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<html><head><title>CGI Overview (Perl in a Nutshell, 2nd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Stephen Spainhour" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596002416L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Perl in a Nutshell, 2nd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Java and XSLT" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part4.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228" /><td align="right" valign="top" width="228"><a href="ch09_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><h1 class="chapter">Chapter 9. CGI Overview</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4> <p> <a href="#perlnut2-CHP-9-SECT-1">A Typical CGI Interaction</a><br /><a href="ch09_02.htm">URL Encoding</a><br /><a href="ch09_03.htm">Extra Path Information</a><br /><a href="ch09_04.htm">CGI Environment Variables</a><br /></p></div><p>Perl is the most commonly used language for CGI programming on theWorld Wide Web<a name="INDEX-1607" /></a><a name="INDEX-1608" /></a>.The Common Gateway Interface (CGI) isan essential tool for creating and managing comprehensive web sites.With CGI, you can write scripts that create interactive, user-drivenapplications.</p><p>CGI allows the web server to communicate with other programs that arerunning on the same machine. For example, with CGI, the web servercan invoke an external program, while passing user-specific data tothe program (such as what host the user is connecting from, or inputthe user has supplied through an HTML form). The program thenprocesses that data, and the server passes theprogram's response back to the web browser.</p><p>Rather than limiting the Web to documents written ahead of time, CGIenables web pages to be created on the fly, based upon the input ofusers. You can use CGI scripts to create a wide range ofapplications, from surveys to search tools, from Internet servicegateways to quizzes and games. You can increment the number of userswho access a document or let them sign an electronic guestbook. Youcan provide users with all types of information, collect theircomments, and respond to them.</p><p>For Perl programmers, there are two approaches you can take to CGI.They are:</p><ul><li><p>Programs that handle all CGI interaction directly, without the use ofa module such as CGI.pm. While often frowned upon by Perl programmersbecause it's more likely to introduce bugs,bypassing the modules has the advantage of avoiding the overhead ofCGI.pm for quick, dirty tasks. This chapter explains the concepts ofCGI necessary if you intend to write CGI programs from scratch.</p></li><li><p><a name="INDEX-1609" /></a>CGI.pm is aPerl module designed to facilitate CGI programming. For non-trivialCGI programs, especially ones that need to maintain state overmultiple transactions, CGI.pm is indispensable, and is included inthe standard Perl distribution as of Perl 5.004. Rather than discussit in <a href="ch08_01.htm">Chapter 8, "Standard Modules"</a>, with the rest of the standardlibraries, however, its complexity and importance made it a candidatefor its own chapter, <a href="ch10_01.htm">Chapter 10, "The CGI.pm Module"</a>.</p></li></ul><p><a name="INDEX-1610" /></a>One performance hit for CGI programsis that the Perl interpreter needs to be started up each and everytime a CGI script is called. For improving performance on Apachesystems, the <em class="emphasis">mod_perl</em> Apache module embeds thePerl interpreter directly into the server, avoiding the startupoverhead. <a href="ch11_01.htm">Chapter 11, "Web Server Programmingwith mod_perl"</a> talks about installing andusing <em class="emphasis">mod_perl</em>.</p><div class="sect1"><a name="perlnut2-CHP-9-SECT-1" /></a><h2 class="sect1">9.1. A Typical CGI Interaction</h2><p><a name="INDEX-1611" /></a>For an example of a CGI application,suppose you create a guestbook for your website. The guestbook pageasks users to submit their first name and last name using a fill-inform composed of two input text fields. <a href="ch09_01.htm#perlnut2-CHP-9-FIG-1">Figure 9-1</a>shows the form you might see in your browser window.</p><a name="perlnut2-CHP-9-FIG-1" /></a><div class="figure"><img src="figs/pns2_0901.gif" alt="Figure 9-1" width="406" height="159" /></div><h4 class="objtitle">Figure 9-1. HTML form</h4><p>The HTML that produces this form might read as follows: </p><blockquote><pre class="code"><HTML><HEAD><TITLE>Guestbook</TITLE></HEAD><BODY><H1>Fill in my guestbook!</H1><FORM METHOD="GET" ACTION="/cgi-bin/guestbook.pl"><PRE>First Name: <INPUT TYPE="TEXT" NAME="firstname">Last Name: <INPUT TYPE="TEXT" NAME="lastname"><INPUT TYPE="SUBMIT"> <INPUT TYPE="RESET"></FORM></pre></blockquote><p>The form is written using special"form" tags, as follows:</p><ul><li><p><a name="INDEX-1612" /></a><a name="INDEX-1613" /></a>The<tt class="literal"><form></tt> tag defines the method used for theform (either GET or POST) and the action to take when the form issubmitted—that is, the URL of the CGI program to pass theparameters to.</p></li><li><p>The <tt class="literal"><input></tt> tag can be used in manydifferent ways. In its first two invocations, it creates a text inputfield and defines the variable name to associate with thefield's contents when the form is submitted. Thefirst field is given the variable name <tt class="literal">firstname</tt>and the second field is given the name <tt class="literal">lastname</tt>.</p></li><li><p>In its last two invocations, the <tt class="literal"><input></tt> tagcreates a "submit" button and a"reset" button.</p></li><li><p>The <tt class="literal"></form></tt> tag indicates the end of theform.</p></li></ul><p>When the user presses the "submit"button, data entered into the <tt class="literal"><input></tt> textfields is passed to the CGI program specified by the<tt class="literal">action</tt> attribute of the<tt class="literal"><form></tt> tag (in this case, the<em class="emphasis">/cgi-bin/guestbook.pl</em> program).