Live CD
Encyclopedia
A live CD, live DVD, or live disc is a CD or DVD
containing a bootable
computer operating system. Live CDs are unique in that they have the ability to run a complete, modern operating system on a computer lacking mutable secondary storage
, such as a hard disk drive. Live USB
flash drives are similar to live CDs, but often have the added functionality of automatically and transparently writing changes back to their bootable medium. Also, the solid-state device pure electronic operation gives a significant speed advantage by eliminating the CD reader's intrinsic mechanical latencies. Write-locked Live SD WORM
systems are the direct solid-state counterpart to live CD's and can be booted natively in a media card slot or by using a USB adapter. Write-locked Live SD systems avoid excessive write cycles or corruption by ill-conditioned software, like viruses, which can prematurely "ruin" a live USB.
The term "live" derives from the fact that these CDs each contain a complete functioning operating system on the distribution medium.
While a live CD typically does not alter any operating system or files already installed on a computer's secondary storage (such as hard disk drives), many live CDs include mechanisms and utilities for altering the host computer's data stores, including installation of an operating system. This is important for the system management aspect of live CDs, such as removing malware, drive imaging, and system recovery.
The default option, however, is to allow the user to return the computer to its previous state when the live CD is ejected and the computer is reboot
ed. 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
, though this does cut down on the RAM available to applications.
s were used as boot disk
s, which generally had a small operating system and limited tools.
Programmers adapted the compact disc
s (originally developed for storing audio
) for use as media for storing and distributing large amounts of computer data. This data may also include application and operating-system software, sometimes packaged and archived in compressed formats. Later, it became convenient and useful to boot the computer directly from compact disc, often with a minimal working system in order to install a full system onto a hard drive.
The first Compact Disc drives on personal computers were generally much too slow for running complex operating systems. Often, the computer could not boot from optical disc
s. When operating systems were distributed on compact discs, either a boot floppy or the CD itself would boot specifically, and only, in order to install onto a hard drive. The first live CD was FM Towns OS first released in 1989.
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.
However in the case of these distributions built on top of the Linux kernel
, the free
operating system was meeting resistance in the consumer market because of the perceived difficulty, effort, and risk involved in installing an additional partition on the hard disk, in parallel with an existing operating system installation.
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 on the hard disk.
This was a new and different situation for Linux than other operating systems, 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.
The first Linux
-based 'Live CD' was Yggdrasil Linux
first released in beta form 1992~1993 (ceased production in 1995), though in practice its functionality was hampered due to the low throughput of contemporary CD-ROM drives. DemoLinux
, released in 1998, was the first Linux distribution specially designed as a live CD. The Linuxcare
bootable business card
, first released in 1999, was the first Live CD to focus on system administration, and the first to be distributed in the bootable business card form factor. , Finnix
(first released in 2000) is the oldest Live CD still in production. Knoppix
, a Debian
-derived Linux distribution, 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 distribution
s now include a live CD variant, which in some cases is also the preferred installation medium.
or open source
operating system), there are live CDs made for many different uses.
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 with large live CD images, but for smaller live CD images loading the filesystem directly into RAM can be highly practical. Loading the filesystem image into RAM can provide a significant performance boost as RAM is several orders of magnitude faster than a hard drive. Also, since RAM has no moving parts, a system running from a live CD loaded into RAM can run with improved power efficiency. Experienced users of the operating system may also use a live CD to determine whether and to what extent a particular operating system or version is compatible with a particular hardware configuration and certain peripherals, or as a way to know beforehand which computer or peripheral will work before buying. Users may also use a live CD to troubleshoot hardware, especially when a hard drive fails, and more generally as a recovery disc in case of problems. Some live CDs can save user-created files in a Windows partition, a USB drive, a network drive, or other accessible media.
A few additional uses include:
Some of these topics covers sub topics, e.g. IT administration breaks down to firewall, rescue, security, etc. type of live CDs. In some cases a particular LiveDVD covers more than one topic.
as an installable live CD can often be
beneficial as a single image can run on real hardware in addition to
most types of virtual machines.
This allows developers to avoid the complexities involved in supporting
multiple incompatible virtual machine
images formats and
focus on the lowest common denominator instead.
Typically after booting the machine from the live CD, the appliance will
either run in non-persistent demo mode or install itself, at the user's request, to an available storage device.
can be accessed in Microsoft Windows
with a disk image emulator such as Daemon Tools
, or in Unix
variants by mounting a loop device
.
