Live CD

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For musical recordings, see Live album.
Live CD of ubuntu, running Firefox, OpenOffice.org and Pidgin
Live CD of ubuntu, running Firefox, OpenOffice.org and Pidgin

A Live CD or LiveCD or CD Live Distro is a computer operating system that is executed upon boot, without installation to a hard disk drive. Typically, the LiveDistro is named after the bootable medium it is stored on, such as a CD-ROM or DVD (Live CD/DVD) or a USB flash drive (Live USB).

The term "live" derives from the fact that these "distros", or software distributions, each contain a complete, functioning and operational operating system on the distribution medium. A LiveDistro does not alter the operating system or files already installed on the computer hard drive unless instructed to do so. LiveDistros often include mechanisms and utilities for more permanent installation, including disk partitioning tools. The default option, however, is to allow the user to return the computer to its previous state when the LiveDistro is ejected and the computer is rebooted. It is able to run without permanent installation by placing the files that typically would be stored on a hard drive into RAM, typically in a RAM disk. However, this does cut down on the RAM available to applications, reducing performance somewhat. As of 2007, certain LiveDistros run a graphical user interface in as little as 32MB RAM.

In some LiveDistros, the user can optionally install the OS from the removable media to the hard disk drive (they are called installable LiveDistros).

Contents

[edit] History

On computers, optical discs were originally intended for storing video. Then the principles of storing information into the optical disc were changed to provide storing different types of information. In the case of operating systems those were mostly installation files, often packaged and archived in compressed formats. Later, it became convenient and useful to boot the computer from compact disc, often with a minimal working system in order to install a full system onto a hard drive, and troubleshoot hardware. Although early Linux developers and users were able to take advantage of cheap optical disks and rapidly declining prices of CD drives for personal computers, the Linux distribution CDs or "distros" were generally treated as a collection of installation packages that must first be permanently installed to hard disks on the target machine.

The first Compact Disc drives on personal computers were generally much too slow for running complex operating systems such as Linux. Often, the computer could not even boot from optical discs. When operating systems were distributed on discs, either a boot floppy or the CD itself would boot specifically, and only, in order to install onto a hard drive. There was little reason not to install directly on a hard disk. However, in the case of Linux, the free operating system was meeting resistance in the consumer market because of the difficulty and effort and risk involved with installing an additional partition on the hard disk, particularly the ext2 filesystem. The term "Live CD" was coined because after typical PC RAM was large enough and 52x speed CD drives and CD burners were widespread among PC owners, it finally became convenient and practical to boot the kernel, run X11, a window manager and GUI applications directly from a CD without disturbing the OS (generally Windows on FAT32 or NTFS) on the hard disk. This was a new and different situation for Linux than other OSes, because the updates/upgrades were being released so quickly, different distributions and versions were being offered online, and especially because users were burning their own CDs. Copying Linux from the installation media was also encouraged instead of actively hindered and discouraged with such things as requiring the input of long and elaborate serial numbers and lengthy and complicated installation procedures.

The first Linux-based Live CD was Yggdrasil Linux (went out of production in 1995), though in practice it did not function well due to the low throughput of then-current CD-ROM drives. The Debian-derived Linux distribution Knoppix was released in 2003, and found popularity as both a rescue disk system and as a primary distribution in its own right. Since 2003, the popularity of Live CDs has increased substantially, partly due to Linux Live scripts and remastersys which made it very easy to build customized live systems.

Most of the popular Linux distributions now include a Live CD variant, which in some cases is also the preferred installation medium.

[edit] Uses

Some LiveDistros are designed to "demo" or "test drive" a particular operating system (usually Linux or another free or open source operating system).

Although some live CDs can load into memory in order to free the optical drive for other uses, loading the data off a CD-ROM is still slower than a typical hard drive boot, so this is rarely the default.[1] Experienced users of the operating system may also use a LiveDistro to determine whether and to what extent a particular operating system or version is compatible with a particular hardware configuration and certain peripherals.[1] Users may also use a LiveDistro to troubleshoot hardware, especially when a hard drive fails. Some Live CDs can save user-created files in a Windows partition, a USB drive, a network drive, or other accessible media.

