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7 --><title>Apache Tutorial: Dynamic Content with CGI - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Apache Tutorial: Dynamic Content with CGI</h1></div><div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#intro">Introduction</a></li><li><img alt="" src="../images/down.gif" /> <a href="#configuring">Configuring Apache to permit CGI</a></li><li><img alt="" src="../images/down.gif" /> <a href="#writing">Writing a CGI program</a></li><li><img alt="" src="../images/down.gif" /> <a href="#troubleshoot">But it's still not working!</a></li><li><img alt="" src="../images/down.gif" /> <a href="#behindscenes">What's going on behind the scenes?</a></li><li><img alt="" src="../images/down.gif" /> <a href="#libraries">CGI modules/libraries</a></li><li><img alt="" src="../images/down.gif" /> <a href="#moreinfo">For more information</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="intro" id="intro">Introduction</a></h2>
10 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code></li><li><code class="module"><a href="../mod/mod_cgi.html">mod_cgi</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code></li><li><code class="directive"><a href="../mod/core.html#options">Options</a></code></li><li><code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code></li></ul></td></tr></table>
12 <p>The CGI (Common Gateway Interface) defines a way for a web
13 server to interact with external content-generating programs,
14 which are often referred to as CGI programs or CGI scripts. It
15 is the simplest, and most common, way to put dynamic content on
16 your web site. This document will be an introduction to setting
17 up CGI on your Apache web server, and getting started writing
19 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="configuring" id="configuring">Configuring Apache to permit CGI</a></h2>
22 <p>In order to get your CGI programs to work properly, you'll
23 need to have Apache configured to permit CGI execution. There
24 are several ways to do this.</p>
26 <h3><a name="scriptalias" id="scriptalias">ScriptAlias</a></h3>
30 <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
32 directive tells Apache that a particular directory is set
33 aside for CGI programs. Apache will assume that every file in
34 this directory is a CGI program, and will attempt to execute
35 it, when that particular resource is requested by a
39 <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
41 directive looks like:</p>
43 <div class="example"><p><code>ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/</code></p></div>
45 <p>The example shown is from your default
46 <code>httpd.conf</code>
48 configuration file, if you installed Apache in the default
49 location. The <code>ScriptAlias</code> directive is much like the
50 <code>Alias</code> directive, which defines a URL prefix that
51 is to mapped to a particular directory. <code>Alias</code>
52 and <code>ScriptAlias</code> are usually used for directories
53 that are outside of the <code>DocumentRoot</code> directory.
54 The difference between <code>Alias</code> and
55 <code>ScriptAlias</code> is that <code>ScriptAlias</code>
56 has the added meaning that everything under that URL prefix
57 will be considered a CGI program. So, the example above tells
58 Apache that any request for a resource beginning with
59 <code>/cgi-bin/</code> should be served from the directory
60 <code>/usr/local/apache/cgi-bin/</code>, and should be treated
63 <p>For example, if the URL
64 <code>http://www.example.com/cgi-bin/test.pl</code>
65 is requested, Apache will attempt to execute the file
66 <code>/usr/local/apache/cgi-bin/test.pl</code>
67 and return the output. Of course, the file will have to
68 exist, and be executable, and return output in a particular
69 way, or Apache will return an error message.</p>
72 <h3><a name="nonscriptalias" id="nonscriptalias">CGI outside of ScriptAlias directories</a></h3>
75 <p>CGI programs are often restricted to
76 <code>ScriptAlias</code>'ed directories for security reasons.