After mounting the live CD's filesystem, software on the live CD can be run directly (I.e., without booting) by chroot
ing into the live CD's mounted filesystem.
A live CD ISO image can also be mounted by Virtual Machine
software such as VirtualBox
and VMware Workstation
or can be converted to a Live USB
using SYSLINUX
. Tools such as UNetbootin
can automate this process.
. Most live CDs can access the information on internal and/or external hard drives, diskettes and USB flash drives.
Generally live CDs 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 Puppy Linux
which has the ability to save files to the live CD itself or other multisession media, allowing users to carry data, and more importantly, added programs and customized settings, along with them on optical disc.
Most live CDs 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 live CD for other operating systems, such as OpenSolaris
, BeleniX
and others based on Solaris. Other "live" operating systems include AmigaOS 4
, FreeBSD
, FreeDOS
, Mac OS
, Microsoft Windows
, OS/2, ReactOS
, NetBSD
, OpenBSD
, MINIX 3
, Plan 9 from Bell Labs
, and MorphOS
.
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.. 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 and OS/2 "live CDs" were specifically designed to run without installation onto other media like a hard disk drive. The live CD 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 live CDs 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. Live CDs have a reputation for supporting advanced auto-configuration
and plug-and-play
functionality. This came out of necessity to avoid requiring the user to configure the system each time it boots and to make it easily usable by those who are new to the operating 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 "
s) and "
) are kept in ramdisk, because the system updates them frequently. Puppy linux has a savable layer so if you choose to, the next time you boot you can resume (pick right back up again) from where you left off. Each time the CD boots, it looks for the file and then uses it if it has the right name.
In modern live CDs, a read-only file system is merged with ramdisk using transparent
techniques such as UnionFS
, AuFS
or EWF
. In MS-DOS and OS/2 systems, ramdrive.sys, can be loaded at boot for this purpose. Boot loaders like syslinux can boot ISO files from USB memory devices.
Live CDs have to be able to detect and use a wide variety of hardware (including network card
s, graphic cards etc.) in realtime. This is easily achieved nowadays by udev
, hotplug, hal, udisk etc.. which is a common part of all distributions based on Linux kernel
2.6.
s.
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
containing a bootable
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
computer operating system. Live CDs are unique in that they have the ability to run a complete, modern operating system on a computer lacking mutable secondary storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
, such as a hard disk drive. Live USB
Live USB
A live USB is a USB flash drive or a USB external hard disk drive containing a full operating system that can be booted. Live USBs are closely related to live CDs, but sometimes have the ability to persistently save settings and permanently install software packages back onto the USB device...
flash drives are similar to live CDs, but often have the added functionality of automatically and transparently writing changes back to their bootable medium. Also, the solid-state device pure electronic operation gives a significant speed advantage by eliminating the CD reader's intrinsic mechanical latencies. Write-locked Live SD WORM
Write Once Read Many
A Write Once Read Many or WORM drive is a data storage device where information, once written, cannot be modified. On ordinary data storage devices, the number of times data can be modified is not limited, except by the rated lifespan of the device, as modification involves physical changes that...
systems are the direct solid-state counterpart to live CD's and can be booted natively in a media card slot or by using a USB adapter. Write-locked Live SD systems avoid excessive write cycles or corruption by ill-conditioned software, like viruses, which can prematurely "ruin" a live USB.
The term "live" derives from the fact that these CDs each contain a complete functioning operating system on the distribution medium.
While a live CD typically does not alter any operating system or files already installed on a computer's secondary storage (such as hard disk drives), many live CDs include mechanisms and utilities for altering the host computer's data stores, including installation of an operating system. This is important for the system management aspect of live CDs, such as removing malware, drive imaging, and system recovery.
The default option, however, is to allow the user to return the computer to its previous state when the live CD is ejected and the computer is reboot
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
ed. 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
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
, though this does cut down on the RAM available to applications.
History
Prior to the wide use of compact discs in computers, floppy diskFloppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
s were used as boot disk
Boot disk
A boot disk is a removable digital data storage medium from which a computer can load and run an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting certain standards.Boot disks are used for:* Operating...
s, which generally had a small operating system and limited tools.
Programmers adapted the compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
s (originally developed for storing audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
) for use as media for storing and distributing large amounts of computer data. This data may also include application and operating-system software, sometimes packaged and archived in compressed formats. Later, it became convenient and useful to boot the computer directly from compact disc, often with a minimal working system in order to install a full system onto a hard drive.
