lwptut.3

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.\" ========================================================================
.\"
.IX Title "lwptut 3"
.TH lwptut 3 "2004-11-30" "perl v5.10.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
lwptut \-\- An LWP Tutorial
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\s-1LWP\s0 (short for \*(L"Library for \s-1WWW\s0 in Perl\*(R") is a very popular group of
Perl modules for accessing data on the Web. Like most Perl
module-distributions, each of \s-1LWP\s0's component modules comes with
documentation that is a complete reference to its interface. However,
there are so many modules in \s-1LWP\s0 that it's hard to know where to start
looking for information on how to do even the simplest most common
things.
.PP
Really introducing you to using \s-1LWP\s0 would require a whole book \*(-- a book
that just happens to exist, called \fIPerl & \s-1LWP\s0\fR. But this article
should give you a taste of how you can go about some common tasks with
\&\s-1LWP\s0.
.Sh "Getting documents with LWP::Simple"
.IX Subsection "Getting documents with LWP::Simple"
If you just want to get what's at a particular \s-1URL\s0, the simplest way
to do it is LWP::Simple's functions.
.PP
In a Perl program, you can call its \f(CW\*(C`get($url)\*(C'\fR function.  It will try
getting that \s-1URL\s0's content.  If it works, then it'll return the
content; but if there's some error, it'll return undef.
.PP
.Vb 2
\&  my $url = \*(Aqhttp://freshair.npr.org/dayFA.cfm?todayDate=current\*(Aq;
\&    # Just an example: the URL for the most recent /Fresh Air/ show
\&
\&  use LWP::Simple;
\&  my $content = get $url;
\&  die "Couldn\*(Aqt get $url" unless defined $content;
\&
\&  # Then go do things with $content, like this:
\&
\&  if($content =~ m/jazz/i) {
\&    print "They\*(Aqre talking about jazz today on Fresh Air!\en";
\&  }
\&  else {
\&    print "Fresh Air is apparently jazzless today.\en";
\&  }
.Ve
.PP
The handiest variant on \f(CW\*(C`get\*(C'\fR is \f(CW\*(C`getprint\*(C'\fR, which is useful in Perl
one-liners.  If it can get the page whose \s-1URL\s0 you provide, it sends it
to \s-1STDOUT\s0; otherwise it complains to \s-1STDERR\s0.
.PP
.Vb 1
\&  % perl \-MLWP::Simple \-e "getprint \*(Aqhttp://cpan.org/RECENT\*(Aq"
.Ve
.PP
That is the \s-1URL\s0 of a plaintext file that lists new files in \s-1CPAN\s0 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
\&\f(CW\*(C`Acme::\*(C'\fR modules:
.PP
.Vb 2
\&  % perl \-MLWP::Simple \-e "getprint \*(Aqhttp://cpan.org/RECENT\*(Aq"  \e
\&     | grep "/by\-module/Acme" | mail \-s "New Acme modules! Joy!" $USER
.Ve
.PP
There are other useful functions in LWP::Simple, including one function
for running a \s-1HEAD\s0 request on a \s-1URL\s0 (useful for checking links, or
getting the last-revised time of a \s-1URL\s0), and two functions for
saving/mirroring a \s-1URL\s0 to a local file. See the LWP::Simple
documentation for the full details, or chapter 2 of \fIPerl
& \s-1LWP\s0\fR for more examples.
.Sh "The Basics of the \s-1LWP\s0 Class Model"
.IX Subsection "The Basics of the LWP Class Model"
LWP::Simple's functions are handy for simple cases, but its functions
don't support cookies or authorization, don't support setting header
lines in the \s-1HTTP\s0 request, generally don't support reading header lines
in the \s-1HTTP\s0 response (notably the full \s-1HTTP\s0 error message, in case of an
error). To get at all those features, you'll have to use the full \s-1LWP\s0
class model.
.PP
While \s-1LWP\s0 consists of dozens of classes, the main two that you have to
understand are LWP::UserAgent and HTTP::Response. LWP::UserAgent
is a class for \*(L"virtual browsers\*(R" which you use for performing requests,
and HTTP::Response is a class for the responses (or error messages)
that you get back from those requests.
.PP
The basic idiom is \f(CW\*(C`$response = $browser\->get($url)\*(C'\fR, or more fully
illustrated:
.PP
.Vb 1
\&  # 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 = \*(Aqhttp://freshair.npr.org/dayFA.cfm?todayDate=current\*(Aq;
\&  
\&  my $response = $browser\->get( $url );
\&  die "Can\*(Aqt get $url \-\- ", $response\->status_line
\&   unless $response\->is_success;
\&
\&  die "Hey, I was expecting HTML, not ", $response\->content_type
\&   unless $response\->content_type eq \*(Aqtext/html\*(Aq;
\&     # or whatever content\-type you\*(Aqre equipped to deal with
\&
\&  # Otherwise, process the content somehow:
\&  
\&  if($response\->decoded_content =~ m/jazz/i) {
\&    print "They\*(Aqre talking about jazz today on Fresh Air!\en";
\&  }
\&  else {
\&    print "Fresh Air is apparently jazzless today.\en";
\&  }
.Ve
.PP
There are two objects involved: \f(CW$browser\fR, which holds an object of
class LWP::UserAgent, and then the \f(CW$response\fR 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:
.IP "\(bu" 4
A status code indicating
success or failure
(which you can test with \f(CW\*(C`$response\->is_success\*(C'\fR).
.IP "\(bu" 4
An \s-1HTTP\s0 status
line that is hopefully informative if there's failure (which you can
see with \f(CW\*(C`$response\->status_line\*(C'\fR,
returning something like \*(L"404 Not Found\*(R").
.IP "\(bu" 4
A \s-1MIME\s0 content-type like \*(L"text/html\*(R", \*(L"image/gif\*(R",
\&\*(L"application/xml\*(R", etc., which you can see with 
\&\f(CW\*(C`$response\->content_type\*(C'\fR
.IP "\(bu" 4
The actual content of the response, in \f(CW\*(C`$response\->decoded_content\*(C'\fR.
If the response is \s-1HTML\s0, that's where the \s-1HTML\s0 source will be; if
it's a \s-1GIF\s0, then \f(CW\*(C`$response\->decoded_content\*(C'\fR will be the binary
\&\s-1GIF\s0 data.
.IP "\(bu" 4
And dozens of other convenient and more specific methods that are
documented in the docs for HTML::Response, and its superclasses
HTML::Message and HTML::Headers.
.Sh "Adding Other \s-1HTTP\s0 Request Headers"
.IX Subsection "Adding Other HTTP Request Headers"
The most commonly used syntax for requests is \f(CW\*(C`$response =
$browser\->get($url)\*(C'\fR, but in truth, you can add extra \s-1HTTP\s0 header
lines to the request by adding a list of key-value pairs after the \s-1URL\s0,
like so:
.PP
.Vb 1
\&  $response = $browser\->get( $url, $key1, $value1, $key2, $value2, ... );
.Ve