78 administrators can tightly control who is allowed to use CGI
79 programs. However, if the proper security precautions are
80 taken, there is no reason why CGI programs cannot be run from
81 arbitrary directories. For example, you may wish to let users
82 have web content in their home directories with the
83 <code>UserDir</code> directive. If they want to have their own
84 CGI programs, but don't have access to the main
85 <code>cgi-bin</code> directory, they will need to be able to
86 run CGI programs elsewhere.</p>
89 <h3><a name="options" id="options">Explicitly using Options to permit CGI execution</a></h3>
92 <p>You could explicitly use the <code>Options</code>
93 directive, inside your main server configuration file, to
94 specify that CGI execution was permitted in a particular
97 <div class="example"><p><code><Directory /usr/local/apache/htdocs/somedir> <br />
98 Options +ExecCGI<br />
99 </Directory></code></p></div>
101 <p>The above directive tells Apache to permit the execution
102 of CGI files. You will also need to tell the server what
103 files are CGI files. The following
104 <code>AddHandler</code>
106 directive tells the server to treat all files with the
107 <code>cgi</code> or <code>pl</code> extension as CGI programs:</p>
109 <div class="example"><p><code>AddHandler cgi-script cgi pl</code></p></div>
112 <h3><a name="htaccess" id="htaccess">.htaccess files</a></h3>
115 <p>A <code>.htaccess</code> file is a way to set configuration
116 directives on a per-directory basis. When Apache serves a
117 resource, it looks in the directory from which it is serving
118 a file for a file called <code>.htaccess</code>, and, if it
119 finds it, it will apply directives found therein.
121 <code>.htaccess</code> files can be permitted with the
122 <code>AllowOverride</code> directive, which specifies what
123 types of directives can
124 appear in these files, or if they are not allowed at all. To
125 permit the directive we will need for this purpose, the
126 following configuration will be needed in your main server
129 <div class="example"><p><code>AllowOverride Options</code></p></div>
131 <p>In the <code>.htaccess</code> file, you'll need the
132 following directive:</p>
134 <div class="example"><p><code>Options +ExecCGI</code></p></div>
136 <p>which tells Apache that execution of CGI programs is
137 permitted in this directory.</p>
139 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="writing" id="writing">Writing a CGI program</a></h2>
142 <p>There are two main differences between ``regular''
143 programming, and CGI programming.</p>
145 <p>First, all output from your CGI program must be preceded by
146 a MIME-type header. This is HTTP header that tells the client
147 what sort of content it is receiving. Most of the time, this
150 <div class="example"><p><code>Content-type: text/html</code></p></div>
152 <p>Secondly, your output needs to be in HTML, or some other
153 format that a browser will be able to display. Most of the
154 time, this will be HTML, but occasionally you might write a CGI
155 program that outputs a gif image, or other non-HTML
158 <p>Apart from those two things, writing a CGI program will look
159 a lot like any other program that you might write.</p>
161 <h3><a name="firstcgi" id="firstcgi">Your first CGI program</a></h3>
164 <p>The following is an example CGI program that prints one
165 line to your browser. Type in the following, save it to a
166 file called <code>first.pl</code>, and put it in your
167 <code>cgi-bin</code> directory.</p>
169 <div class="example"><p><code>#!/usr/bin/perl<br />
170 print "Content-type: text/html\n\n";<br />
171 print "Hello, World.";
174 <p>Even if you are not familiar with Perl, you should be able
175 to see what is happening here. The first line tells Apache
176 (or whatever shell you happen to be running under) that this
177 program can be executed by feeding the file to the
178 interpreter found at the location <code>/usr/bin/perl</code>.
179 The second line prints the content-type declaration we
180 talked about, followed by two carriage-return newline pairs.
181 This puts a blank line after the header, to indicate the end
182 of the HTTP headers, and the beginning of the body. The third
183 line prints the string ``Hello, World.'' And that's the end
186 <p>If you open your favorite browser and tell it to get the
189 <div class="example"><p><code>http://www.example.com/cgi-bin/first.pl</code></p></div>
191 <p>or wherever you put your file, you will see the one line
192 <code>Hello, World.</code>
194 appear in your browser window. It's not very exciting, but
195 once you get that working, you'll have a good chance of
196 getting just about anything working.</p>
198 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="troubleshoot" id="troubleshoot">But it's still not working!</a></h2>
201 <p>There are four basic things that you may see in your browser
202 when you try to access your CGI program from the web:</p>
205 <dt>The output of your CGI program</dt>
207 <dd>Great! That means everything worked fine.
210 <dt>The source code of your CGI program or a "POST Method Not
211 Allowed" message</dt>
213 <dd>That means that you have not properly configured Apache
214 to process your CGI program. Reread the section on
215 <a href="#configuringapachetopermitcgi">configuring
218 and try to find what you missed.
221 <dt>A message starting with "Forbidden"</dt>
223 <dd>That means that there is a permissions problem. Check the
225 <a href="#errorlogs">Apache error log</a>
227 and the section below on
228 <a href="#permissions">file permissions</a>.
