A unique feature of Linux is that you can burn the OS to a CD or DVD and boot from that medium. This bootable version of Linux is called a live CD/DVD. This is a piece of medium that contains a complete and bootable OS. This is very different from items like the boot floppies of the past. In the case of boot floppies, a completely functional OS was just not possible (except in the early days of the disk operation systems [DOS]). With a live CD or DVD, you can run a fully featured, fully functional OS that gives the same experience as the OS installed on the hard drive of a computer. For all intents and purposes, you can say that just about every distribution of Linux is available in a live CD/DVD format.

One of the bigger benefits of a live CD/DVD is that you can boot a computer off a live CD/DVD without making any alterations to the existing OS on the computer. When running a live CD/DVD, the computer boots off the given medium and uses the OS, which is running totally off the removable medium. This can be useful for evaluating the OS prior to making changes to the computer in any way. You could also use this for evaluating hardware support and compatibility. You may also use a live CD/DVD to troubleshoot hardware—for example, when a piece of hardware fails—or to recover a corrupted OS.

Here are some common uses of live distributions:

• Installing Linux on a new system

• Testing new software

• Evaluating hardware configurations

• Repairing damaged systems

• Providing guest systems

• Providing portable systems

• Cracking passwords

• Stealing passwords

• Resetting passwords

• Conducting penetration testing

• Multibooting

• Performing forensic investigations

• Providing a secure, unalterable OS

• Setting up kiosks

• Creating persistent desktops

As with most live distributions, the ability to return the system to whatever state it happened to be in prior to the installation is standard. The process is simple: Boot off the live medium, and use the OS. When you are finished, shut down the OS, eject the medium, and reboot, and you are back where you started. The downside of live distributions is performance. Because the entire OS is being run from physical memory, the performance will be less than if it were installed on the physical hard drive. Essentially, the entire OS is running from random access memory (RAM) along with all the applications, which means less RAM to go around. However, the amount of RAM required for Linux is quite low, with some Linux distributions able to run with as little as 32 MB (megabytes) of memory.

Although most live CDs/DVDs are designed for you to test-drive an OS, there are some designed for other uses. Live CDs/DVDs are available for forensic purposes, malware removal, system recovery, password reset, and other uses.

Although the majority of live CDs/DVDs can run in memory to free the optical drive or other media for other uses, loading the data off a CD or DVD will always be slower than a hard drive–based installation. With larger OSs, there will be a substantial penalty incurred while the required information is loaded off the media, but with smaller images, loading the OS directly into RAM can be fast and efficient. Loading the image into physical memory provides substantial performance benefits because RAM is much faster than a hard drive.