Linux is, among other things, a customizable operating system. Clever developers can craft a Linux whose kernel and packages are configured for a specific purpose, to serve as a sort of vertical-market operating system. The benefit to users is somewhat akin to walking into a hardware store. On the shelves are tools, each suited to a specific task. And it's particularly nice that all the tools are free.

In this article, we examine three kinds of "Linux as tool" distributions that can help you in a pinch: small-footprint Linuxes, whose boot and runtime images fit in cramped spaces; Linuxes for old hardware, which are designed to execute on systems you might otherwise push to the back of a closet; and system-rescue Linuxes for recovering lost data from crashed systems.

From each category, we have selected a pair of representatives and probed their features. In a follow-up article, we'll look at several Linuxes designed to run as firewalls, security systems, and even storage servers for your local network. Some are full gateway servers, sporting proxy, e-mail, print services, VPN, and other essentials for the small business network.

Small-footprint Linux

Other than the technical satisfaction of cramming as much capability into a small space, what is the attraction of a Linux whose boot image can fit into less than 100MB? Obviously, one benefit is portability. Imagine walking into an Internet café, plugging in a pen drive, and booting into your own personal environment.

Also, small-footprint Linuxes place minimal burdens on memory and processor. Consequently, they run well in a LiveCD configuration. Just as it sounds, a LiveCD Linux boots and runs directly from a CD; you do not need to install it on your hard disk. Consequently, if you need to run Linux only occasionally, you can run a small-footprint system from LiveCD without having to devote any hard-disk space to it.

Low memory requirements make small-footprint Linuxes ideal for execution in a virtual machine (such as VMware Workstation or Sun xVM VirtualBox). In most cases, 256MB of RAM is more than enough. By comparison, running a full-blown Windows OS in a virtual machine can consume upward of a gigabyte.

Finally, because small-footprint Linuxes place minimal burdens on memory and processor, they stand a good chance of working on older hardware.

Two excellent Linuxes in this category are Puppy Linux and Damn Small Linux.

Puppy Linux, the brainchild of Barry Kauler, is supported by a community that has not only created a large library of Puppy-installable packages but also has produced many specialized Puppy Linux variants. The boot image is around 90MB, and Puppy Linux can boot from LiveCD, a pen drive, or a network. The recommended memory size for Puppy Linux is 256MB. However, I have run a 2.x version of Puppy Linux on a 196MB laptop for more than a year now, and most applications execute directly from RAM, the exception being the Xine media player.