All fields are optional, but type should usually be set since it describes the form of the data, and ndata must be an accurate count (possibly zero) of the number of bytes of data. A missing field is represented by an empty line.
The file is a sequence of rule sets, each of which is a set of one-line rules called patterns and actions. There must be at least one pattern and one action in each rule set. (The only exception is that a rule set may contain nothing but plumb to rules; such a rule set declares the named ports but has no other effect.) A blank line terminates a rule set. Lines beginning with a # character are commentary and are regarded as blank lines.
A line of the form
include filesubstitutes the contents of file for the line, much as in a C #include statement. Unlike in C, the file name is not quoted. If file is not an absolute path name, or one beginning ./ or ../, file is looked for first in the directory in which the plumber is executing, and then in /sys/lib/plumb.
When a message is received by the plumber, the rule sets are examined in order. For each rule set, if the message matches all the patterns in the rule set, the actions associated with the rule set are triggered to dispose of the message. If a rule set is triggered, the rest are ignored for this message. If none is triggered, the message is discarded (giving a write error to the sender) unless it has a dst field that specifies an existing port, in which case the message is emitted, unchanged, from there.
Patterns and actions all consist of three components: an object, a verb, and arguments. These are separated by white space on the line. The arguments may contain quoted strings and variable substitutions, described below, and in some cases contain multiple words. The object and verb are single words from a pre-defined set.
The object in a pattern is the name of an element of the message, such as src or data, or the special case arg, which refers to the argument component of the current rule. The object in an action is always the word plumb.
The verbs in the pattern rules describe how the objects and arguments are to be interpreted. Within a rule set, the patterns are evaluated in sequence; if one fails, the rule set fails. Some verbs are predicates that check properties of the message; others rewrite components of the message and implicitly always succeed. Such rewritings are permanent, so rules that specify them should be placed after all pattern-matching rules in the rule set.
The matches verb has special properties that enable the rules to select which portion of the data is to be sent to the destination. By default, a data matches rule requires that the entire text matches the regular expression. If, however, the message has an attribute named click, that reports that the message was produced by a mouse click within the text and that the regular expressions in the rule set should be used to identify what portion of the data the user intended. Typically, a program such as an editor will send a white-space delimited block of text containing the mouse click, using the value of the click attribute (a number starting from 0) to indicate where in the textual data the user pointed.
When the message has a click attribute, the data matches rules extract the longest leftmost match to the regular expression that contains or abuts the textual location identified by the click. For a sequence of such rules within a given rule set, each regular expression, evaluated by this specification, must match the same subset of the data for the rule set to match the message. For example, here is a pair of patterns that identify a message whose data contains the name of an existing file with a conventional ending for an encoded picture file:
data matches '[a-zA-Z0-9_-./]+' data matches '([a-zA-Z0-9_-./]+).(jpe?g|gif|bit|ps|pdf)'The first expression extracts the largest subset of the data around the click that contains file name characters; the second sees if it ends with, for example, .jpeg. If only the second pattern were present, a piece of text horse.gift could be misinterpreted as an image file named horse.gif.
If a click attribute is specified in a message, it will be deleted by the plumber before sending the message if the data matches rules expand the selection.
The action rules all have the object plumb. There are only three verbs for action rules:
The arguments to all rules may contain quoted strings, exactly as in They may also contain simple string variables, identified by a leading dollar sign $. Variables may be set, between rule sets, by assignment statements in the style of rc. Only one variable assignment may appear on a line. The plumber also maintains some built-in variables:
# these are generally in order from most specific to least, # since first rule that fires wins. addr=':(#?[0-9]+)' protocol='(https?|ftp|file|gopher|mailto|news|nntp|telnet|wais)' domain='[a-zA-Z0-9_@]+([.:][a-zA-Z0-9_@]+)*/?[a-zA-Z0-9_?,%#~&/\-]+' file='([:.][a-zA-Z0-9_?,%#~&/\-]+)*' # image files go to page type is text data matches '[a-zA-Z0-9_\-./]+' data matches '([a-zA-Z0-9_\-./]+).(jpe?g|gif|bit)' arg isfile $0 plumb to image plumb start page -w $file # URLs go to web browser type is text data matches $protocol://$domain$file plumb to web plumb start window webbrowser $0 # existing files, possibly tagged by line number, go to edit/sam type is text data matches '([.a-zA-Z0-9_/-]+[a-zA-Z0-9_/\-])('$addr')?' arg isfile $1 data set $file attr add addr=$3 plumb to edit plumb start window sam $file # .h files are looked up in /sys/include and passed to edit/sam type is text data matches '([a-zA-Z0-9]+\.h)('$addr')?' arg isfile /sys/include/$1 data set $file attr add addr=$3 plumb to edit plumb start window sam $file
The following simple plumbing rules file is a good beginning set of rules.
# to update: cp /usr/$user/lib/plumbing /mnt/plumb/rules editor = acme # or editor = sam include basic