Live Linux CDs


Jarret W. Buse

Live Linux CDs

Most Linux users have come across Live CDs. For those who haven't, a Live CD is a bootable CD (or DVD) which runs a version of Linux. When the Linux Operating System is run, it is running in memory only. The data on the hard disks are not touched or changed, unless a specific Live CD is used, which are for desired purposes such as data rescue.

The main purpose for a Live CD is to test drive a version of Linux which may need to be installed. By downloading and burning the ISO image to a CD/DVD, the disc can then be used to boot the PC. Once booted, the OS is loaded into memory and the OS can be used in a similar fashion as if it were installed on the local hardware. Hardware compatibility can be tested as well as software issues with required applications needed on the local PC.

There are many uses for a Live CD/DVD which can include, but not being limited to, the following:

· astronomy
· bioinformatics
· clustering
· cnc-metalworking
· desktop
· development
· diagnostics
· education
· firewall
· forensics
· gaming
· gis
· hobby
· home-entertainment
· kiosk
· media-production
· medical
· os-installation
· rescue
· robotics
· science
· secure-desktop
· security
· server
· system-administration
· thin-client
· voip
· windows-antivirus

These categories are used at the website The website has an extensive list of Live CD/DVDs which includes a home page and download page for each Live CD/DVD.

Now, to get back to what a Live CD is and how it works. First, the ISO image contains the files necessary to boot from the media to which the ISO is placed.

NOTE: The term CD/DVD is used, but it is possible to place the contents of the image onto a USB thumb drive and boot from it. The ISO images can also be used to load into a virtual system such as Virtual Box.

The boot files cause the CD/DVD device to be used as boot media which causes the local OS to be loaded into memory. Once loaded, the OS can be used without changing any local data on the hard disk (unless the boot media is for that purpose, such as a Live CD of Gparted or the like).

Live CDs can also be beneficial for systems with no internal hard drive or storage media.

The initial part of the Live CD (and any Linux distro) is the Initial RAMDisk (initrd). The initrd is a compressed file which is extracted to a RAM Disk to be executed as the initial bootloader.

A RAM Disk is a portion of Random Access Memory (RAM) which is used as if it were a hard disk. Since the RAM Disk is in RAM, everything in it runs faster since RAM has faster access time than a physical hard disk.

NOTE: Even with an OS installed on a physical hard disk, the initrd is used for the boot process. The initrd places the kernel into memory to continue the booting of the rest of the OS.

Some distros may use an Initial RAM FileSystem (initramfs) file instead of an initrd file.

On some Ubuntu Live CDs, a file called “initrd.lz” contains the initrd file. The “lz” file is a compressed file using the lzma compression method. The kernel itself is in a compressed file called vmlinuz.

Let's look at the steps of the system boot process. The steps are as follows:

· Stage 1 (Master Boot Record)
· Stage 2 (GRUB)
· Kernel (Linux)
· Init (User space)

For a Live CD, Stage 2 is where the initrd is loaded from the Live CD and the RAM Disk is created. The configuration files on the CD show to continue to the use the RAM Disk as the main media and not to use the local hard disk if one exists.

Once the RAM Disk is created, the initrd file is uncompressed to it and control is passed to it. The initrd uncompresses the vmlinuz file onto the RAM disk which is the kernel. Control is passed to the kernel which then allows the OS to continue to load. For a Live CD, the OS is placed onto the RAM Disk which is then a completely working version of Linux.

Once Linux is loaded, each distro falls into a specific category as mentioned above. Each desktop as well as incorporated applications vary depending on the distro Live CD. For example, Rescue Live CDs will have minimal applications except those pertaining to the rescue methods. If the Live CD is to help rescue data from a hard disk, then only applications needed to restore data are included. Linux distros meant for a specific purpose such as gaming, will have games included as applications.


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George Mulak

Wow, this is also a great resource. Thanks for that.

Nathan Rodriguez

Which there was Live distros for development, with Unity3D for instance.
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