systemd/udev.8

.TH UDEV 8 "October 2003" "" "Linux Administrator's Manual"
.SH NAME
udev \- Linux configurable dynamic device naming support
.SH SYNOPSIS
.BI udev " hotplug-subsystem"
.P
The environment must provide the following variables:
.TP
.B ACTION
.IR add " or " remove
signifies the connection or disconnection of a device.
.TP
.B DEVPATH
The sysfs devpath of the device without the mountpoint but a leading slash.
.P
Additional optional environment variables are:
.TP
.B UDEV_CONFIG_FILE
Overrides the default location of the
.B udev
config file.
.TP
.B UDEV_NO_SLEEP
The default behavior of
.B udev
is to wait until all the sysfs files of the device chain are populated. If set
.B udev
will continue, regardless of the state of the device representation.
.SH "DESCRIPTION"
.B udev
creates or removes device node files usually located in the /dev directory.
It provides a dynamic device directory that contains only the files for
devices that are actually present.
.P
As part of the
.B hotplug
subsystem,
.B udev
is executed if a kernel device is added or removed from the system.
On device creation,
.B 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.
.B udev
maintains a database for devices present on the system.
.br
On device removal,
.B udev
queries its database for the name of the device file to be deleted.
.SH "CONFIGURATION"
All
.B udev
configuration files consist of a set of lines of text.  All empty
lines, and lines beginning with a '#' will be ignored.
.P

.B udev
expects its main configuration file at
.IR /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:
.TP
.B udev_root
This is the where in the filesystem to place the device nodes.  The default
value for this is
.IR /udev/ .
.TP
.B udev_db
The name and location of the udev database.  The default value for this is
.IR /udev/.udev.tdb .
.TP
.B udev_rules
This is the location of the udev rules file.  The default value for this is
.IR /etc/udev/udev.rules .
If a directory is specified, the whole directory is
scanned for files ending with
.I .rules
and all rule files are read in lexical order.
.TP
.B udev_permissions
This is the location of the udev permission file. The default value for this is
.IR /etc/udev/udev.permissions .
If a directory is specified, the whole directory is scanned for files ending with
.I .permissions
and all permission files are read in lexical order.
.TP
.B 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
.IR yes .
.TP
.B 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
.IR 0666 .
.TP
.B 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
.IR root .
.TP
.B 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
.IR root .
.br
.P
.RI "A sample " udev.conf " might look like this:
.sp
.nf
# 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"
.fi
.P
The rules for udev to use when naming devices may specified at
.I /etc/udev/udev.rules
or specified by the
.I udev_rules
value in the
.I /etc/udev/udev.conf
file.
.P
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.
.br
If no matching rule is found, the default kernel device name is used.
.P
Every rule consists of a list a comma separated fields:
.sp
.IR "key " ,[ "key " ,...] " name " [, " symlink" ]
.sp
where possible fields are:
.TP
.B BUS
Match the bus type of the device.
(The sysfs device bus must be able to be determined by a "device" symlink.)
.TP
.B KERNEL
Match the kernel device name.
.TP
.B ID
Match the device number on the bus, like PCI bus id.
.TP
.B PLACE
Match the topological position on bus, like physical port of USB device
.TP
.BI 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.
.br
Trailing whitespace characters in the sysfs attribute value are ignored, if
the key doesn't have any trailing whitespace characters by itself.
.TP
.B PROGRAM
Call external program. This key is valid if the program returns successful.
The environment variables of
.B udev
are also available for the program.
.br
The string returned by the program may additionally matched with the
.B RESULT
key.
.TP
.B RESULT
Match the returned string of the last
.B PROGRAM
call. This key may used in any following rule after a
.B PROGRAM
call.
.TP
.B NAME
The name of the node to be created.
.br
If given with the attribute
.BR NAME{ all_partitions }
it will  create all 15 partitions of a blockdevice.
This may be useful for removable media devices.
.TP
.B SYMLINK
The name of a symlink targeting the node. Multiple symlinks may be
specified by separating the names by the space character.
.br
If both the name and the symlink fields are omitted or its
values empty, the device will be ignored and no node will be created.
.br
If only the symlink field is given and the name field is omitted,
the rule will not be applied immediatly, but the symlink field is added
to the symlink list of the rule which will create the node.
This makes it possible to specify additional symlinks in a possibly
separate rules file, while the device nodes are maintained by the
distribution provided rules file.
.P
.RB "The " NAME " ," SYMLINK " and " PROGRAM
fields support simple printf-like string substitution:
.TP
.B %n
The "kernel number" of the device.
For example, 'sda3' has a "kernel number" of '3'.
.TP
.B %k
The "kernel name" for the device.
.TP
.B %M
The kernel major number for the device.
.TP
.B %m
The kernel minor number for the device.
.TP
.B %b
The bus id for the device.
.TP
.B %c
The
.B PROGRAM
returned string.
(This does not work within the
.B PROGRAM
field for the obvious reason.)
.br
A single part of the string, separated by the space character
my be selected by specifying the part number as a attribute:
.BI %c{ part }
.TP
.BI %s{ filename }
The content of a sysfs attribute.
.TP
.B %%
The '%' character itself.
.P
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.
.P
.RI "A sample " udev.rules " might look like this:"
.sp
.nf
# 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"
.fi
.P
Permissions and ownership for the created device files may specified at
.I /etc/udev/udev.permissions
or specified by the
.I udev_permission
value in the
.I /etc/udev/udev.conf
file.
.br
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.
.sp
.RI "A sample " udev.permissions " might look like this:"
.sp
.nf
#name:user:group:mode
input/*:root:root:644
ttyUSB1:0:8:0660
video*:root:video:0660
dsp1:::0666
.fi
.P
The value
.I $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
.I default_owner
value is used in lieu.
.P
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:
.TP
.B *
Matches zero, one, or more characters.
.TP
.B ?
Matches any single character, but does not match zero characters.
.TP
.B [ ]
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.
.SH "FILES"
.nf
/sbin/udev                           udev program
/etc/udev/*                          udev config files
/etc/hotplug.d/default/udev.hotplug  hotplug symlink to udev program
.fi
.LP
.SH "SEE ALSO"
.BR udevinfo (8),
.BR udevd (8),
.BR hotplug (8)
.PP
The
.I http://linux\-hotplug.sourceforge.net/
web site.
.SH AUTHORS
.B udev
was developed by Greg Kroah-Hartman <greg@kroah.com> with much help from
Dan Stekloff <dsteklof@us.ibm.com>, Kay Sievers <kay.sievers@vrfy.org>, and
many others.