Provided by: urlwatch_2.28-3_all 
NAME
urlwatch-filters - Filtering output and diff data of urlwatch jobs
SYNOPSIS
urlwatch --edit
DESCRIPTION
Each job can have two filter stages configured, with one or more filters processed after each other:
• Applied to the downloaded page before diffing the changes (filter)
• Applied to the diff result before reporting the changes (diff_filter)
While creating your filter pipeline, you might want to preview what the filtered output looks like. You
can do so by first configuring your job and then running urlwatch with the --test-filter command, passing
in the index (from --list) or the URL/location of the job to be tested:
urlwatch --test-filter 1 # Test the first job in the list
urlwatch --test-filter https://example.net/ # Test the job with the given URL
The output of this command will be the filtered plaintext of the job, this is the output that will (in a
real urlwatch run) be the input to the diff algorithm.
The filter is only applied to new content, the old content was already filtered when it was retrieved.
This means that changes to filter are not visible when reporting unchanged contents (see Display for
details), and the diff output will be between (old content with filter at the time old content was
retrieved) and (new content with current filter).
Once urlwatch has collected at least 2 historic snapshots of a job (two different states of a webpage)
you can use the command-line option --test-diff-filter to test your diff_filter settings; this will use
historic data cached locally.
BUILT-IN FILTERS
The list of built-in filters can be retrieved using:
urlwatch --features
At the moment, the following filters are built-in:
• beautify: Beautify HTML
• css: Filter XML/HTML using CSS selectors
• csv2text: Convert CSV to plaintext
• element-by-class: Get all HTML elements by class
• element-by-id: Get an HTML element by its ID
• element-by-style: Get all HTML elements by style
• element-by-tag: Get an HTML element by its tag
• format-json: Convert to formatted json
• grep: Filter only lines matching a regular expression
• grepi: Remove lines matching a regular expression
• hexdump: Convert binary data to hex dump format
• html2text: Convert HTML to plaintext
• pdf2text: Convert PDF to plaintext
• pretty-xml: Pretty-print XML
• ical2text: Convert iCalendar <https://en.wikipedia.org/wiki/ICalendar> to plaintext
• ocr: Convert text in images to plaintext using Tesseract OCR
• re.sub: Replace text with regular expressions using Python's re.sub
• reverse: Reverse input items
• sha1sum: Calculate the SHA-1 checksum of the content
• shellpipe: Filter using a shell command
• sort: Sort input items
• remove-duplicate-lines: Remove duplicate lines (case sensitive)
• strip: Strip leading and trailing whitespace
• striplines: Strip leading and trailing whitespace in each line
• xpath: Filter XML/HTML using XPath expressions
• jq: Filter, transform and extract values from JSON
PICKING OUT ELEMENTS FROM A WEBPAGE
You can pick only a given HTML element with the built-in filter, for example to extract <div
id="something">.../<div> from a page, you can use the following in your urls.yaml:
url: http://example.org/idtest.html
filter:
- element-by-id: something
Also, you can chain filters, so you can run html2text on the result:
url: http://example.net/id2text.html
filter:
- element-by-id: something
- html2text
CHAINING MULTIPLE FILTERS
The example urls.yaml file also demonstrates the use of built-in filters, here 3 filters are used:
html2text, line-grep and whitespace removal to get just a certain info field from a webpage:
url: https://example.net/version.html
filter:
- html2text
- grep: "Current.*version"
- strip
EXTRACTING ONLY THE <BODY> TAG OF A PAGE
If you want to extract only the body tag you can use this filter:
url: https://example.org/bodytag.html
filter:
- element-by-tag: body
FILTERING BASED ON AN XPATH EXPRESSION
To filter based on an XPath <https://www.w3.org/TR/1999/REC-xpath-19991116/> expression, you can use the
xpath filter like so:
url: https://example.net/xpath.html
filter:
- xpath: /html/body/marquee
This filters only the <marquee> elements directly below the <body> element, which in turn must be below
the <html> element of the document, stripping out everything else.
See Microsoft’s XPath Examples <https://msdn.microsoft.com/en-us/library/ms256086(v=vs.110).aspx> page
for some other examples. You can also find an XPath of an <html> node in the Chromium/Google Chrome
developer tools by right clicking on the node and selecting copy XPath.
