The environment must provide the following variables:
ACTION add " or " remove signifies the connection or disconnection of a device.
DEVPATH The sysfs devpath of the device without the mountpoint but a leading slash.
Additional optional environment variables are:
UDEV_CONFIG_FILE Overrides the default location of the udev config file.
UDEV_NO_SLEEP The default behavior of udev is to wait until all the sysfs files of the device chain are populated. If set udev will continue, regardless of the state of the device representation.
As part of the hotplug subsystem, udev is executed if a kernel device is added or removed from the system. On device creation, udev reads the sysfs directory of the given device to collect device attributes like label, serial number or bus device number. These attributes may used as keys to determine a unique name for device file creation. udev maintains a database for devices present on the system.
On device removal, udev queries its database for the name of the device file to be deleted.
udev expects its main configuration file at /etc/udev/udev.conf . The file consists of a set of variables and values that allow the user to override default udev values. The current set of variables that can be overridden in this file is:
udev_root This is the where in the filesystem to place the device nodes. The default value for this is /udev/ .
udev_db The name and location of the udev database. The default value for this is /udev/.udev.tdb .
udev_rules This is the location of the udev rules file. The default value for this is /etc/udev/udev.rules . If a directory is specified, the whole directory is scanned for files ending with .rules and all rule files are read in lexical order.
udev_permissions This is the location of the udev permission file. The default value for this is /etc/udev/udev.permissions . If a directory is specified, the whole directory is scanned for files ending with .permissions and all permission files are read in lexical order.
udev_log If you want udev to log some information to the syslog for every node created or removed. The default value for this is yes .
default_mode This is the default mode for all nodes that have no explicit match in the permissions file. The default value for this is 0666 .
default_owner This is the default owner for all nodes that have no explicit match in the permissions file. The default value for this is root .
default_group This is the default group for all nodes that have no explicit match in the permissions file. The default value for this is root .
"A sample " udev.conf " might look like this:
# udev_root - where in the filesystem to place the device nodes udev_root="/udev/" # udev_db - The name and location of the udev database. udev_db="/udev/.udev.tdb" # udev_rules - The location of the directory where to look for files which names ending with .rules udev_rules="/etc/udev/" # udev_permissions - The name and location of the udev permission file udev_permissions="/etc/udev/udev.permissions" # udev_log - set to "yes" if you want logging, else "no" udev_log="yes" # default_mode - set the default mode for all nodes that have no # explicit match in the permissions file default_mode="0666" # default_owner - set the default owner for all nodes that have no # explicit match in the permissions file default_owner="root" # default_group - set the default group for all nodes that have no # explicit match in the permissions file default_group="root"
The rules for udev to use when naming devices may specified at /etc/udev/udev.rules or specified by the udev_rules value in the /etc/udev/udev.conf file.
Every line in the rules file defines the mapping between device attributes and the device file name. One ore more keys are specified to match a rule with the current device. If all keys are matching, the rule will be applied and the name is used for the device node.
If no matching rule is found, the default kernel device name is used.
Every rule consists of a list a comma separated fields: "key " ,[ "key " ,...] " name " [, " symlink" ] where possible fields are:
BUS Match the bus type of the device. (The sysfs device bus must be able to be determined by a "device" symlink.)
KERNEL Match the kernel device name.
ID Match the device number on the bus, like PCI bus id.
PLACE Match the topological position on bus, like physical port of USB device
SYSFS{ filename } Match sysfs device attribute like label, vendor, USB serial number, SCSI UUID or file system label. Up to 5 different sysfs files can be checked, with all of the values being required in order to match the rule.
Trailing whitespace characters in the sysfs attribute value are ignored, if the key doesn't have any trailing whitespace characters by itself.
PROGRAM Call external program. This key is valid if the program returns successful. The environment variables of udev are also available for the program.
The string returned by the program may additionally matched with the RESULT key.
RESULT Match the returned string of the last PROGRAM call. This key may used in any following rule after a PROGRAM call.
NAME The name of the node to be created. If the name field is omitted or its value is empty, the device will be ignored and no node will be created.
If given with the attribute NAME{ all_partitions } it will create all 15 partitions of a blockdevice. This may be useful for removable media devices.
SYMLINK The name of a symlink targeting the node. Multiple symlinks may be specified by separating the names by the space character.
"The " NAME " ," SYMLINK " and " PROGRAM fields support simple printf-like string substitution:
%n The "kernel number" of the device. For example, 'sda3' has a "kernel number" of '3'.
%k The "kernel name" for the device.
%M The kernel major number for the device.
%m The kernel minor number for the device.
%b The bus id for the device.
%c The PROGRAM returned string. (This does not work within the PROGRAM field for the obvious reason.)
A single part of the string, separated by the space character my be selected by specifying the part number as a attribute: %c{ part }
%s{ filename } The content of a sysfs attribute.
%% The '%' character itself.
The count of charcters to insert may be limited by specifying the format length value. For example, '%3s{file}' will only insert the first three characters of the sysfs attribute.
"A sample " udev.rules " might look like this:"
# if /sbin/scsi_id returns "OEM 0815" device will be called disk1 BUS="scsi", PROGRAM="/sbin/scsi_id", RESULT="OEM 0815", NAME="disk1" # USB printer to be called lp_color BUS="usb", SYSFS{serial}="W09090207101241330", NAME="lp_color" # SCSI disk with a specific vendor and model number is to be called boot BUS="scsi", SYSFS{vendor}="IBM", SYSFS{model}="ST336", NAME="boot%n" # sound card with PCI bus id 00:0b.0 to be called dsp BUS="pci", ID="00:0b.0", NAME="dsp" # USB mouse at third port of the second hub to be called mouse1 BUS="usb", PLACE="2.3", NAME="mouse1" # ttyUSB1 should always be called pda with two additional symlinks KERNEL="ttyUSB1", NAME="pda", SYMLINK="palmtop handheld" # multiple USB webcams with symlinks to be called webcam0, webcam1, ... BUS="usb", SYSFS{model}="XV3", NAME="video%n", SYMLINK="webcam%n"
Permissions and ownership for the created device files may specified at /etc/udev/udev.permissions or specified by the udev_permission value in the /etc/udev/udev.conf file.
Every line lists a device name followed by owner, group and permission mode. All values are separated by colons. The name field may contain a pattern to apply the values to a whole class of devices. "A sample " udev.permissions " might look like this:"
#name:user:group:mode input/*:root:root:644 ttyUSB1:0:8:0660 video*:root:video:0660 dsp1:::0666
The value $local can be substituted for a specific username. In that case, udev will determine the current local user at the time of device node creation and substitute that username as the owner of the new device node. This is useful, for example, to let hot-plugged devices, such as cameras, be owned by the user at the current console. Note that if no user is currently logged in, or if udev otherwise fails to determine a current user, the default_owner value is used in lieu.
A number of different fields in the above configuration files support a simple form of shell style pattern matching. It supports the following pattern characters:
* Matches zero, one, or more characters.
? Matches any single character, but does not match zero characters.
[ ] Matches any single character specified within the brackets. For example, the pattern string "tty[SR]" would match either "ttyS" or "ttyR". Ranges are also supported within this match with the '-' character. For example, to match on the range of all digits, the pattern [0-9] would be used. If the first character following the '[' is a '!' then any character not enclosed is matched.
/sbin/udev udev program /etc/udev/* udev config files /etc/hotplug.d/default/udev.hotplug hotplug symlink to udev program
The http://linux-hotplug.sourceforge.net/ web site.