The first Compact Disc drives on personal computers were generally much too slow for running complex operating systems. Often, the computer could not boot from optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
s. When operating systems were distributed on compact discs, either a boot floppy or the CD itself would boot specifically, and only, in order to install onto a hard drive. The first live CD was FM Towns OS first released in 1989.
Origin of Linux live CDs
Although early developers and users of distributions built on top of the Linux kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
were able to take advantage of cheap optical disks and rapidly declining prices of CD drives for personal computers, the Linux distribution
Linux distribution
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...
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.
However in the case of these distributions built on top of the Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
, the free
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
operating system was meeting resistance in the consumer market because of the perceived difficulty, effort, and risk involved in installing an additional partition on the hard disk, in parallel with an existing operating system installation.
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 on the hard disk.
This was a new and different situation for Linux than other operating systems, 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.
The first Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
-based 'Live CD' was Yggdrasil Linux
Yggdrasil Linux
Yggdrasil Linux/GNU/X, or LGX , was an early Linux distribution developed by Yggdrasil Computing, Incorporated, a company founded by Adam J. Richter in Berkeley, California.Yggdrasil was the first company to create a Live CD Linux distribution...
first released in beta form 1992~1993 (ceased production in 1995), though in practice its functionality was hampered due to the low throughput of contemporary CD-ROM drives. DemoLinux
DemoLinux
DemoLinux has been one of the first Live CD Linux distributions.It was created by Roberto Di Cosmo, Vincent Balat and Jean-Vincent Loddo, in 1998.The DemoLinux CD was created to make it possible to use Linux without having to install it on the hard disk...
, released in 1998, was the first Linux distribution specially designed as a live CD. The Linuxcare
Linuxcare
Linuxcare was founded in San Francisco in 1998 by Dave Sifry, Arthur Tyde and Dave LaDuke. The company's initial goal was to be "the 800 number for Linux" and operate 24 hours a day...
bootable business card
Bootable business card
A bootable business card is a CD-ROM that has been cut, pressed, or molded to the size and shape of a business card . Alternative names for this form factor include "credit card", "hockey rink", and "wallet-size". The cards are designed to hold about 50 MB...
, first released in 1999, was the first Live CD to focus on system administration, and the first to be distributed in the bootable business card form factor. , Finnix
Finnix
Finnix is a Debian-based Live CD operating system, developed by Ryan Finnie and intended for system administrators for tasks such as filesystem recovery, network monitoring and OS installation...
(first released in 2000) is the oldest Live CD still in production. Knoppix
Knoppix
Knoppix, or KNOPPIX , is an operating system based on Debian designed to be run directly from a CD / DVD or a USB key , one of the first of its kind for any operating system. Knoppix was developed by Linux consultant Klaus Knopper. When starting a program, it is loaded from the removable medium...
, a Debian
Debian
Debian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...
-derived Linux distribution, was released in 2003, and found popularity as both a rescue disk
Data recovery
Data recovery is the process of salvaging data from damaged, failed, corrupted, or inaccessible secondary storage media when it cannot be accessed normally. Often the data are being salvaged from storage media such as internal or external hard disk drives, solid-state drives , USB flash drive,...
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
Remastersys
remastersys is a free and open source programfor Debian, Ubuntu-based, or derivative software systems that can:* Create a customized Live CD/DVD of Debian and its derivatives....
which made it very easy to build customized live systems. Most of the popular Linux distribution
Linux distribution
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...
s now include a live CD variant, which in some cases is also the preferred installation medium.
Uses
While some live CDs are designed to "demo" or "test drive" a particular operating system (usually Linux or another freeFree software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
or open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
operating system), there are live CDs made for many different uses.
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 with large live CD images, but for smaller live CD images loading the filesystem directly into RAM can be highly practical. Loading the filesystem image into RAM can provide a significant performance boost as RAM is several orders of magnitude faster than a hard drive. Also, since RAM has no moving parts, a system running from a live CD loaded into RAM can run with improved power efficiency. Experienced users of the operating system may also use a live CD to determine whether and to what extent a particular operating system or version is compatible with a particular hardware configuration and certain peripherals, or as a way to know beforehand which computer or peripheral will work before buying. Users may also use a live CD to troubleshoot hardware, especially when a hard drive fails, and more generally as a recovery disc in case of problems. Some live CDs can save user-created files in a Windows partition, a USB drive, a network drive, or other accessible media.
