lwpcook - libwww-perl cookbook

DESCRIPTION

This document contain some examples that show typical usage of the libwww-perl library. You should consult the documentation for the individual modules for more detail.

All examples should be runnable programs. You can, in most cases, test the code sections by piping the program text directly to perl.

GET

It is very easy to use this library to just fetch documents from the net. The LWP::Simple module provides the get() function that return the document specified by its URL argument:

  use LWP::Simple;
  $doc = get 'http://www.sn.no/libwww-perl/';

or, as a perl one-liner using the getprint() function:

  perl -MLWP::Simple -e 'getprint "http://www.sn.no/libwww-perl/";'

or, how about fetching the latest perl by running this:

  perl -MLWP::Simple -e '
    getstore "ftp://ftp.sunet.se/pub/lang/perl/CPAN/src/latest.tar.gz";,
             "perl.tar.gz"'

You will probably first want to find a CPAN site closer to you by running something like the following command:

  perl -MLWP::Simple -e 'getprint "http://www.perl.com/perl/CPAN/CPAN.html";'

Enough of this simple stuff! The LWP object oriented interface gives you more control over the request sent to the server. Using this interface you have full control over headers sent and how you want to handle the response returned.

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $ua->agent("$0/0.1 " . $ua->agent);
  # $ua->agent("Mozilla/5.0") # pretend you are some very new Netscape browser

  $req = new HTTP::Request 'GET' => 'http://www.sn.no/libwww-perl';
  $req->header('Accept' => 'text/html');

  # send request
  $res = $ua->request($req);

  # check the outcome
  if ($res->is_success) {
     print $res->content;
  } else {
     print "Error: " . $res->code . " " . $res->message;
  }

The lwp-request program (alias GET) that is distributed with the library can also be used to fetch documents from WWW servers.

HEAD

If you just want to check if a document is present (i.e. the URL is valid) try to run code that looks like this:

  use LWP::Simple;

  if (head($url)) {
     # ok document exists
  }

The head() function really returns a list of meta-information about the document. The first three values of the list returned are the document type, the size of the document, and the age of the document.

More control over the request or access to all header values returned require that you use the object oriented interface described for GET above. Just s/GET/HEAD/g.

POST

There is no simple interface for posting data to a WWW server. You must use the object oriented interface for this. The most common POST operation is to access a WWW form application:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = new HTTP::Request 'POST','http://www.perl.com/cgi-bin/BugGlimpse';
  $req->content_type('application/x-www-form-urlencoded');
  $req->content('match=www&errors=0');

  my $res = $ua->request($req);
  print $res->as_string;

If your application has the key/value pairs to be posted in an associative array, then we can exploit the URI::URL module to create the content for the POST request message (it handles all the escaping issues):

  %form = ( search => 'www', errors => 0 );

  use URI::URL;
  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = new HTTP::Request 'POST', 'http://www.perl.com/cgi-bin/BugGlimpse';
  $req->content_type('application/x-www-form-urlencoded');

  my $curl = url("http:";);      # create an empty HTTP URL object
  $curl->query_form(%form);
  $req->content($curl->equery); # %form content as escaped query string

  print $ua->request($req)->as_string;

The lwp-request program (alias POST) that is distributed with the library can also be used for posting data.

PROXIES

Some sites use proxies to go through fire wall machines, or just as cache in order to improve performance. Proxies can also be used for accessing resources through protocols not supported directly (or supported badly :-) by the libwww-perl library.

You should initialize your proxy setting before you start sending requests:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $ua->env_proxy; # initialize from environment variables
  # or
  $ua->proxy(ftp  => 'http://proxy.myorg.com');
  $ua->proxy(wais => 'http://proxy.myorg.com');
  $ua->no_proxy(qw(no se fi));

  my $req = new HTTP::Request 'wais://xxx.com/';
  print $ua->request($req)->as_string;

The LWP::Simple interface will call env_proxy() for you automatically. Applications that use the $ua->env_proxy() method will normally not use the $ua->proxy() and $ua->no_proxy() methods.

