Deconstructing Linux udev Rules

udev is one of those pieces of Linux that is fairly well documented and not very well understood. This note isn't intended as a general introduction to writing udev rules, but, rather, a brief introduction to the topic by way of specific example.

Why Bother With udev?

There are many clever uses for udev documented on the Web, but the most common use is to ensure that when you connect a device - disk, tape, usb thumbdrive, camera... whatever - to a Linux system, that device shows up with the same name every time.

Original Unix derivatives had a static tree of devices the system could support. This was encoded in the /dev file tree hierarchy. This was pretty inflexible in the face of devices being added- and removed- from the system as it ran. For this reason, modern device handling in Linux and most other Unix derivatives is dynamic - the content of /dev changes to reflect the actual state of the system as things get connected or disconnected. (Exactly how this is done is outside the purpose of this document, but if you care, investigate the Linux /sys filesystem works.)

Our Example Problem

While the example below is "cooked", it is very much rooted in real world udev application. We want to do the following things:

• Make sure that a particular disk always shows up under the same name under /dev no matter how many disks have been connected before it. We do that by uniquely identifying a disk and then creating a symbolic link to it that never changes.
• Change the user and group ownership of that disk to something other than the default of root:disk.
• Create a corresponding "raw" character device under /dev/raw. This is something you commonly see on database servers and it's suffiently useful that's it's worth mentioning here.

Where Do udev Rules Live?

Our Example Rules

Author

Tim Daneliuk - tundra@tundraware.com

Comments and/or improvements welcome!

Document Revision Information

$Id: Deconstructing_Linux_udev_Rules.rst,v 1.101 2013/10/31 21:40:42 tundra Exp $

You can find the latest version of this document at:

http://www.tundraware.com/TechnicalNotes/Deconstructing-Linux-udev-Rules