FILTERING BASED ON CSS SELECTORS
To filter based on a CSS selector <https://www.w3.org/TR/2011/REC-css3-selectors-20110929/>, you can use
the css filter like so:
url: https://example.net/css.html
filter:
- css: ul#groceries > li.unchecked
This would filter only <li class="unchecked"> tags directly below <ul id="groceries"> elements.
Some limitations and extensions exist as explained in cssselect’s documentation
<https://cssselect.readthedocs.io/en/latest/#supported-selectors>.
USING XPATH AND CSS FILTERS WITH XML AND EXCLUSIONS
By default, XPath and CSS filters are set up for HTML documents. However, it is possible to use them for
XML documents as well (these examples parse an RSS feed and filter only the titles and publication
dates):
url: https://example.com/blog/xpath-index.rss
filter:
- xpath:
path: '//item/title/text()|//item/pubDate/text()'
method: xml
url: http://example.com/blog/css-index.rss
filter:
- css:
selector: 'item > title, item > pubDate'
method: xml
- html2text: re
To match an element in an XML namespace <https://www.w3.org/TR/xml-names/>, use a namespace prefix before
the tag name. Use a : to separate the namespace prefix and the tag name in an XPath expression, and use a
| in a CSS selector.
url: https://example.net/feed/xpath-namespace.xml
filter:
- xpath:
path: '//item/media:keywords/text()'
method: xml
namespaces:
media: http://search.yahoo.com/mrss/
url: http://example.org/feed/css-namespace.xml
filter:
- css:
selector: 'item > media|keywords'
method: xml
namespaces:
media: http://search.yahoo.com/mrss/
- html2text
Alternatively, use the XPath expression //*[name()='<tag_name>'] to bypass the namespace entirely.
Another useful option with XPath and CSS filters is exclude. Elements selected by this exclude
expression are removed from the final result. For example, the following job will not have any <a> tag in
its results:
url: https://example.org/css-exclude.html
filter:
- css:
selector: body
exclude: a
LIMITING THE RETURNED ITEMS FROM A CSS SELECTOR OR XPATH
If you only want to return a subset of the items returned by a CSS selector or XPath filter, you can use
two additional subfilters:
• skip: How many elements to skip from the beginning (default: 0)
• maxitems: How many elements to return at most (default: no limit)
For example, if the page has multiple elements, but you only want to select the second and third matching
element (skip the first, and return at most two elements), you can use this filter:
url: https://example.net/css-skip-maxitems.html
filter:
- css:
selector: div.cpu
skip: 1
maxitems: 2
Dealing with duplicated results
If you get multiple results on one page, but you only expected one (e.g. because the page contains both a
mobile and desktop version in the same HTML document, and shows/hides one via CSS depending on the
viewport size), you can use maxitems: 1 to only return the first item.
FIXING LIST REORDERINGS WITH CSS SELECTOR OR XPATH FILTERS
In some cases, the ordering of items on a webpage might change regularly without the actual content
changing. By default, this would show up in the diff output as an element being removed from one part of
the page and inserted in another part of the page.
In cases where the order of items doesn't matter, it's possible to sort matched items lexicographically
to avoid spurious reports when only the ordering of items changes on the page.
The subfilter for css and xpath filters is sort, and can be true or false (the default):
url: https://example.org/items-random-order.html
filter:
- css:
selector: span.item
sort: true
FILTERING PDF DOCUMENTS
To monitor the text of a PDF file, you use the pdf2text filter. It requires the installation of the
pdftotext <https://github.com/jalan/pdftotext/blob/master/README.md#pdftotext> library and any of its
OS-specific dependencies <https://github.com/jalan/pdftotext/blob/master/README.md#os-dependencies>.
This filter must be the first filter in a chain of filters, since it consumes binary data and outputs
text data.
url: https://example.net/pdf-test.pdf
filter:
- pdf2text
- strip
If the PDF file is password protected, you can specify its password:
url: https://example.net/pdf-test-password.pdf
filter:
- pdf2text:
password: urlwatchsecret
- strip
DEALING WITH CSV INPUT
The csv2text filter can be used to turn CSV data to a prettier textual representation This is done by
supplying a format_string which is a python format string
<https://docs.python.org/3/library/string.html#format-string-syntax>.