</p><a name="perlnut2-CHP-9-SECT-1.1" /></a><div class="sect2"><h3 class="sect2">9.1.1. Transferring the Form Data</h3><p><a name="INDEX-1614" /></a>Parameters to a CGI program aretransferred either in the URL or in the body text of the request. Themethod used to pass parameters is determined by the<tt class="literal">method</tt> attribute to the<tt class="literal"><form></tt> tag. The GET method says to transferthe data within the URL itself; for example, under the GET method,the browser might initiate the HTTP transaction as follows:</p><blockquote><pre class="code">GET /cgi-bin/guestbook.pl?firstname=Joe&lastname=Schmoe HTTP/1.0</pre></blockquote><p><a name="INDEX-1615" /></a>The POSTmethod says to use the body portion of the HTTP request to passparameters. The same transaction with the POST method would read asfollows:</p><blockquote><pre class="code">POST /cgi-bin/guestbook.pl HTTP/1.0 <em class="replaceable"><tt>... [More headers here]</tt></em>firstname=Joe&lastname=Schmoe</pre></blockquote><p>In both of these examples, you should recognize the<tt class="literal">firstname</tt> and <tt class="literal">lastname</tt> variablenames that were defined in the HTML form, coupled with the valuesentered by the user. An ampersand (&) is used to separate thevariable=value pairs.</p><p>The server now passes the variable=value pairs to the CGI program. Itdoes this either through Unix environment variables or in standardinput (STDIN). If the CGI program is called with the GET method, thenparameters are expected to be embedded into the URL of the request,and the server transfers them to the program by assigning them to<a name="INDEX-1616" /></a>theQUERY_STRING environment variable.The CGI program can then retrieve the parameters from QUERY_STRING asit would read any environment variable (for example, from the<tt class="literal">%ENV</tt> hash in Perl). If the CGI program is calledwith the POST method, parameters are expected to be embedded into thebody of the request, and the server passes the body text to theprogram as standard input (STDIN).</p><p>Other environment variables defined by the server for CGI store suchinformation as the format and length of the input, the remote host,the user, and various client information. They also store the servername, the communication protocol, and the name of the softwarerunning the server. (We provide a list of the most common CGIenvironment variables later in this chapter.)</p><p>The CGI program needs to retrieve the information as appropriate andthen process it. The sky's the limit on what the CGIprogram actually does with the information it retrieves. It mightreturn an anagram of the user's name, or tell herhow many times her name uses the letter"t," or it might just compile thename into a list that the programmer regularly sells totelemarketers. Only the programmer knows for sure.</p></div><a name="perlnut2-CHP-9-SECT-1.2" /></a><div class="sect2"><h3 class="sect2">9.1.2. Creating Virtual Documents</h3><p><a name="INDEX-1617" /></a><a name="INDEX-1618" /></a>Regardless of what the CGI program doeswith its input, it's responsible for giving thebrowser something to display when it's done. It musteither create a new document to be served to the browser or point toan existing document. On Unix, programs send their output to standardoutput (STDOUT) as a data stream that consists of two parts. Thefirst part is either a full or partial HTTP header that (at minimum)describes the format of the returned data (e.g., HTML, ASCII text,GIF, etc.). A blank line signifies the end of the header section. Thesecond part is the body of the output, which contains the dataconforming to the format type reflected in the header. For example:</p><blockquote><pre class="code">Content-type: text/html<HTML><HEAD><TITLE>Thanks!</TITLE></HEAD><BODY><H1>Thanks for signing my guest book!</H1> ...</BODY></HTML></pre></blockquote><p><a name="INDEX-1619" /></a>In this case, the only header linegenerated is Content-type, which gives the media format of the outputas HTML (<tt class="literal">text/html</tt>). This line is essential forevery CGI program, since it tells the browser what kind of format toexpect. The blank line separates the header from the body text(which, in this case, is in HTML format as advertised).</p><p>The server transfers the results of the CGI program back to thebrowser. The body text is not modified or interpreted by the serverin any way, but the server generally supplies additional headers withinformation such as the date, the name and version of the server,etc.</p><p>CGI programs can also supply a complete HTTP header itself, in whichcase the server does not add any additional headers but insteadtransfers the response verbatim as returned by the CGI program. Theserver needs to be configured to allow this behavior; see your serverdocumentation on NPH (no-parsed headers) scripts for moreinformation.</p><p>Here is the sample output of a program generating an HTML virtualdocument, with a complete HTTP header:</p><blockquote><pre class="code">HTTP/1.0 200 OKDate: Thursday, 28-June-96 11:12:21 GMTServer: NCSA/1.4.2Content-type: text/htmlContent-length: 2041<HTML><HEAD><TITLE>Thanks!</TITLE></HEAD><BODY><H1>Thanks for signing my guestbook!</H1> ...</BODY></HTML></pre></blockquote><p>The header contains the communication protocol, the date and time ofthe response, and the server name and version. (<tt class="literal">200OK</tt> is a status code generated by the HTTP protocol tocommunicate the status of a request, in this case successful.) Mostimportantly, the header also contains the content type and the numberof characters (equivalent to the number of bytes) of the encloseddata.</p><p>The result is that after users click the"submit" button, they see themessage contained in the HTML section of the response thanking themfor signing the guestbook<a name="INDEX-1620" /></a>. </p></div></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part4.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td align="right" valign="top" width="228"><a href="ch09_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td align="left" valign="top" width="228">IV. CGI</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td align="right" valign="top" width="228">9.2. URL Encoding</td></tr></table></div><hr width="684" align="left" /><img src="../gifs/navbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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