Dipping a toe in the waters of Linux can be a daunting prospect. Friendly installers on major distributions take the edge off, but there's still plenty lurking beneath the surface to trip up the unsuspecting new user.

For many potential Linux converts a hard disk install is too much of an investment. In response to this problem, there are now several 'live' distributions that can be run directly from CD.

There's no need to install anything - just set your PC to boot from the CD drive and go. The operating system loads from CD and sets up a temporary Ramdisk in system memory to hold the necessary dynamic files. Though different live options target different markets, most offer more or less automatic network configuration and a graphical desktop environment with supplied office suites, browsers and applications.

There's also mileage in live distributions for the experienced user. It's possible to customise some live variants, burn them to a fresh CD and use them as a portable, instant personalised Linux environment with a writeable home directory stored on a USB memory key. And a read-only OS is invaluable for kiosk applications and administrators investigating security breaches.

Running from CD isn't a free ride: applications run much more slowly than from a hard disk, and customising and updating the environment means building a new CD image.

However, live distributions have their place for those trying out Linux and for specialist applications. This article looks at several of the currently available live distributions, comparing features, performance and convenience.

Your favourite distribution on CD
Most of the products reviewed here aren't Linux distributions developed from the ground up. Typically, live editions are based on an existing distribution and are tweaked to run from CD.

MandrakeMove is, unsurprisingly, based on Mandrake and SuSE Live on SuSE. Several others, including Knoppix, Gnoppix and Morphix, are based on Debian. In fact, a hard disk install of any of the Debian-based live implementations can be the quickest way of getting Debian on your system without wrestling with the native installer.

If you already have an affinity to a particular Linux distribution, you'll find the environment of the associated live distribution most familiar, but the beauty of a CD-based OS is that it's easy to try different options, and it's worth looking at several products.

Some of the editions are geared heavily towards the try-before-you-buy audience, so they don't offer a full spread of applications. The thoroughbred live distributions, however, offer a complete Linux environment with little functional compromise.

Keeping it mainstream
Linux hardware detection has been driven in part by the growth of live distributions. Once a luxury for hard-disk Linux installers, proper automatic configuration of hardware is now an essential part of a useful live distribution.

In general the products reviewed here did an impressive job of figuring out our test systems well enough to launch X-Windows and configure networking. But, as with everything Linux, working with newer or more esoteric hardware was not always so easy.

We gave up testing on a laptop with only an external FireWire CD drive, because none of the live distributions we tried could boot all the way up; on an older motherboard with an nVidia nForce chipset results were variable, with some environments working perfectly and others failing to boot properly; and none of the distributions could automatically configure the 3Com Gigabit Ethernet interface on an Asus A7V600 motherboard.

Most of these kinks can be worked out and new images built of any of the customisable live distributions, but if your hardware is very new or you know it to be poorly supported in current Linux distributions you may struggle with live setups. However, if you have a mainstream system, you'll probably have no trouble.

Performance
Live distributions aren't ideal if your hardware is limited. CPU speed isn't of paramount importance, but you need a reasonable amount of Ram and a fast CD-Rom drive. The most modest machine we used in testing was a 1.1GHz AMD Duron with 256MB Ram and a 52X CD-Rom drive.

In all the distributions the performance of running applications was fine, and the bottleneck was always slow boot and load times from CD-Rom. We wouldn't, however, recommend running any of the full-size distributions with less than 256MB Ram.

Linux live reloaded
A stock live distribution may well contain everything you need. With different options for office applications, gaming and desktop environments, the products featured in this article do an excellent job of covering the common bases.

But requirements differ and if you need a package that isn't provided in the off-the-shelf live distributions, or a completely customised environment, you'll need to dig a little deeper. The good news is that the project teams for live distributions typically expect you to want to customise the product beyond just holding a persistent home directory on removeable media.

Rolling your own version of a live distribution is easiest with a host Linux machine with enough space to house the guts of the distribution, the prepared ISO image for burning to CD and, of course, a CD recorder. In practice that means almost any recent machine will do for hardware.

Of course if trying a live distribution is your way of dipping your toe in the Linux waters then you probably don't have a Linux machine handy. If you do, though, it's not too tricky to lash together your own custom distribution based on one of the products here. Morphix in particular provides good outline documentation describing how the system is constructed and suggesting simple places to add your own modules.

Home on the USB key
For many applications, a writeable home directory takes a live Linux distribution from oddity to seriously useful. And the most convenient format for carrying around data is on USB memory key, offering a good combination of size, performance and durability.

Over the last few years keyring-sized USB keys of reasonable capacity have become cheap, with 256MB keys available for around £50.

Several of the distributions featured here include simple mechanisms for setting up a home directory on a USB key, and instructions for using that directory when booting the OS. Since the distribution itself is inherently read-only this involves manually appending an option at boot time, but it's a small price to pay for the added convenience.

You don't need to devote your key to act solely as a Linux home directory - you can create an image file which, like a mini ISO image, contains your home directory and all the files in it. Then all you need is a generic copy of your chosen live distribution to boot into your preferred environment.

If your system supports booting from USB you can go one step further and install the whole operating system on a USB key. Of course that takes a lot more space, and does mean using your key just as bootable Linux device. But if you have a 512MB or 1GB key it's possible to cram on a full-size distribution with a full suite of office applications. Try that with Windows.

Working with ISO images
All the distributions featured here are supplied as .ISO images. An ISO image is a large, single file containing the exact data to burn to CD, laid out in the proper file system. It doesn't matter which OS you use to burn an image, since the data is all pre-prepared inside the .ISO file.

Once you've downloaded a CD image you should verify it before burning to CD. Download or project sites typically display MD5 checksums of each file posted. The MD5 sums a hash of the contents of a file. If the contents change, the checksum will change. So you checksum the downloaded file and compare the two checksums. If they match you're good to go, otherwise there's a problem with your CD image.

On Linux you'll typically find the program 'md5sum' already installed. For other operating systems, search for 'md5 checksum' on your preferred download site - there are many such utilities, and they're usually free.

If you're using Windows or a Mac your CD software will have an option such as 'Burn ISO image'. Just point it to the downloaded .ISO file and tell it to record. If everything's successful you should end up with a working, bootable CD.

Be sure to look for the specific ISO burning mode, rather than just copying the .ISO file to a CD. If you don't choose the correct mode you'll end up with a CD with a single directory containing the .ISO file, which isn't what you want. Telling your CD software you want to burn from an ISO image tells it too look inside the file and lay out the burned CD accordingly, rather than just copying across the .ISO file itself.

Don't miss Linux live and kicking - Part 2, in which we take a closer look at nine major Linux live distributions.