udev(7)


NAME

   udev - Dynamic device management

DESCRIPTION

   udev supplies the system software with device events, manages
   permissions of device nodes and may create additional symlinks in the
   /dev directory, or renames network interfaces. The kernel usually just
   assigns unpredictable device names based on the order of discovery.
   Meaningful symlinks or network device names provide a way to reliably
   identify devices based on their properties or current configuration.

   The udev daemon, systemd-udevd.service(8), receives device uevents
   directly from the kernel whenever a device is added or removed from the
   system, or it changes its state. When udev receives a device event, it
   matches its configured set of rules against various device attributes
   to identify the device. Rules that match may provide additional device
   information to be stored in the udev database or to be used to create
   meaningful symlink names.

   All device information udev processes is stored in the udev database
   and sent out to possible event subscribers. Access to all stored data
   and the event sources is provided by the library libudev.

RULES FILES

   The udev rules are read from the files located in the system rules
   directory /lib/udev/rules.d, the volatile runtime directory
   /run/udev/rules.d and the local administration directory
   /etc/udev/rules.d. All rules files are collectively sorted and
   processed in lexical order, regardless of the directories in which they
   live. However, files with identical filenames replace each other. Files
   in /etc have the highest priority, files in /run take precedence over
   files with the same name in /lib. This can be used to override a
   system-supplied rules file with a local file if needed; a symlink in
   /etc with the same name as a rules file in /lib, pointing to /dev/null,
   disables the rules file entirely. Rule files must have the extension
   .rules; other extensions are ignored.

   Every line in the rules file contains at least one key-value pair.
   Except for empty lines or lines beginning with "#", which are ignored.
   There are two kinds of keys: match and assignment. If all match keys
   match against their values, the rule gets applied and the assignment
   keys get the specified values assigned.

   A matching rule may rename a network interface, add symlinks pointing
   to the device node, or run a specified program as part of the event
   handling.

   A rule consists of a comma-separated list of one or more key-value
   pairs. Each key has a distinct operation, depending on the used
   operator. Valid operators are:

   "=="
       Compare for equality.

   "!="
       Compare for inequality.

   "="
       Assign a value to a key. Keys that represent a list are reset and
       only this single value is assigned.

   "+="
       Add the value to a key that holds a list of entries.

   "-="
       Remove the value from a key that holds a list of entries.

   ":="
       Assign a value to a key finally; disallow any later changes.

   The following key names can be used to match against device properties.
   Some of the keys also match against properties of the parent devices in
   sysfs, not only the device that has generated the event. If multiple
   keys that match a parent device are specified in a single rule, all
   these keys must match at one and the same parent device.

   ACTION
       Match the name of the event action.

   DEVPATH
       Match the devpath of the event device.

   KERNEL
       Match the name of the event device.

   NAME
       Match the name of a network interface. It can be used once the NAME
       key has been set in one of the preceding rules.

   SYMLINK
       Match the name of a symlink targeting the node. It can be used once
       a SYMLINK key has been set in one of the preceding rules. There may
       be multiple symlinks; only one needs to match.

   SUBSYSTEM
       Match the subsystem of the event device.

   DRIVER
       Match the driver name of the event device. Only set this key for
       devices which are bound to a driver at the time the event is
       generated.

   ATTR{filename}, SYSCTL{kernel parameter}
       Match sysfs attribute values of the event device. Trailing
       whitespace in the attribute values is ignored unless the specified
       match value itself contains trailing whitespace.  Match a kernel
       parameter value.

   KERNELS
       Search the devpath upwards for a matching device name.

   SUBSYSTEMS
       Search the devpath upwards for a matching device subsystem name.

   DRIVERS
       Search the devpath upwards for a matching device driver name.

   ATTRS{filename}
       Search the devpath upwards for a device with matching sysfs
       attribute values. If multiple ATTRS matches are specified, all of
       them must match on the same device. Trailing whitespace in the
       attribute values is ignored unless the specified match value itself
       contains trailing whitespace.

   TAGS
       Search the devpath upwards for a device with matching tag.

   ENV{key}
       Match against a device property value.

   TAG
       Match against a device tag.

   TEST{octal mode mask}
       Test the existence of a file. An octal mode mask can be specified
       if needed.

   PROGRAM
       Execute a program to determine whether there is a match; the key is
       true if the program returns successfully. The device properties are
       made available to the executed program in the environment. The
       program's standard output is available in the RESULT key.

       This can only be used for very short-running foreground tasks. For
       details, see RUN.

   RESULT
       Match the returned string of the last PROGRAM call. This key can be
       used in the same or in any later rule after a PROGRAM call.

   Most of the fields support shell glob pattern matching and alternate
   patterns. The following special characters are supported:

   "*"
       Matches zero or more characters.

   "?"
       Matches any single character.

   "[]"
       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 via the "-" character. For
       example, to match on the range of all digits, the pattern "[0-9]"
       could be used. If the first character following the "[" is a "!",
       any characters not enclosed are matched.

   "|"
       Separates alternative patterns. For example, the pattern string
       "abc|x*" would match either "abc" or "x*".

   The following keys can get values assigned:

   NAME
       The name to use for a network interface. See systemd.link(5) for a
       higher-level mechanism for setting the interface name. The name of
       a device node cannot be changed by udev, only additional symlinks
       can be created.

   SYMLINK
       The name of a symlink targeting the node. Every matching rule adds
       this value to the list of symlinks to be created.

       The set of characters to name a symlink is limited. Allowed
       characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
       sequences, and "\x00" hex encoding. All other characters are
       replaced by a "_" character.

       Multiple symlinks may be specified by separating the names by the
       space character. In case multiple devices claim the same name, the
       link always points to the device with the highest link_priority. If
       the current device goes away, the links are re-evaluated and the
       device with the next highest link_priority becomes the owner of the
       link. If no link_priority is specified, the order of the devices
       (and which one of them owns the link) is undefined.