233 <dt>A message saying "Internal Server Error"</dt>
236 <a href="#errorlogs">Apache error log</a>, you will probably
237 find that it says "Premature end of
238 script headers", possibly along with an error message
239 generated by your CGI program. In this case, you will want to
240 check each of the below sections to see what might be
241 preventing your CGI program from emitting the proper HTTP
245 <h3><a name="permissions" id="permissions">File permissions</a></h3>
248 <p>Remember that the server does not run as you. That is,
249 when the server starts up, it is running with the permissions
250 of an unprivileged user - usually ``nobody'', or ``www'' -
251 and so it will need extra permissions to execute files that
252 are owned by you. Usually, the way to give a file sufficient
253 permissions to be executed by ``nobody'' is to give everyone
254 execute permission on the file:</p>
256 <div class="example"><p><code>chmod a+x first.pl</code></p></div>
258 <p>Also, if your program reads from, or writes to, any other
259 files, those files will need to have the correct permissions
262 <p>The exception to this is when the server is configured to
263 use <a href="../suexec.html">suexec</a>.
265 This program allows CGI programs to be run under different
266 user permissions, depending on which virtual host or user
267 home directory they are located in. Suexec has very strict
268 permission checking, and any failure in that checking will
269 result in your CGI programs failing with an "Internal Server
270 Error". In this case, you will need to check the suexec log
271 file to see what specific security check is failing.</p>
274 <h3><a name="pathinformation" id="pathinformation">Path information</a></h3>
277 <p>When you run a program from your command line, you have
278 certain information that is passed to the shell without you
279 thinking about it. For example, you have a path, which tells
280 the shell where it can look for files that you reference.</p>
282 <p>When a program runs through the web server as a CGI
283 program, it does not have that path. Any programs that you
284 invoke in your CGI program (like 'sendmail', for example)
285 will need to be specified by a full path, so that the shell
286 can find them when it attempts to execute your CGI
289 <p>A common manifestation of this is the path to the script
290 interpreter (often <code>perl</code>) indicated in the first
291 line of your CGI program, which will look something like:</p>
293 <div class="example"><p><code>#!/usr/bin/perl</code></p></div>
295 <p>Make sure that this is in fact the path to the
299 <h3><a name="syntaxerrors" id="syntaxerrors">Syntax errors</a></h3>
302 <p>Most of the time when a CGI program fails, it's because of
303 a problem with the program itself. This is particularly true
304 once you get the hang of this CGI stuff, and no longer make
305 the above two mistakes. Always attempt to run your program
306 from the command line before you test if via a browser. This
307 will eliminate most of your problems.</p>
310 <h3><a name="errorlogs" id="errorlogs">Error logs</a></h3>
313 <p>The error logs are your friend. Anything that goes wrong
314 generates message in the error log. You should always look
315 there first. If the place where you are hosting your web site
316 does not permit you access to the error log, you should
317 probably host your site somewhere else. Learn to read the
318 error logs, and you'll find that almost all of your problems
319 are quickly identified, and quickly solved.</p>
321 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="behindscenes" id="behindscenes">What's going on behind the scenes?</a></h2>
324 <p>As you become more advanced in CGI programming, it will
325 become useful to understand more about what's happening behind
326 the scenes. Specifically, how the browser and server
327 communicate with one another. Because although it's all very
328 well to write a program that prints ``Hello, World.'', it's not
329 particularly useful.</p>
331 <h3><a name="env" id="env">Environment variables</a></h3>
334 <p>Environment variables are values that float around you as
335 you use your computer. They are useful things like your path
336 (where the computer searches for a the actual file
337 implementing a command when you type it), your username, your
338 terminal type, and so on. For a full list of your normal,
339 every day environment variables, type
340 <code>env</code> at a command prompt.</p>
342 <p>During the CGI transaction, the server and the browser
343 also set environment variables, so that they can communicate
344 with one another. These are things like the browser type
345 (Netscape, IE, Lynx), the server type (Apache, IIS, WebSite),
346 the name of the CGI program that is being run, and so on.</p>
348 <p>These variables are available to the CGI programmer, and
349 are half of the story of the client-server communication. The
350 complete list of required variables is at
351 <a href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">
352 http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a>
355 <p>This simple Perl CGI program will display all of the
356 environment variables that are being passed around. Two
357 similar programs are included in the
360 directory of the Apache distribution. Note that some
361 variables are required, while others are optional, so you may
362 see some variables listed that were not in the official list.