LiveDistros can be used for many purposes, including providing an environment for optionally installing a Linux distribution to a hard drive, testing new versions of software, testing hardware, system repair and restoration, high security/non-invasive environment for a guest, cracking/stealing passwords, network security testing, and as a fall back when a main drive fails. Everyday use of LiveDistros is rare because CDs are slower than hard drives, because user files in a RAM disk are lost when the power is turned off, and because systems may not have access to a swap disk for opening many large applications.

[edit] Mounting without burning

The files on a Live Distro ISO image can be accessed in Microsoft Windows with a disk image emulator such as Daemon Tools, or in Unix variants by mounting a loop device.

[edit] Common traits

Some LiveDistros come with an installation utility launchable from a desktop icon that can optionally install the system on a hard drive or USB flash drive. Most LiveDistros can access the information on internal and/or external hard drives, diskettes and USB flash drives. Generally LiveDistros are booted from read-only media, requiring either copying to rewriteable media (i.e. a hard drive) or complete remastering to install additional software; however, there are exceptions such as Morphix and Puppy Linux which are one of the few Linux Live CD distributions able to save files to the Live CD itself or other multisession medium, allowing users to carry data, and more importantly, added programs and customized settings, along with them on optical disc.

Most LiveDistros are based on Linux, as this was the operating system that had the most to gain by offering free trials and demonstrations without regard to sales or copyright. Now others are using the term LiveDistro for other operating systems, such as OpenSolaris, BeleniX and others based on Solaris. Other "live" operating systems include ReactOS, NetBSD, FreeBSD, OpenBSD, MINIX 3, Plan 9 from Bell Labs, MorphOS and FreeDOS. Although Mac OS and various Microsoft Windows releases can function as "live CDs" they are not referred to as such. This may be because these systems are only intended to be run from CD for troubleshooting, repair and installation. There are a growing number of Windows-based utilities like Windows PE, Windows RE and BartPE that resemble and function as LiveDistros, however. The first personal computer operating system on a CD to support "live" operations might have been the AmigaOS, which could be booted from CD on an Amiga CDTV in 1990.[citation needed]. Earlier examples of live OS are of course the operating systems used from floppy, and most widely spread is DOS. Unlike previous operating systems on optical media, though, Linux "Live CDs" were specifically designed to run without installation onto other media like a hard disk drive. The LiveDistro concept was meant to promote Linux and showcase the abilities of the free, open source operating system on conventional personal computers with Microsoft Windows already installed.

On a PC, a bootable Compact Disc generally conforms to the El Torito specification. Many Linux based LiveDistros use a compressed filesystem image, often with the cloop compressed loopback driver, or squashfs compressed filesystem, generally doubling effective storage capacity, although slowing application start up. The resulting environment can be quite rich: typical Knoppix systems include around 1,200 separate software packages. LiveDistros have a reputation for supporting advanced auto-configuration and plug-and-play functionality. This out of necessity so as to avoid requiring the user to configure the system each time it boots, and to make them easily usable by those who are new to the operating system.


[edit] Technique

A read-only file system, such as on a CD-ROM has the drawback of being unable to save any current working data. For this reason, a read-only file system is often merged with a temporary writable file system in the form of a RAM disk. Often the default Linux directories "/home" (containing users' personal files and configuration files) and "/var" (containing variable data) are kept in ramdisk, because the system updates them frequently.

In modern LiveDistros, a read-only file system is merged with ramdisk using transparent techniques such as UnionFS or AuFS. In MS-DOS systems, a DOS utility, ramdrive.sys, can be loaded at boot for this purpose.

LiveDistros have to be able to detect a wide variety of hardware (including network cards, graphic cards etc.). This is easily achieved nowadays by udev or hotplug, which is a common part of all distributions based on Linux kernel 2.6.

[edit] Cheat code

Codes that are introduced during Live CD initialization to change the booting behavior. They vary from distribution to distribution but can most often be accessed upon first boot screen by one of the function keys.

[edit] See also

[edit] References

  1. ^ a b Brickner, David (2005). Test Driving Linux: From Windows to Linux in 60 Seconds. O'Reilly. ISBN 059600754X. 

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