A few additional uses include:
- installing a Linux distributionLinux distributionA Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...
to a hard drive - testing new versions of software
- listing & testing hardware
- system repair and restorationData recoveryData recovery is the process of salvaging data from damaged, failed, corrupted, or inaccessible secondary storage media when it cannot be accessed normally. Often the data are being salvaged from storage media such as internal or external hard disk drives, solid-state drives , USB flash drive,...
- high security/non-invasive environment for a guest
- cracking/stealing passwords
- network security testing
- being the primary or backup operating system for any computer
- quick and simple clustering of computers
- computer forensicsComputer forensicsComputer forensics is a branch of digital forensic science pertaining to legal evidence found in computers and digital storage media...
- playing video games
- providing a secure server platform where crucial files cannot be permanently altered
- providing a secure, reliable platform for the performance of high-vulnerability tasks such as internet banking;
- Internet kiosks, which can be brought back to their original state by a reboot
Thematic Live CDs
Several live CDs are dedicated to specific type of applications according to the requirements of thematic user communities. These CDs are tailored to the needs of the applications in subject including general knowledge, tutorial, specifications and trial data too.Some of these topics covers sub topics, e.g. IT administration breaks down to firewall, rescue, security, etc. type of live CDs. In some cases a particular LiveDVD covers more than one topic.
Live CD software appliances
Packaging a software applianceSoftware appliance
A software appliance is a software application that might be combined with just enough operating system for it to run optimally on industry standard hardware or in a virtual machine....
as an installable live CD can often be
beneficial as a single image can run on real hardware in addition to
most types of virtual machines.
This allows developers to avoid the complexities involved in supporting
multiple incompatible virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
images formats and
focus on the lowest common denominator instead.
Typically after booting the machine from the live CD, the appliance will
either run in non-persistent demo mode or install itself, at the user's request, to an available storage device.
Mounting without burning
The files on a live CD ISO imageISO image
An ISO image is an archive file of an optical disc, composed of the data contents of every written sector of an optical disc, including the optical disc file system...
can be accessed in Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
with a disk image emulator such as Daemon Tools
Daemon Tools
Daemon Tools is a disk image emulator and optical disc authoring program for Microsoft Windows. Daemon Tools was originally a furtherance in the development of another program, Generic SafeDisc emulator, and incorporated all of its features. The program claims to be able to defeat most copy...
, or in Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
variants by mounting a loop device
Loop device
In Unix-like operating systems, a loop device, vnd , or lofi is a pseudo-device that makes a file accessible as a block device....
.
After mounting the live CD's filesystem, software on the live CD can be run directly (I.e., without booting) by chroot
Chroot
A chroot on Unix operating systems is an operation that changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree. The term "chroot" may refer to the chroot...
ing into the live CD's mounted filesystem.
A live CD ISO image can also be mounted by Virtual Machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
software such as VirtualBox
VirtualBox
Oracle VM VirtualBox is an x86 virtualization software package, originally created by software company Innotek GmbH, purchased by Sun Microsystems, and now developed by Oracle Corporation as part of its family of virtualization products...
and VMware Workstation
VMware Workstation
VMware Workstation is a virtual machine software suite for x86 and x86-64 computers from VMware, a division of EMC Corporation, which allows users to set up multiple x86 and x86-64 virtual machines and use one or more of these virtual machines simultaneously with the hosting operating system...
or can be converted to a Live USB
Live USB
A live USB is a USB flash drive or a USB external hard disk drive containing a full operating system that can be booted. Live USBs are closely related to live CDs, but sometimes have the ability to persistently save settings and permanently install software packages back onto the USB device...
using SYSLINUX
SYSLINUX
The SYSLINUX Project is a suite of lightweight IBM PC MBR bootloaders for starting up computers with the Linux kernel. It is the work of H. Peter Anvin, and consists of several separate systems, the best-known of which is ISOLINUX.-List:...
. Tools such as UNetbootin
UNetbootin
UNetbootin is a cross-platform utility that can create live USB systems and can load a variety of system utilities or install various Linux distributions and other operating systems without a CD.- USB Install :...
can automate this process.
Common traits
Some live CDs come with an installation utility launchable from a desktop icon that can optionally install the system on a hard drive or USB flash driveUSB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...