363 In addition, Apache provides many different ways for you to
364 <a href="../env.html">add your own environment variables</a>
366 to the basic ones provided by default.</p>
368 <div class="example"><p><code>
369 #!/usr/bin/perl<br />
370 print "Content-type: text/html\n\n";<br />
371 foreach $key (keys %ENV) {<br />
372 print "$key --> $ENV{$key}<br>";<br />
376 <h3><a name="stdin" id="stdin">STDIN and STDOUT</a></h3>
379 <p>Other communication between the server and the client
380 happens over standard input (<code>STDIN</code>) and standard
381 output (<code>STDOUT</code>). In normal everyday context,
382 <code>STDIN</code> means the keyboard, or a file that a
383 program is given to act on, and <code>STDOUT</code>
384 usually means the console or screen.</p>
386 <p>When you <code>POST</code> a web form to a CGI program,
387 the data in that form is bundled up into a special format
388 and gets delivered to your CGI program over <code>STDIN</code>.
389 The program then can process that data as though it was
390 coming in from the keyboard, or from a file</p>
392 <p>The ``special format'' is very simple. A field name and
393 its value are joined together with an equals (=) sign, and
394 pairs of values are joined together with an ampersand
395 (&). Inconvenient characters like spaces, ampersands, and
396 equals signs, are converted into their hex equivalent so that
397 they don't gum up the works. The whole data string might look
400 <div class="example"><p><code>
401 name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
404 <p>You'll sometimes also see this type of string appended to
405 the a URL. When that is done, the server puts that string
406 into the environment variable called
407 <code>QUERY_STRING</code>. That's called a <code>GET</code>
408 request. Your HTML form specifies whether a <code>GET</code>
409 or a <code>POST</code> is used to deliver the data, by setting the
410 <code>METHOD</code> attribute in the <code>FORM</code> tag.</p>
412 <p>Your program is then responsible for splitting that string
413 up into useful information. Fortunately, there are libraries
414 and modules available to help you process this data, as well
415 as handle other of the aspects of your CGI program.</p>
417 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="libraries" id="libraries">CGI modules/libraries</a></h2>
420 <p>When you write CGI programs, you should consider using a
421 code library, or module, to do most of the grunt work for you.
422 This leads to fewer errors, and faster development.</p>
424 <p>If you're writing CGI programs in Perl, modules are
425 available on <a href="http://www.cpan.org/">CPAN</a>. The most
426 popular module for this purpose is CGI.pm. You might
427 also consider CGI::Lite, which implements a minimal set of
428 functionality, which is all you need in most programs.</p>
430 <p>If you're writing CGI programs in C, there are a variety of
431 options. One of these is the CGIC library, from
432 <a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>
434 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="moreinfo" id="moreinfo">For more information</a></h2>
437 <p>There are a large number of CGI resources on the web. You
438 can discuss CGI problems with other users on the Usenet group
439 comp.infosystems.www.authoring.cgi. And the -servers mailing
440 list from the HTML Writers Guild is a great source of answers
441 to your questions. You can find out more at
442 <a href="http://www.hwg.org/lists/hwg-servers/">
443 http://www.hwg.org/lists/hwg-servers/</a>
446 <p>And, of course, you should probably read the CGI
447 specification, which has all the details on the operation of
448 CGI programs. You can find the original version at the
449 <a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">
452 and there is an updated draft at the
453 <a href="http://web.golux.com/coar/cgi/">Common Gateway
454 Interface RFC project</a>.</p>
456 <p>When you post a question about a CGI problem that you're
457 having, whether to a mailing list, or to a newsgroup, make sure
458 you provide enough information about what happened, what you
459 expected to happen, and how what actually happened was
460 different, what server you're running, what language your CGI
461 program was in, and, if possible, the offending code. This will
462 make finding your problem much simpler.</p>
464 <p>Note that questions about CGI problems should <strong>never</strong>
465 be posted to the Apache bug database unless you are sure you
466 have found a problem in the Apache source code.</p>
467 </div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>