Introduction

This document serves as an instruction to build a minimal Linux system for use on rescue diskettes, installation diskettes and embedded projects.

The Bootdisk Howto from the Linux Documentation Project also describes how to build a bootable diskette, but a diskette created in this way has a few shortcomings:

• It carries a lot of ballast such as inittab, getty and login. For a simple to use rescue disk, these programs and their associated configuration files are not necessary.
• It contains full featured GNU shell utilities that occupy a lot of space.
• It contains the real GNU C libraries, which are very large indeed.

Therefore a boot disk created in this way contains not many useful programs and it needs a comparatively large RAM disk to load. If you have 8MB of RAM, this works, but you can forget this on a 4MB or even 2MB machine.

There are two excellent software packages that are specially designed for saving precious memory and disk space when you are on a tight budget:

• Busybox is a combination of a Unix shell and many common Unix utility programs in a single executable. The size of Busybox is about half that of the typical bash shell and at that it already contains a few dozen Unix commands, ranging from ls to gzip. These are trimmed down versions of course, but adequate in most cases.
• uClibc is a scaled down Unix library. The GNU C library is huge, has every feature on the planet and adheres to every conceivable standard on every conceivable platform. uClibc is adequate for many applications and uses much less space.

While the use of Busybox on rescue disks is common, uClibc is rarely used. There are toolkits to create Busybox based diskettes, but IMHO there is no good instructional document that describes how to create a bootable Linux diskette with both Busybox and uClibc. The BootE diskette is a bootable Linux diskette with Busybox and uClibc. When I wrote the original text, there used to be no instructions on the site how to customize that disk to your needs, but now at least there is a tar archive with useful scripts and documentation.

In order to achieve huge space savings, we need to recompile everything from source.

The Mission

Suppose we have an old 386 machine in the basement that comes with only 4MB of RAM. We want to show our friends that this old beast can still run Linux.

We must be able to perform the following tasks:

• Partition a hard disk.
• Make a swap partition on the hard disk an use it.
• Create an ext2 file system on the hard disk and make it usable as the root file system.
• Mount diskettes and a hard disk.
• Edit files.

So basically we have everything to install a Linux root file system on the hard disk.

What needs to be on the diskette?

In order to get a Linux system up and running we need the following items:

• A boot loader. This program is loaded by the PC BIOS and this makes it possible to load another program, such as the Linux kernel.
• the Linux kernel. This is the heart of the operating system.
• a root file system. This is the file system that is mounted when the kernel is started. The first program that runs (typically /sbin/init has to be in the root file system. The root file system can exist on a diskette, it can be loaded in RAM at boot time or it can exist on the hard disk.

The root file system has to contain the following items:

• Binaries that we want to run.
• Startup scripts and other configuration files.
• Shared libraries.
• Device files.
• Mount points.

The Host and Target System

The host system is the computer on which we build the bootable diskette. It is assumed to be a fairly modern PC with a modern installation of Linux. We will assume that it is a Pentium with at least 32MB of RAM.

Also we assume that it contains a modern Linux system that contains the following software:

• Linux kernel 2.4
• recent gcc (2.95 or later)
• Loop devices (a kernel feature that allows you to mount a file system on a file instead of a block device).
• Bzip2
• a recent LILO

You will need lots of hard disk space. Around 300MB would be enough. The unpacked Linux kernel source tree alone takes around 150MB these days. Paradoxically enough the end result will fit on a single 1.44MB diskette.

The target system is an old 386 PC with 4MB of RAM or more and zero, one or two hard disks.