       Symlink names must never conflict with the kernel's default device
       node names, as that would result in unpredictable behavior.

   OWNER, GROUP, MODE
       The permissions for the device node. Every specified value
       overrides the compiled-in default value.

   SECLABEL{module}
       Applies the specified Linux Security Module label to the device
       node.

   ATTR{key}
       The value that should be written to a sysfs attribute of the event
       device.

   SYSCTL{kernel parameter}
       The value that should be written to kernel parameter.

   ENV{key}
       Set a device property value. Property names with a leading "."  are
       neither stored in the database nor exported to events or external
       tools (run by, for example, the PROGRAM match key).

   TAG
       Attach a tag to a device. This is used to filter events for users
       of libudev's monitor functionality, or to enumerate a group of
       tagged devices. The implementation can only work efficiently if
       only a few tags are attached to a device. It is only meant to be
       used in contexts with specific device filter requirements, and not
       as a general-purpose flag. Excessive use might result in
       inefficient event handling.

   RUN{type}
       Add a program to the list of programs to be executed after
       processing all the rules for a specific event, depending on "type":

       "program"
           Execute an external program specified as the assigned value. If
           no absolute path is given, the program is expected to live in
           /lib/udev; otherwise, the absolute path must be specified.

           This is the default if no type is specified.

       "builtin"
           As program, but use one of the built-in programs rather than an
           external one.

       The program name and following arguments are separated by spaces.
       Single quotes can be used to specify arguments with spaces.

       This can only be used for very short-running foreground tasks.
       Running an event process for a long period of time may block all
       further events for this or a dependent device.

       Starting daemons or other long-running processes is not appropriate
       for udev; the forked processes, detached or not, will be
       unconditionally killed after the event handling has finished.

   LABEL
       A named label to which a GOTO may jump.

   GOTO
       Jumps to the next LABEL with a matching name.

   IMPORT{type}
       Import a set of variables as device properties, depending on
       "type":

       "program"
           Execute an external program specified as the assigned value
           and, if it returns successfully, import its output, which must
           be in environment key format. Path specification,
           command/argument separation, and quoting work like in RUN.

       "builtin"
           Similar to "program", but use one of the built-in programs
           rather than an external one.

       "file"
           Import a text file specified as the assigned value, the content
           of which must be in environment key format.

       "db"
           Import a single property specified as the assigned value from
           the current device database. This works only if the database is
           already populated by an earlier event.

       "cmdline"
           Import a single property from the kernel command line. For
           simple flags the value of the property is set to "1".

       "parent"
           Import the stored keys from the parent device by reading the
           database entry of the parent device. The value assigned to
           IMPORT{parent} is used as a filter of key names to import (with
           the same shell glob pattern matching used for comparisons).

       This can only be used for very short-running foreground tasks. For
       details see RUN.

   OPTIONS
       Rule and device options:

       link_priority=value
           Specify the priority of the created symlinks. Devices with
           higher priorities overwrite existing symlinks of other devices.
           The default is 0.

       string_escape=none|replace
           Usually, control and other possibly unsafe characters are
           replaced in strings used for device naming. The mode of
           replacement can be specified with this option.

       static_node=
           Apply the permissions specified in this rule to the static
           device node with the specified name. Also, for every tag
           specified in this rule, create a symlink in the directory
           /run/udev/static_node-tags/tag pointing at the static device
           node with the specified name. Static device node creation is
           performed by systemd-tmpfiles before systemd-udevd is started.
           The static nodes might not have a corresponding kernel device;
           they are used to trigger automatic kernel module loading when
           they are accessed.

       watch
           Watch the device node with inotify; when the node is closed
           after being opened for writing, a change uevent is synthesized.

       nowatch
           Disable the watching of a device node with inotify.

   The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
   fields support simple string substitutions. The RUN substitutions are
   performed after all rules have been processed, right before the program
   is executed, allowing for the use of device properties set by earlier
   matching rules. For all other fields, substitutions are performed while
   the individual rule is being processed. The available substitutions
   are:

   $kernel, %k
       The kernel name for this device.

   $number, %n
       The kernel number for this device. For example, "sda3" has kernel
       number "3".

   $devpath, %p
       The devpath of the device.

   $id, %b
       The name of the device matched while searching the devpath upwards
       for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.

   $driver
       The driver name of the device matched while searching the devpath
       upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.

   $attr{file}, %s{file}
       The value of a sysfs attribute found at the device where all keys
       of the rule have matched. If the matching device does not have such
       an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
       test selected a parent device, then the attribute from that parent
       device is used.

       If the attribute is a symlink, the last element of the symlink
       target is returned as the value.

   $env{key}, %E{key}
       A device property value.

   $major, %M
       The kernel major number for the device.

   $minor, %m
       The kernel minor number for the device.

   $result, %c
       The string returned by the external program requested with PROGRAM.
       A single part of the string, separated by a space character, may be
       selected by specifying the part number as an attribute: "%c{N}". If
       the number is followed by the "+" character, this part plus all
       remaining parts of the result string are substituted: "%c{N+}".

   $parent, %P
       The node name of the parent device.

   $name
       The current name of the device. If not changed by a rule, it is the
       name of the kernel device.

   $links
       A space-separated list of the current symlinks. The value is only
       set during a remove event or if an earlier rule assigned a value.

   $root, %r
       The udev_root value.

   $sys, %S
       The sysfs mount point.

   $devnode, %N
       The name of the device node.

   %%
       The "%" character itself.

   $$
       The "$" character itself.

SEE ALSO

   systemd-udevd.service(8), udevadm(8), systemd.link(5)





Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.