excertp from 'man udev';

udev - dynamic device management

DESCRIPTION
       udev provides a dynamic device directory containing only the files for
       actually present devices. It creates or removes device node files in
       the /dev directory, or it renames network interfaces.

       Usually udev runs as udevd(8) and receives uevents directly from the
       kernel if a device is added or removed from the system.

       If udev receives a device event, it matches its configured rules
       against the available device attributes provided in sysfs to identify
       the device. Rules that match may provide additional device information
       or specify a device node name and multiple symlink names and instruct
       udev to run additional programs as part of the device event handling.

CONFIGURATION
       udev configuration files are placed in /etc/udev/ and /lib/udev/. All
       empty lines, or lines beginning with '#' will be ignored.

   Configuration file
       udev expects its main configuration file at /etc/udev/udev.conf. It
       consists of a set of variables allowing the user to override default
       udev values. The following variables can be set:

       udev_root
           Specifies where to place the device nodes in the filesystem. The
           default value is /dev.

       udev_log
           The logging priority. Valid values are the numerical syslog
           priorities or their textual representations: err, info and debug.

   Rules files
       The udev rules are read from the files located in the default rules
       directory /lib/udev/rules.d/, the custom rules directory
       /etc/udev/rules.d/ and the temporary rules directory
       /dev/.udev/rules.d/. All rule files are sorted and processed in lexical
       order, regardless in which of these directories they live.

       Rule files are required to have a unique name, duplicate file names are
       ignored. Files in /etc/udev/rules.d/ have precedence over files with
       the same name in /lib/udev/rules.d/. This can be used to ignore a
       default rules file if needed.

       Every line in the rules file contains at least one key value pair.
       There are two kind of keys, match and assignment keys. If all match
       keys are matching against its value, the rule gets applied and the
       assign keys get the specified value assigned.

       A matching rule may specify the name of the device node, add a symlink
       pointing to the node, or run a specified program as part of the event
       handling. If no matching rule is found, the default device node name is
       used.