User-Mode Linux

User-mode Linux lets you do tricks like run a safely isolated Debian 3.1 on Fedora 4.

It's hosed up pretty good at this point. In fact, you can't even run halt, because the halt program itself is gone. From another command window, kill the system with:

killall -9 vmlinux-

Then, see what happens when you try to boot it up again using the same command (Listing 3).

That's gotta hurt. So, as a lesson, do not do that on a real system. But because this is a UML guest with a COW file, you simply can delete the DangerDanger.cow file, and this guest system will boot up back to its initial state.

More on COW Files

The utility uml_moo included in the UML utilities will read a filesystem image and an associated COW file and create a new merged filesystem image. This allows you to merge changes stored in the COW file into a new master filesystem image. This makes it easy to clone working guest filesystem images when you have them set up the way you want.


User-mode Linux is fun to play with, but it also has some real-world uses. You can use it to test unknown or untrusted applications while limiting possible damage to the running host system. You can create virtual networks of UMLs by starting up multiple guests at once. This allows you to create a test-lab-in-a-box environment with very little time and effort, so you can try all those “Stupid Linux Tricks” you were afraid to try on a real system!

The code for this article is available at

Resources for this article: /article/8883.

Matthew E. Hoskins is a Senior UNIX System Administrator for The New Jersey Institute of Technology where he maintains many of the corporate administrative systems. He enjoys trying to get wildly different systems and software working together, usually with a thin layer of Perl (locally known as “MattGlue”). When not hacking systems, he often can be found hacking in the kitchen. Matt is a member of the Society of Professional Journalists. He is eager to hear your feedback and can be reached at



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