. Most live CDs can access the information on internal and/or external hard drives, diskettes and USB flash drives.
Generally live CDs are booted from read-only
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers 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 Puppy Linux
Puppy Linux
Puppy Linux is a lightweight Linux distribution that focuses on ease of use. The entire system can be run from RAM, allowing the boot medium to be removed after the operating system has started...
which has the ability to save files to the live CD itself or other multisession media, allowing users to carry data, and more importantly, added programs and customized settings, along with them on optical disc.
Most live CDs 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 live CD for other operating systems, such as OpenSolaris
OpenSolaris
OpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...
, BeleniX
BeleniX
BeleniX is an operating system distribution built using the OpenSolaris source base. It can be used as a Live CD as well as installed to hard disk. From the information provided on the BeleniX website, it may appear that currently, BeleniX is compiled only for 32-bit execution...
and others based on Solaris. Other "live" operating systems include AmigaOS 4
AmigaOS 4
AmigaOS 4, , is a line of Amiga operating systems which runs on PowerPC microprocessors. It is mainly based on AmigaOS 3.1 source code, and partially on version 3.9 developed by Haage & Partner...
, FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
, FreeDOS
FreeDOS
FreeDOS is an operating system for IBM PC compatible computers. FreeDOS is made up of many different, separate programs that act as "packages" to the overall FreeDOS Project...
, Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
, Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, OS/2, ReactOS
ReactOS
ReactOS is an open source computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up...
, NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
, OpenBSD
OpenBSD
OpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...
, MINIX 3
MINIX 3
MINIX 3 is a project to create a small, highly reliable and functional Unix-like operating system. It is published under the BSD license.The main goal of the project is for the system to be fault-tolerant by detecting and repairing its own faults on the fly, without user intervention...
, Plan 9 from Bell Labs
Plan 9 from Bell Labs
Plan 9 from Bell Labs is a distributed operating system. It was developed primarily for research purposes as the successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002...
, and MorphOS
MorphOS
MorphOS is an Amiga-compatible computer operating system. It is a mixed proprietary and open source OS produced for the Pegasos PowerPC processor based computer, PowerUP accelerator equipped Amiga computers, and a series of Freescale development boards that use the Genesi firmware, including the...
.
The first personal computer operating system on a CD to support "live" operations might have been the AmigaOS
AmigaOS
AmigaOS is the default native operating system of the Amiga personal computer. It was developed first by Commodore International, and initially introduced in 1985 with the Amiga 1000...
, which could be booted from CD on an Amiga CDTV in 1990.. Earlier examples of live OS are of course the operating systems used from floppy, and most widely spread is DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
.
Unlike previous operating systems on optical media, though, Linux and OS/2 "live CDs" were specifically designed to run without installation onto other media like a hard disk drive. The live CD concept was meant to promote Linux and showcase the abilities of the free, open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
operating system on conventional personal computers with Microsoft Windows already installed.
On a PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
, a bootable Compact Disc generally conforms to the El Torito
El Torito (CD-ROM standard)
The El Torito Bootable CD Specification is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM...
specification. Many Linux based live CDs use a compressed filesystem image, often with the cloop
Cloop
The compressed loopback device or cloop is a module for the Linux kernel. It adds support for transparently decompressed, read-only block devices. It is not a compressed file system in itself....
compressed loopback driver, or squashfs
SquashFS
SquashFS is a compressed read-only file system for Linux. SquashFS compresses files, inodes and directories, and supports block sizes up to 1 MB for greater compression...
compressed filesystem, generally doubling effective storage capacity, although slowing application start up.
The resulting environment can be quite rich: typical Knoppix
Knoppix
Knoppix, or KNOPPIX , is an operating system based on Debian designed to be run directly from a CD / DVD or a USB key , one of the first of its kind for any operating system. Knoppix was developed by Linux consultant Klaus Knopper. When starting a program, it is loaded from the removable medium...
systems include around 1,200 separate software packages. Live CDs have a reputation for supporting advanced auto-configuration
Auto-configuration
Auto-configuration is the automatic configuration of devices without manual intervention, without any need for software configuration programs or jumpers. Ideally, auto-configuring devices should just "plug and play"...
and plug-and-play
Plug-and-play
In computing, plug and play is a term used to describe the characteristic of a computer bus, or device specification, which facilitates the discovery of a hardware component in a system, without the need for physical device configuration, or user intervention in resolving resource conflicts.Plug...
functionality. This came out of necessity to avoid requiring the user to configure the system each time it boots and to make it easily usable by those who are new to the operating system.