ACCESS TO PROTECTED DOCUMENTS

Documents protected by basic authorization can easily be accessed like this:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $req = new HTTP::Request GET => 'http://www.sn.no/secret/';
  $req->authorization_basic('aas', 'mypassword');
  print $ua->request($req)->as_string;

The other alternative is to provide a subclass of LWP::UserAgent that overrides the get_basic_credentials() method. Study the lwp-request program for an example of this.

MIRRORING

If you want to mirror documents from a WWW server, then try to run code similar to this at regular intervals:

  use LWP::Simple;

  %mirrors = (
     'http://www.sn.no/'             => 'sn.html',
     'http://www.perl.com/'          => 'perl.html',
     'http://www.sn.no/libwww-perl/' => 'lwp.html',
     'gopher://gopher.sn.no/'        => 'gopher.html',
  );

  while (($url, $localfile) = each(%mirrors)) {
     mirror($url, $localfile);
  }

Or, as a perl one-liner:

  perl -MLWP::Simple -e 'mirror("http://www.perl.com/";, "perl.html")';

The document will not be transfered unless it has been updated.

LARGE DOCUMENTS

If the document you want to fetch is too large to be kept in memory, then you have two alternatives. You can instruct the library to write the document content to a file (second $ua->request() argument is a file name):

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = new HTTP::Request 'GET',
                'http://www.sn.no/~aas/perl/www/libwww-perl-5.00.tar.gz';
  $res = $ua->request($req, "libwww-perl.tar.gz");
  if ($res->is_success) {
     print "ok\n";
  }

Or you can process the document as it arrives (second $ua->request() argument is a code reference):

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $URL = 'ftp://ftp.unit.no/pub/rfc/rfc-index.txt';

  my $expected_length;
  my $bytes_received = 0;
  $ua->request(HTTP::Request->;new('GET', $URL),
               sub {
                   my($chunk, $res) = @_;
                   $bytes_received += length($chunk);
                   unless (defined $expected_length) {
                      $expected_length = $res->content_length || 0;
                   }
                   if ($expected_length) {
                        printf STDERR "%d%% - ",
                                  100 * $bytes_received / $expected_length;
                   }
                   print STDERR "$bytes_received bytes received\n";

                   # XXX Should really do something with the chunk itself
                   # print $chunk;
               });

HTML FORMATTING

It is easy to convert HTML code to ``readable'' text.

  use LWP::Simple;
  use HTML::Parse;
  print parse_html(get 'http://www.sn.no/libwww-perl/')->format;

PARSE URLS

To access individual elements of a URL, try this:

  use URI::URL;
  $host = url("http://www.sn.no/";)->host;

  use URI::URL;
  $u = url("ftp://ftp.sn.no/test/aas;type=i");
  print "Protocol scheme is ", $u->scheme, "\n";
  print "Host is ", $u->host, " at port ", $u->port, "\n";

or even

  use URI::URL;
  my($host,$port) = (url("ftp://ftp.sn.no/test/aas;type=i")->crack)[3,4];

EXPAND RELATIVE URLS

This code reads URLs and print expanded version.

  use URI::URL;
  $BASE = "http://www.sn.no/some/place?query";;
  while (<>) {
     print url($_, $BASE)->abs->as_string, "\n";
  }

We can expand URLs in an HTML document by using the parser to build a tree that we then traverse:

  %link_elements =
  (
   'a'    => 'href',
   'img'  => 'src',
   'form' => 'action',
   'link' => 'href',
  );

  use HTML::Parse;
  use URI::URL;

  $BASE = "http://somewhere/root/";;
  $h = parse_htmlfile("xxx.html");
  $h->traverse(\&expand_urls, 1);

  print $h->as_HTML;

  sub expand_urls
  {
     my($e, $start) = @_;
     return 1 unless $start;
     my $attr = $link_elements{$e->tag};
     return 1 unless defined $attr;
     my $url = $e->attr($attr);
     return 1 unless defined $url;
     $e->attr($attr, url($url, $BASE)->abs->as_string);
  }

BASE URL

If you want to resolve relative links in a page you will have to determine which base URL to use. The HTTP::Response objects now has a base() method.

  $BASE = $res->base;