lwptut(3pm) User Contributed Perl Documentation lwptut(3pm)
NAME
lwptut -- An LWP Tutorial
DESCRIPTION
LWP (short for "Library for WWW in Perl") is a very popular group of
Perl modules for accessing data on the Web. Like most Perl module-
distributions, each of LWP's component modules comes with documentation
that is a complete reference to its interface. However, there are so
many modules in LWP that it's hard to know where to start looking for
information on how to do even the simplest most common things.
Really introducing you to using LWP would require a whole book -- a book
that just happens to exist, called Perl & LWP. But this article should
give you a taste of how you can go about some common tasks with LWP.
Getting documents with LWP::Simple
If you just want to get what's at a particular URL, the simplest way to
do it is LWP::Simple's functions.
In a Perl program, you can call its get($url) function. It will try
getting that URL's content. If it works, then it'll return the content;
but if there's some error, it'll return undef.
my $url = 'http://www.npr.org/programs/fa/?todayDate=current';
# Just an example: the URL for the most recent /Fresh Air/ show
use LWP::Simple;
my $content = get $url;
die "Couldn't get $url" unless defined $content;
# Then go do things with $content, like this:
if($content =~ m/jazz/i) {
print "They're talking about jazz today on Fresh Air!\n";
}
else {
print "Fresh Air is apparently jazzless today.\n";
}
The handiest variant on "get" is "getprint", which is useful in Perl
one-liners. If it can get the page whose URL you provide, it sends it
to STDOUT; otherwise it complains to STDERR.
% perl -MLWP::Simple -e "getprint 'http://www.cpan.org/RECENT'"
That is the URL of a plain text file that lists new files in CPAN in the
past two weeks. You can easily make it part of a tidy little shell
command, like this one that mails you the list of new "Acme::" modules:
% perl -MLWP::Simple -e "getprint 'http://www.cpan.org/RECENT'" \
| grep "/by-module/Acme" | mail -s "New Acme modules! Joy!" $USER
There are other useful functions in LWP::Simple, including one function
for running a HEAD request on a URL (useful for checking links, or
getting the last-revised time of a URL), and two functions for
saving/mirroring a URL to a local file. See the LWP::Simple
documentation for the full details, or chapter 2 of Perl & LWP for more
examples.
The Basics of the LWP Class Model
LWP::Simple's functions are handy for simple cases, but its functions
don't support cookies or authorization, don't support setting header
lines in the HTTP request, generally don't support reading header lines
in the HTTP response (notably the full HTTP error message, in case of an
error). To get at all those features, you'll have to use the full LWP
class model.
While LWP consists of dozens of classes, the main two that you have to
understand are LWP::UserAgent and HTTP::Response. LWP::UserAgent is a
class for "virtual browsers" which you use for performing requests, and
HTTP::Response is a class for the responses (or error messages) that you
get back from those requests.
The basic idiom is "$response = $browser->get($url)", or more fully
illustrated:
# Early in your program:
use LWP 5.64; # Loads all important LWP classes, and makes
# sure your version is reasonably recent.
my $browser = LWP::UserAgent->new;
...
# Then later, whenever you need to make a get request:
my $url = 'http://www.npr.org/programs/fa/?todayDate=current';
my $response = $browser->get( $url );
die "Can't get $url -- ", $response->status_line
unless $response->is_success;
die "Hey, I was expecting HTML, not ", $response->content_type
unless $response->content_type eq 'text/html';
# or whatever content-type you're equipped to deal with
# Otherwise, process the content somehow:
if($response->decoded_content =~ m/jazz/i) {
print "They're talking about jazz today on Fresh Air!\n";
}
else {
print "Fresh Air is apparently jazzless today.\n";
}
There are two objects involved: $browser, which holds an object of class
LWP::UserAgent, and then the $response object, which is of class
HTTP::Response. You really need only one browser object per program; but
every time you make a request, you get back a new HTTP::Response object,
which will have some interesting attributes:
• A status code indicating success or failure (which you can test with
"$response->is_success").
• An HTTP status line that is hopefully informative if there's failure
(which you can see with "$response->status_line", returning
something like "404 Not Found").