Technique
A read-only file systemFile system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
, 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
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
. Often the default Linux directories "
/home" (containing users' personal files and configuration fileConfiguration file
In computing, configuration files, or config files configure the initial settings for some computer programs. They are used for user applications, server processes and operating system settings. The files are often written in ASCII and line-oriented, with lines terminated by a newline or carriage...
s) and "
/var" (containing variable dataData (computing)
In computer science, data is information in a form suitable for use with a computer. Data is often distinguished from programs. A program is a sequence of instructions that detail a task for the computer to perform...
) are kept in ramdisk, because the system updates them frequently. Puppy linux has a savable layer so if you choose to, the next time you boot you can resume (pick right back up again) from where you left off. Each time the CD boots, it looks for the file and then uses it if it has the right name.
In modern live CDs, a read-only file system is merged with ramdisk using transparent
Transparency (computing)
Any change in a computing system, such as new feature or new component, is transparent if the system after change adheres to previous external interface as much as possible while changing its internal behaviour. The purpose is to shield from change all systems on the other end of the interface...
techniques such as UnionFS
UnionFS
UnionFS is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system...
, AuFS
Aufs
aufs implements a union mount for Linux file systems.Developed by Junjiro Okajima in 2006 aufs is a complete rewriting of the earlier UnionFS...
or EWF
Enhanced Write Filter
Enhanced Write Filter is a component of Windows XP Embedded which filters writes to another medium instead of being physically written to the volume itself. EWF allows the writes to be discarded or committed to the physical volume later...
. In MS-DOS and OS/2 systems, ramdrive.sys, can be loaded at boot for this purpose. Boot loaders like syslinux can boot ISO files from USB memory devices.
Live CDs have to be able to detect and use a wide variety of hardware (including network card
Network card
A network interface controller is a computer hardware component that connects a computer to a computer network....
s, graphic cards etc.) in realtime. This is easily achieved nowadays by udev
Udev
udev is the device manager for the Linux kernel. Primarily, it manages device nodes in /dev. It is the successor of devfs and hotplug, which means that it handles the /dev directory and all user space actions when adding/removing devices, including firmware load.-History:udev was new in Linux...
, hotplug, hal, udisk etc.. which is a common part of all distributions based on Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
2.6.
Boot code
During live CD initialization, a user typically may resort to using one or more boot codes to change the booting behavior. These vary from distribution to distribution but can most often be accessed upon first boot screen by one of the function keyFunction key
A function key is a key on a computer or terminal keyboard which can be programmed so as to cause an operating system command interpreter or application program to perform certain actions...
s.
See also
- Comparison of Linux distributions (Live media)
- GIS Live DVDGIS Live DVDGIS Live DVD is a type of the thematic Live CDs containing GIS/RS applications and related tutorials, and sample data sets. The general sense of a GIS Live DVD is to demonstrate the power of FLOSS GIS and encourage users to start on FLOSS GIS. However a disc can be used GIS data processing and...
- List of live CDs
- List of remastering software
- Live USBLive USBA live USB is a USB flash drive or a USB external hard disk drive containing a full operating system that can be booted. Live USBs are closely related to live CDs, but sometimes have the ability to persistently save settings and permanently install software packages back onto the USB device...
- Windows To GoWindows To GoWindows To Go is a feature in Windows 8 that allows the entire system to run from USB mass storage devices such as flash drives and external hard drives....
- Software applianceSoftware applianceA software appliance is a software application that might be combined with just enough operating system for it to run optimally on industry standard hardware or in a virtual machine....
External links
- The LiveCD List tracks LiveCDs, LiveDVDs, and LiveUSB operating systems.
- Bootable CDROM HowTo
- Build your own Custom Live CD
- Linux Live scripts
- Live CD News
- LiveDistro.org Howto create LiveCDs, build tool directory, and download LiveCDs
- Pondering Live CDs - A BSD perspective on the concept of Live CDs
- The magic of live CDs Article published on Free Software Magazine about Live CDs
- A FreeBSD LiveCD project - it also contains valuable information on how to make LiveCD's in FreeBSD
- The Ultimate Boot CD - PC hardware diagnosis tools into one bootable CD to analyze and repair hard drives and boot problems
- The Ultimate Boot CD for Windows - A live CD of Windows XP, with system-restore utilities