A web crawler creating link soups
noncrawl is a crawler that saves only links. It crawls the web but does not attempt to do everything. Instead, its only purpose is to recursively check sites for links to other sites, which are then also checked for links to other sites, etc. So, if site Y links to site X, that piece of information is saved, and if site X has not been checked yet, it will be crawled just like site Y was. For this to work, one must specify one or more startpages. By default, noncrawl will attempt to crawl several sites simultaneously using threading, but this can be disabled. It is also possible to set a limit to the number of threads.
noncrawl is free software under the terms of the GNU General Public License version 3 (or any later version). The author of noncrawl is Niels Serup, contactable at firstname.lastname@example.org. This is version 0.1 of the program.
External libraries included with noncrawl are GPL-compatible.
Just run this (requires that you have python-setuptools installed):
$ sudo easy_install noncrawl
noncrawl has no real dependencies not included in a default Python install. Python 2.5+ is probably required, though.
If present, noncrawl will use these Python modules:
- Web address: http://pypi.python.org/pypi/htmlentitiesdecode/
- $ sudo easy_install htmlentitiesdecode
(A copy of this module is included in the noncrawl distribution, so you’ll be fine without it)
noncrawl consists of two parts: the crawler and the parser. The crawler must be accessed using a command-line utility called noncrawler. Extracting information from projects can be done either on the command-line using the noncrawlget script or by importing the noncrawl.parser module in a Python program.
noncrawler can be run like this:
$ noncrawler [options] startpages
noncrawler has several options. Run noncrawler --help to see a list of them. When creating a new noncrawl project, noncrawler will create a directory in which all data will be saved. All projects can be resumed if they have been saved properly (which should always happen). White-and-black-listing is supported using line-separated regular expressions with keywords. The syntax of these expressions will be described in a moment.
noncrawlget can be run like this:
$ noncrawlget [options] expression
The program then looks for entries that match the expression. The syntax of these expressions is explained in the next subsection.
The expressions used by noncrawl consists of operator-separated two-word-groups consisting of one keyword and one Python regular expression or one string with UNIX-style wildcards prefixed with an ‘*’, with everything eventually prefixed with a negating character.
An expression looks like this: [y|n] (filter regex|wildcards [operator])+
“y” or “n” specifies whether to accept the result of a match or not. If there is a match between the regex/wildcards and a string, using a “n” negates the return value. It is optional to set this keyword, and it defaults to “y”, meaning that results are not modified.
Filters in groups signify how string testing for matches should be filtered. “url” means not changing them, “domain” means extracting the domain name from the url and using that.
Regular expressions can be studied in the Python documentation at http://docs.python.org/library/re.html
Strings with wildcards should be parsable by the Python fnmatch module, documented at http://docs.python.org/library/fnmatch.html
Operators can be either &&, meaning logical AND, or ||, meaning logical OR.
Expressions beginning with a ‘#’ character are ignored completely.
Note that black-and-white lists prioritize non-negating expressions. That is, specifying an expression that blacklists all urls in existence doesn’t overrule an expression that whitelists something.
The following expressions examplify what is possible:
# Disallow everything using the wildcard '*' (prefixed by another # '*' because it's not a regular expression) n url ** # Disallow search pages because of their dynamic nature n url .+?\?.*?q=.* # Still disallow them, but only on one site n url .+?\?.*?q=.* && domain example.com # Allow urls containing the string "examples" on example.com, or # something similar on Wikipedia. domain example.com && url **examples* || domain wikipedia.org && url **wiki* # Allow all example.* domains except for .org domain .*?example\.(?!org)
noncrawl comes with a base inclusion-exclusion list that it uses per default. For more examples, see the list in the file named “whiteblacklist.py” of this distribution.
noncrawl uses Git for branches. To get the latest branch, get it from gitorious.org like this:
$ git clone git://gitorious.org/noncrawl/noncrawl.git
noncrawl is written in Python.
The logo of noncrawl, found in the “logo” directory, is available under the terms of the Creative Commons Attribution-ShareAlike 3.0 (or any later version) Unported license. A copy of this license is available at http://creativecommons.org/licenses/by-sa/3.0/
Copyright (C) 2010 Niels Serup
Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.
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