• A MIME content-type like "text/html", "image/gif",
"application/xml", etc., which you can see with
"$response->content_type"
• The actual content of the response, in "$response->decoded_content".
If the response is HTML, that's where the HTML source will be; if
it's a GIF, then "$response->decoded_content" will be the binary GIF
data.
• And dozens of other convenient and more specific methods that are
documented in the docs for HTTP::Response, and its superclasses
HTTP::Message and HTTP::Headers.
Adding Other HTTP Request Headers
The most commonly used syntax for requests is "$response =
$browser->get($url)", but in truth, you can add extra HTTP header lines
to the request by adding a list of key-value pairs after the URL, like
so:
$response = $browser->get( $url, $key1, $value1, $key2, $value2, ... );
For example, here's how to send some commonly used headers, in case
you're dealing with a site that would otherwise reject your request:
my @ns_headers = (
'User-Agent' => 'Mozilla/4.76 [en] (Win98; U)',
'Accept' => 'image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*',
'Accept-Charset' => 'iso-8859-1,*,utf-8',
'Accept-Language' => 'en-US',
);
...
$response = $browser->get($url, @ns_headers);
If you weren't reusing that array, you could just go ahead and do this:
$response = $browser->get($url,
'User-Agent' => 'Mozilla/4.76 [en] (Win98; U)',
'Accept' => 'image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*',
'Accept-Charset' => 'iso-8859-1,*,utf-8',
'Accept-Language' => 'en-US',
);
If you were only ever changing the 'User-Agent' line, you could just
change the $browser object's default line from "libwww-perl/5.65" (or
the like) to whatever you like, using the LWP::UserAgent "agent" method:
$browser->agent('Mozilla/4.76 [en] (Win98; U)');
Enabling Cookies
A default LWP::UserAgent object acts like a browser with its cookies
support turned off. There are various ways of turning it on, by setting
its "cookie_jar" attribute. A "cookie jar" is an object representing a
little database of all the HTTP cookies that a browser knows about. It
can correspond to a file on disk or an in-memory object that starts out
empty, and whose collection of cookies will disappear once the program
is finished running.
To give a browser an in-memory empty cookie jar, you set its
"cookie_jar" attribute like so:
use HTTP::CookieJar::LWP;
$browser->cookie_jar( HTTP::CookieJar::LWP->new );
To save a cookie jar to disk, see "dump_cookies" in HTTP::CookieJar. To
load cookies from disk into a jar, see "load_cookies" in
HTTP::CookieJar.
Posting Form Data
Many HTML forms send data to their server using an HTTP POST request,
which you can send with this syntax:
$response = $browser->post( $url,
[
formkey1 => value1,
formkey2 => value2,
...
],
);
Or if you need to send HTTP headers:
$response = $browser->post( $url,
[
formkey1 => value1,
formkey2 => value2,
...
],
headerkey1 => value1,
headerkey2 => value2,
);
For example, the following program makes a search request to AltaVista
(by sending some form data via an HTTP POST request), and extracts from
the HTML the report of the number of matches:
use strict;
use warnings;
use LWP 5.64;
my $browser = LWP::UserAgent->new;
my $word = 'tarragon';
my $url = 'http://search.yahoo.com/yhs/search';
my $response = $browser->post( $url,
[ 'q' => $word, # the Altavista query string
'fr' => 'altavista', 'pg' => 'q', 'avkw' => 'tgz', 'kl' => 'XX',
]
);
die "$url error: ", $response->status_line
unless $response->is_success;
die "Weird content type at $url -- ", $response->content_type
unless $response->content_is_html;
if( $response->decoded_content =~ m{([0-9,]+)(?:<.*?>)? results for} ) {
# The substring will be like "996,000</strong> results for"
print "$word: $1\n";
}
else {
print "Couldn't find the match-string in the response\n";
}
Sending GET Form Data
Some HTML forms convey their form data not by sending the data in an
HTTP POST request, but by making a normal GET request with the data
stuck on the end of the URL. For example, if you went to "www.imdb.com"
and ran a search on "Blade Runner", the URL you'd see in your browser
window would be:
http://www.imdb.com/find?s=all&q=Blade+Runner
To run the same search with LWP, you'd use this idiom, which involves
the URI class:
use URI;
my $url = URI->new( 'http://www.imdb.com/find' );
# makes an object representing the URL
$url->query_form( # And here the form data pairs:
'q' => 'Blade Runner',
's' => 'all',
);
my $response = $browser->get($url);
See chapter 5 of Perl & LWP for a longer discussion of HTML forms and of
form data, and chapters 6 through 9 for a longer discussion of
extracting data from HTML.