If the CSV has a header, the format string should use the header names lowercased. For example, let's
say we have a CSV file containing data like this:
Name;Company
Smith;Initech
Doe;Initech
A possible format string for the above CSV (note the lowercase keys):
Mr {name} works at {company}
If there is no header row, you will need to use the numeric array notation:
Mr {0} works at {1}
You can force the use of numeric indices with the flag ignore_header.
The key has_header can be used to force use the first line or first ignore the first line as header,
otherwise csv.Sniffer <https://docs.python.org/3/library/csv.html#csv.Sniffer> will be used.
SORTING OF WEBPAGE CONTENT
Sometimes a web page can have the same data between comparisons but it appears in random order. If that
happens, you can choose to sort before the comparison.
url: https://example.net/sorting.txt
filter:
- sort
The sort filter takes an optional separator parameter that defines the item separator (by default sorting
is line-based), for example to sort text paragraphs (text separated by an empty line):
url: http://example.org/paragraphs.txt
filter:
- sort:
separator: "\n\n"
This can be combined with a boolean reverse option, which is useful for sorting and reversing with the
same separator (using % as separator, this would turn 3%2%4%1 into 4%3%2%1):
url: http://example.org/sort-reverse-percent.txt
filter:
- sort:
separator: '%'
reverse: true
REVERSING OF LINES OR SEPARATED ITEMS
To reverse the order of items without sorting, the reverse filter can be used. By default it reverses
lines:
url: http://example.com/reverse-lines.txt
filter:
- reverse
This behavior can be changed by using an optional separator string argument (e.g. items separated by a
pipe (|) symbol, as in 1|4|2|3, which would be reversed to 3|2|4|1):
url: http://example.net/reverse-separator.txt
filter:
- reverse: '|'
Alternatively, the filter can be specified more verbose with a dict. In this example "\n\n" is used to
separate paragraphs (items that are separated by an empty line):
url: http://example.org/reverse-paragraphs.txt
filter:
- reverse:
separator: "\n\n"
WATCHING GITHUB RELEASES AND GITLAB TAGS
This is an example how to watch the GitHub “releases” page for a given project for the latest release
version, to be notified of new releases:
url: https://github.com/tulir/gomuks/releases
filter:
- xpath: '(//div[contains(@class,"d-flex flex-column flex-md-row my-5 flex-justify-center")]//h1//a)[1]'
- html2text: re
- strip
This is the corresponding version for Github tags:
url: https://github.com/thp/urlwatch/tags
filter:
- xpath:
path: //*[@class="Link--primary"]
maxitems: 1
- html2text:
and for Gitlab tags:
url: https://gitlab.com/chinstrap/gammastep/-/tags
filter:
- xpath: (//a[contains(@class,"item-title ref-name")])[1]
- html2text
Alternatively, jq can be used for filtering:
url: https://api.github.com/repos/voxpupuli/puppet-rundeck/tags
filter:
- jq: '.[0].name'
REMOVE OR REPLACE TEXT USING REGULAR EXPRESSIONS
Just like Python’s re.sub function, there’s the possibility to apply a regular expression and either
remove of replace the matched text. The following example applies the filter 3 times:
1. Just specifying a string as the value will replace the matches with the empty string.
2. Simple patterns can be replaced with another string using “pattern” as the expression and “repl” as
the replacement.
3. You can use groups (()) and back-reference them with \1 (etc..) to put groups into the replacement
string.
All features are described in Python’s re.sub <https://docs.python.org/3/library/re.html#re.sub>
documentation (the pattern and repl values are passed to this function as-is, with the value of repl
defaulting to the empty string).
