevel. Only the system
administrator can use the system in this mode. You can simply exit
from the shell to boot it to the multiuser mode.
2.6 Multiuser Operation
Though the system has been booted to a particular runlevel, none of
the users can login to the system until init spawns getty processes on
terminals. If the system is booted to runlevel 5, init needs to spawn
the graphical login system 'gdm'.
If the system has gone through the above mentioned stages without
any failures, you may say that your system is booted and is ready to
perform the tasks :)
3. Rebooting and Shutting down
We have discussed about the boot procedure so far. It is also important to shutdown the system properly. Otherwise you may end up with loss of data or serious damage to the file system.
You can safely use the commands /sbin/shutdown,
/usr/bin/halt or /usr/bin/reboot to halt or reboot the computer. For
more details of the commands, refer to the manual pages.
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