Absolutizing URLs
The URI class that we just mentioned above provides all sorts of methods
for accessing and modifying parts of URLs (such as asking sort of URL it
is with "$url->scheme", and asking what host it refers to with
"$url->host", and so on, as described in the docs for the URI class.
However, the methods of most immediate interest are the "query_form"
method seen above, and now the "new_abs" method for taking a probably-
relative URL string (like "../foo.html") and getting back an absolute
URL (like "http://www.perl.com/stuff/foo.html"), as shown here:
use URI;
$abs = URI->new_abs($maybe_relative, $base);
For example, consider this program that matches URLs in the HTML list of
new modules in CPAN:
use strict;
use warnings;
use LWP;
my $browser = LWP::UserAgent->new;
my $url = 'http://www.cpan.org/RECENT.html';
my $response = $browser->get($url);
die "Can't get $url -- ", $response->status_line
unless $response->is_success;
my $html = $response->decoded_content;
while( $html =~ m/<A HREF=\"(.*?)\"/g ) {
print "$1\n";
}
When run, it emits output that starts out something like this:
MIRRORING.FROM
RECENT
RECENT.html
authors/00whois.html
authors/01mailrc.txt.gz
authors/id/A/AA/AASSAD/CHECKSUMS
...
However, if you actually want to have those be absolute URLs, you can
use the URI module's "new_abs" method, by changing the "while" loop to
this:
while( $html =~ m/<A HREF=\"(.*?)\"/g ) {
print URI->new_abs( $1, $response->base ) ,"\n";
}
(The "$response->base" method from HTTP::Message is for returning what
URL should be used for resolving relative URLs -- it's usually just the
same as the URL that you requested.)
That program then emits nicely absolute URLs:
http://www.cpan.org/MIRRORING.FROM
http://www.cpan.org/RECENT
http://www.cpan.org/RECENT.html
http://www.cpan.org/authors/00whois.html
http://www.cpan.org/authors/01mailrc.txt.gz
http://www.cpan.org/authors/id/A/AA/AASSAD/CHECKSUMS
...
See chapter 4 of Perl & LWP for a longer discussion of URI objects.
Of course, using a regexp to match hrefs is a bit simplistic, and for
more robust programs, you'll probably want to use an HTML-parsing module
like HTML::LinkExtor or HTML::TokeParser or even maybe
HTML::TreeBuilder.
Other Browser Attributes
LWP::UserAgent objects have many attributes for controlling how they
work. Here are a few notable ones:
• "$browser->timeout(15);"
This sets this browser object to give up on requests that don't
answer within 15 seconds.
• "$browser->protocols_allowed( [ 'http', 'gopher'] );"
This sets this browser object to not speak any protocols other than
HTTP and gopher. If it tries accessing any other kind of URL (like
an "ftp:" or "mailto:" or "news:" URL), then it won't actually try
connecting, but instead will immediately return an error code 500,
with a message like "Access to 'ftp' URIs has been disabled".
• "use LWP::ConnCache; $browser->conn_cache(LWP::ConnCache->new());"
This tells the browser object to try using the HTTP/1.1 "Keep-Alive"
feature, which speeds up requests by reusing the same socket
connection for multiple requests to the same server.
• "$browser->agent( 'SomeName/1.23 (more info here maybe)' )"
This changes how the browser object will identify itself in the
default "User-Agent" line is its HTTP requests. By default, it'll
send "libwww-perl/versionnumber", like "libwww-perl/5.65". You can
change that to something more descriptive like this:
$browser->agent( 'SomeName/3.14 (contact@robotplexus.int)' );
Or if need be, you can go in disguise, like this:
$browser->agent( 'Mozilla/4.0 (compatible; MSIE 5.12; Mac_PowerPC)' );
• "push @{ $ua->requests_redirectable }, 'POST';"
This tells this browser to obey redirection responses to POST
requests (like most modern interactive browsers), even though the
HTTP RFC says that should not normally be done.
