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udev provides a dynamic device directory containing only the files for actually present devices. It creates or removes device node files in the
\fI/dev\fR
directory, or it renames network interfaces.
.PP
Usually udev runs as
\fBudevd\fR(8)
and receives uevents directly from the kernel if a device is added or removed form the system. The program
\fBudev\fR
itself may be used as an event handler in situations, where running the daemon is not appropriate, like in initramfs.
.PP
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.
\fI/etc/udev/*\fR. Every file consist of a set of lines of text. All empty lines or lines beginning with '#' will be ignored.
.SS"Configuration file"
.PP
udev expects its main configuration file at
\fI/etc/udev/udev.conf\fR. It consists of a set of variables allowing the user to override default udev values. The following variables can be set:
.TP
\fBudev_root\fR
Specifies where to place the device nodes in the filesystem. The default value is
\fI/dev\fR.
.TP
\fBudev_rules\fR
The name of the udev rules file or directory to look for files with the suffix
\fI.rules\fR. Multiple rule files are read in lexical order. The default value is
\fI/etc/udev/rules.d\fR.
.TP
\fBudev_log\fR
The logging priority. Valid values are the numerical syslog priorities or their textual representations:
\fBerr\fR,
\fBinfo\fR
and
\fBdebug\fR.
.SS"Rules files"
.PP
The udev rules are read from the files located in the
\fI/etc/udev/rules.d\fR
directory or at the location specified value in the configuraton file. Every line in the rules file contains at least one key value pair. There are two kind of keys, match and assignement 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.
.PP
A rule may consists of a list of one or more key value pairs separated by a comma. Each key has a distinct operation, depending on the used operator. Valid operators are:
Match against the value of an environment key. Depending on the specified operation, this key is also used as a assignment.
.TP
\fBSYSFS{\fR\fB\fIfilename\fR\fR\fB}\fR
Match the sysfs attribute value. Up to five values can be specified. Trailing whitespace is ignored, if the specified match value does not contain trailing whitespace itself.
.TP
\fBPROGRAM\fR
Execute external program. The key is true, if the program returns without exit code zero. The whole event environment is available to the executed program. The program's output printed to stdout is available for the RESULT key.
.TP
\fBRESULT\fR
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.
.PP
Most of the fields support a shell style pattern matching. The following pattern characters are supported:
.TP
\fB*\fR
Matches zero, or any number of characters.
.TP
\fB?\fR
Matches any single character.
.TP
\fB[]\fR
Matches any single character specified within the brackets. 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 '!', any characters not enclosed are matched.
The name of the node to be created, or the name, the network interface should be renamed to. Only one rule can set the a name, all later rules with a NAME key will be ignored.
.TP
\fBSYMLINK\fR
The name of a symlink targeting the node. Every matching rule can add this value to the list of symlinks to be created along with the device node. Multiple symlinks may be specified by separating the names by the space character.
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:
\fB%c{N}\fR. If the number is followed by the '+' char this part plus all remaining parts of the result string are substituted:
If a device node already exists with the name, the smallest next free number is used. This can be used to create compatibility symlinks and enumerate devices of the same type originating from different kernel subsystems.
Note: The use of the enumeration facility is unreliable for events that request a number at the same time. The use of enumerations in todays setups where devices can come and go at any time is not recommended.
The count of characters to be substituted may be limited by specifying the format length value. For example, '%3s{file}' will only insert the first three characters of the sysfs attribute
