Compiling Linux Kernel

Linux is an open source operating system.Linux is free.Linux being open source gives us advantage of customizing the linux and creating our own version.Various flavours of Linux have already evolved.Eg. Ubuntu evolved from Debian.Edubuntu(Educational version of Ubuntu) evolved from Ubuntu.

To customize linux we have to tinker with source and recompile the kernel.

Below are the steps that we need to follow to compile a linux kernel.

Get Latest Linux kernel code

Visit http://kernel.org/ and download the latest source code. File name would be linux-x.y.z.tar.bz2, where x.y.z is actual version number. For example file linux-2.6.25.tar.bz2 represents 2.6.25 kernel version. Use wget command to download kernel source code:
$ cd /tmp
$ wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-x.y.z.tar.bz2

Note: Replace x.y.z with actual version number.

Extract tar (.tar.bz2) file

Type the following command:
# tar -xjvf linux-2.6.25.tar.bz2 -C /usr/src(-C represents the path where kernel src is stored)
# cd /usr/src

Configure kernel

Before you configure kernel make sure you have development tools (gcc compilers and related tools) are installed on your system. If gcc compiler and tools are not installed then use apt-get command under Debian Linux to install development tools.
# apt-get install gcc

and in Red Hat Distributions

#yum install gcc

Now you can start kernel configuration by typing any one of the command:

• $ make menuconfig – Text based color menus, radiolists & dialogs. This option also useful on remote server if you wanna compile kernel remotely.
• $ make xconfig – X windows (Qt) based configuration tool, works best under KDE desktop
• $ make gconfig – X windows (Gtk) based configuration tool, works best under Gnome Dekstop.

For example make menuconfig command launches following screen:

$ make menuconfig

You have to select different options as per your need. Each configuration option has HELP button associated with it so select help button to get help.

Compile kernel

Start compiling to create a compressed kernel image, enter:
$ make
Start compiling to kernel modules:
$ make modules

Install kernel modules (become a root user, use su command):
$ su - (super user)
# make modules_install


Install kernel

So far we have compiled kernel and installed kernel modules. It is time to install kernel itself.
# make install

It will install three files into /boot directory as well as modification to your kernel grub configuration file:

• System.map-2.6.25
• config-2.6.25
• vmlinuz-2.6.25

Create an initrd image

Type the following command at a shell prompt:
# cd /boot
# mkinitrd -o initrd.img-2.6.25 2.6.25

initrd images contains device driver which needed to load rest of the operating system later on. Not all computer requires initrd, but it is safe to create one.

Modify Grub configuration file – /boot/grub/menu.lst

Open file using vi:
# vi /boot/grub/menu.lst

or

#vi /boot/grub/grub.conf

title           Debian GNU/Linux, kernel 2.6.25 Default
root            (hd0,0)
kernel          /boot/vmlinuz root=/dev/hdb1 ro
initrd          /boot/initrd.img-2.6.25
savedefault
boot

Remember to setup correct root=/dev/hdXX device. Save and close the file. If you think editing and writing all lines by hand is too much for you, try out update-grub command to update the lines for each kernel in /boot/grub/menu.lst file. Just type the command:
# update-grub

Reboot computer and boot into your new kernel

Just issue reboot command:
# reboot