For more options and information, see the full documentation for
LWP::UserAgent.
Writing Polite Robots
If you want to make sure that your LWP-based program respects robots.txt
files and doesn't make too many requests too fast, you can use the
LWP::RobotUA class instead of the LWP::UserAgent class.
LWP::RobotUA class is just like LWP::UserAgent, and you can use it like
so:
use LWP::RobotUA;
my $browser = LWP::RobotUA->new('YourSuperBot/1.34', 'you@yoursite.com');
# Your bot's name and your email address
my $response = $browser->get($url);
But HTTP::RobotUA adds these features:
• If the robots.txt on $url's server forbids you from accessing $url,
then the $browser object (assuming it's of class LWP::RobotUA) won't
actually request it, but instead will give you back (in $response) a
403 error with a message "Forbidden by robots.txt". That is, if you
have this line:
die "$url -- ", $response->status_line, "\nAborted"
unless $response->is_success;
then the program would die with an error message like this:
http://whatever.site.int/pith/x.html -- 403 Forbidden by robots.txt
Aborted at whateverprogram.pl line 1234
• If this $browser object sees that the last time it talked to $url's
server was too recently, then it will pause (via "sleep") to avoid
making too many requests too often. How long it will pause for, is
by default one minute -- but you can control it with the
"$browser->delay( minutes )" attribute.
For example, this code:
$browser->delay( 7/60 );
...means that this browser will pause when it needs to avoid talking
to any given server more than once every 7 seconds.
For more options and information, see the full documentation for
LWP::RobotUA.
Using Proxies
In some cases, you will want to (or will have to) use proxies for
accessing certain sites and/or using certain protocols. This is most
commonly the case when your LWP program is running (or could be running)
on a machine that is behind a firewall.
To make a browser object use proxies that are defined in the usual
environment variables ("HTTP_PROXY", etc.), just call the "env_proxy" on
a user-agent object before you go making any requests on it.
Specifically:
use LWP::UserAgent;
my $browser = LWP::UserAgent->new;
# And before you go making any requests:
$browser->env_proxy;
For more information on proxy parameters, see the LWP::UserAgent
documentation, specifically the "proxy", "env_proxy", and "no_proxy"
methods.
HTTP Authentication
Many web sites restrict access to documents by using "HTTP
Authentication". This isn't just any form of "enter your password"
restriction, but is a specific mechanism where the HTTP server sends the
browser an HTTP code that says "That document is part of a protected
'realm', and you can access it only if you re-request it and add some
special authorization headers to your request".
For example, the Unicode.org admins stop email-harvesting bots from
harvesting the contents of their mailing list archives, by protecting
them with HTTP Authentication, and then publicly stating the username
and password (at "http://www.unicode.org/mail-arch/") -- namely username
"unicode-ml" and password "unicode".
For example, consider this URL, which is part of the protected area of
the web site:
http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html
If you access that with a browser, you'll get a prompt like "Enter
username and password for 'Unicode-MailList-Archives' at server
'www.unicode.org'".
In LWP, if you just request that URL, like this:
use LWP;
my $browser = LWP::UserAgent->new;
my $url =
'http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html';
my $response = $browser->get($url);
die "Error: ", $response->header('WWW-Authenticate') || 'Error accessing',
# ('WWW-Authenticate' is the realm-name)
"\n ", $response->status_line, "\n at $url\n Aborting"
unless $response->is_success;
Then you'll get this error:
Error: Basic realm="Unicode-MailList-Archives"
401 Authorization Required
at http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html
Aborting at auth1.pl line 9. [or wherever]
...because the $browser doesn't know any the username and password for
that realm ("Unicode-MailList-Archives") at that host
("www.unicode.org"). The simplest way to let the browser know about
this is to use the "credentials" method to let it know about a username
and password that it can try using for that realm at that host. The
syntax is:
$browser->credentials(
'servername:portnumber',
'realm-name',
'username' => 'password'
);
In most cases, the port number is 80, the default TCP/IP port for HTTP;
and you usually call the "credentials" method before you make any
requests. For example:
$browser->credentials(
'reports.mybazouki.com:80',
'web_server_usage_reports',
'plinky' => 'banjo123'
);
So if we add the following to the program above, right after the
"$browser = LWP::UserAgent->new;" line...
