Manage Linux Hardware with udev - Page 2

By Carla Schroder | Posted Oct 3, 2006
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Writing udev Rules

Current releases of udev come with bales of man pages, and several different useful commands. On Debian systems, find them all with dpkg:

 $ dpkg -L udev 

On Fedora, use the rpm command:

 $ rpm -ql udev 

You'll see there are different command sets for each distribution, and a pox on both of them for sowing useless confusion, so we'll look at the important commands they have in common.

The first step for writing or modifying udev rules is to make sure that your kernel sees the device you want to make the rule for. (If the kernel doesn't see it, you need to find out if your device is supported in Linux at all, then how to enable that support.) First try running the udevinfo command. This example dumps all devices in the udev database:
 $ udevinfo -e 

You may query individual devices, if you know the device path, as this example for an SATA partition shows:

 $ udevinfo -a -p /block/sda/sda1 

How do you know the device path? From udevinfo. You already know the node name for some of your devices, and can query it this way:

 $ udevinfo -q all -n sda 

Both of these commands spit out a lot of information. Just for fun, you can run lspci and match up the disk/by-path/pci values from udevinfo:

 0000:00:0f.0 RAID bus controller: VIA Technologies, Inc.  VIA
VT6420 SATA RAID Controller (rev 80)S: disk/by-path/pci-0000:00:0f.0-scsi-0:0:0:0-part1 

The output of udevinfo is pretty cryptic, so this is one way to match it up to devices with names you recognize. For USB devices compare with the output of lsusb. SCSI devices compare to lsscsi.

Come back next week to learn how to nail down USB network cards, straighten out device permissions, and manage multiple storage drives painlessly with udev.

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