url: https://example.com/regex-substitute.html
filter:
- re.sub: '\s*href="[^"]*"'
- re.sub:
pattern: '<h1>'
repl: 'HEADING 1: '
- re.sub:
pattern: '</([^>]*)>'
repl: '<END OF TAG \1>'
If you want to enable certain flags (e.g. re.MULTILINE) in the call, this is possible by inserting an
"inline flag" documented in flags in re.compile <https://docs.python.org/3/library/re.html#re.compile>,
here are some examples:
• re.MULTILINE: (?m) (Makes ^ match start-of-line and $ match end-of-line)
• re.DOTALL: (?s) (Makes . also match a newline)
• re.IGNORECASE: (?i) (Perform case-insensitive matching)
This allows you, for example, to remove all leading spaces (only space character and tab):
url: http://example.com/leading-spaces.txt
filter:
- re.sub: '(?m)^[ \t]*'
USING A SHELL SCRIPT AS A FILTER
While the built-in filters are powerful for processing markup such as HTML and XML, in some cases you
might already know how you would filter your content using a shell command or shell script. The shellpipe
filter allows you to start a shell and run custom commands to filter the content.
The text data to be filtered will be written to the standard input (stdin) of the shell process and the
filter output will be taken from the shell's standard output (stdout).
For example, if you want to use grep tool with the case insensitive matching option (-i) and printing
only the matching part of the line (-o), you can specify this as shellpipe filter:
url: https://example.net/shellpipe-grep.txt
filter:
- shellpipe: "grep -i -o 'price: <span>.*</span>'"
This feature also allows you to use sed(1), awk(1) and perl(1) one-liners for text processing (of course,
any text tool that works in a shell can be used). For example, this awk(1) one-liner prepends the line
number to each line:
url: https://example.net/shellpipe-awk-oneliner.txt
filter:
- shellpipe: awk '{ print FNR " " $0 }'
You can also use a multi-line command for a more sophisticated shell script (| in YAML denotes the start
of a text block):
url: https://example.org/shellpipe-multiline.txt
filter:
- shellpipe: |
FILENAME=`mktemp`
# Copy the input to a temporary file, then pipe through awk
tee $FILENAME | awk '/The numbers for (.*) are:/,/The next draw is on (.*)./'
# Analyze the input file in some other way
echo "Input lines: $(wc -l $FILENAME | awk '{ print $1 }')"
rm -f $FILENAME
Within the shellpipe script, two environment variables will be set for further customization (this can be
useful if you have an external shell script file that is used as filter for multiple jobs, but needs to
treat each job in a slightly different way):
┌───────────────────────┬───────────────────────────────────────┐
│Environment variable │ Contents │
├───────────────────────┼───────────────────────────────────────┤
│$URLWATCH_JOB_NAME │ The name of the job (name key in jobs │
│ │ YAML) │
├───────────────────────┼───────────────────────────────────────┤
│$URLWATCH_JOB_LOCATION │ The URL of the job, or command line │
│ │ (for shell jobs) │
└───────────────────────┴───────────────────────────────────────┘
CONVERTING TEXT IN IMAGES TO PLAINTEXT
The ocr filter uses the Tesseract OCR engine <https://github.com/tesseract-ocr> to convert text in images
to plain text. It requires two Python modules to be installed: pytesseract
<https://github.com/madmaze/pytesseract> and Pillow <https://python-pillow.org>. Any file formats
supported by Pillow (PIL) are supported.
This filter must be the first filter in a chain of filters, since it consumes binary data and outputs
text data.
url: https://example.net/ocr-test.png
filter:
- ocr:
timeout: 5
language: eng
- strip
The subfilters timeout and language are optional:
• timeout: Timeout for the recognition, in seconds (default: 10 seconds)
• language: Text language (e.g. fra or eng+fra, default: eng)
FILTERING JSON RESPONSE DATA USING JQ SELECTORS
The jq filter uses the Python bindings for jq <https://stedolan.github.io/jq/>, a lightweight JSON
processor. Use of this filter requires the optional jq Python module
<https://github.com/mwilliamson/jq.py> to be installed.
url: https://example.net/jobs.json
filter:
- jq:
query: '.[].title'
The subfilter query is optional:
• query: A valid jq filter string.
Supports aggregations, selections, and the built-in operators like length. For more information on the
operations permitted, see the jq Manual <https://stedolan.github.io/jq/manual/>.
FILES
$XDG_CONFIG_HOME/urlwatch/urls.yaml
SEE ALSO
urlwatch(1), urlwatch-intro(5), urlwatch-jobs(5)
COPYRIGHT
2023 Thomas Perl
May 03, 2023 URLWATCH-FILTERS(5)