$browser->credentials( # add this to our $browser 's "key ring"
'www.unicode.org:80',
'Unicode-MailList-Archives',
'unicode-ml' => 'unicode'
);
...then when we run it, the request succeeds, instead of causing the
"die" to be called.
Accessing HTTPS URLs
When you access an HTTPS URL, it'll work for you just like an HTTP URL
would -- if your LWP installation has HTTPS support (via an appropriate
Secure Sockets Layer library). For example:
use LWP;
my $url = 'https://www.paypal.com/'; # Yes, HTTPS!
my $browser = LWP::UserAgent->new;
my $response = $browser->get($url);
die "Error at $url\n ", $response->status_line, "\n Aborting"
unless $response->is_success;
print "Whee, it worked! I got that ",
$response->content_type, " document!\n";
If your LWP installation doesn't have HTTPS support set up, then the
response will be unsuccessful, and you'll get this error message:
Error at https://www.paypal.com/
501 Protocol scheme 'https' is not supported
Aborting at paypal.pl line 7. [or whatever program and line]
If your LWP installation does have HTTPS support installed, then the
response should be successful, and you should be able to consult
$response just like with any normal HTTP response.
For information about installing HTTPS support for your LWP
installation, see the helpful README.SSL file that comes in the libwww-
perl distribution.
Getting Large Documents
When you're requesting a large (or at least potentially large) document,
a problem with the normal way of using the request methods (like
"$response = $browser->get($url)") is that the response object in memory
will have to hold the whole document -- in memory. If the response is a
thirty megabyte file, this is likely to be quite an imposition on this
process's memory usage.
A notable alternative is to have LWP save the content to a file on disk,
instead of saving it up in memory. This is the syntax to use:
$response = $ua->get($url,
':content_file' => $filespec,
);
For example,
$response = $ua->get('http://search.cpan.org/',
':content_file' => '/tmp/sco.html'
);
When you use this ":content_file" option, the $response will have all
the normal header lines, but "$response->content" will be empty. Errors
writing to the content file (for example due to permission denied or the
filesystem being full) will be reported via the "Client-Aborted" or
"X-Died" response headers, and not the "is_success" method:
if ($response->header('Client-Aborted') eq 'die') {
# handle error ...
Note that this ":content_file" option isn't supported under older
versions of LWP, so you should consider adding "use LWP 5.66;" to check
the LWP version, if you think your program might run on systems with
older versions.
If you need to be compatible with older LWP versions, then use this
syntax, which does the same thing:
use HTTP::Request::Common;
$response = $ua->request( GET($url), $filespec );
SEE ALSO
Remember, this article is just the most rudimentary introduction to LWP
-- to learn more about LWP and LWP-related tasks, you really must read
from the following:
• LWP::Simple -- simple functions for getting/heading/mirroring URLs
• LWP -- overview of the libwww-perl modules
• LWP::UserAgent -- the class for objects that represent "virtual
browsers"
• HTTP::Response -- the class for objects that represent the response
to a LWP response, as in "$response = $browser->get(...)"
• HTTP::Message and HTTP::Headers -- classes that provide more methods
to HTTP::Response.
• URI -- class for objects that represent absolute or relative URLs
• URI::Escape -- functions for URL-escaping and URL-unescaping strings
(like turning "this & that" to and from "this%20%26%20that").
• HTML::Entities -- functions for HTML-escaping and HTML-unescaping
strings (like turning "C. & E. Brontë" to and from "C. & E.
Brontë")
• HTML::TokeParser and HTML::TreeBuilder -- classes for parsing HTML
• HTML::LinkExtor -- class for finding links in HTML documents
• The book Perl & LWP by Sean M. Burke. O'Reilly & Associates, 2002.
ISBN: 0-596-00178-9, <http://oreilly.com/catalog/perllwp/>. The
whole book is also available free online:
<http://lwp.interglacial.com>.
COPYRIGHT
Copyright 2002, Sean M. Burke. You can redistribute this document
and/or modify it, but only under the same terms as Perl itself.
AUTHOR
Sean M. Burke "sburke@cpan.org"
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