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15 <p class="apache">Apache HTTP Server Version 2.1</p>
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19 <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 2.1</a> > <a href="./">How-To / Tutorials</a></div><div id="page-content"><div id="preamble"><h1>Apache Tutorial: Dynamic Content with CGI</h1>
21 <p><span>Available Languages: </span><a href="../en/howto/cgi.html" title="English"> en </a> |
22 <a href="../ja/howto/cgi.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
25 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#intro">Introduction</a></li>
26 <li><img alt="" src="../images/down.gif" /> <a href="#configuring">Configuring Apache to permit CGI</a></li>
27 <li><img alt="" src="../images/down.gif" /> <a href="#writing">Writing a CGI program</a></li>
28 <li><img alt="" src="../images/down.gif" /> <a href="#troubleshoot">But it's still not working!</a></li>
29 <li><img alt="" src="../images/down.gif" /> <a href="#behindscenes">What's going on behind the scenes?</a></li>
30 <li><img alt="" src="../images/down.gif" /> <a href="#libraries">CGI modules/libraries</a></li>
31 <li><img alt="" src="../images/down.gif" /> <a href="#moreinfo">For more information</a></li>
33 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
35 <h2><a name="intro" id="intro">Introduction</a></h2>
38 <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>
40 <p>The CGI (Common Gateway Interface) defines a way for a web
41 server to interact with external content-generating programs,
42 which are often referred to as CGI programs or CGI scripts. It
43 is the simplest, and most common, way to put dynamic content on
44 your web site. This document will be an introduction to setting
45 up CGI on your Apache web server, and getting started writing
47 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
49 <h2><a name="configuring" id="configuring">Configuring Apache to permit CGI</a></h2>
52 <p>In order to get your CGI programs to work properly, you'll
53 need to have Apache configured to permit CGI execution. There
54 are several ways to do this.</p>
56 <h3><a name="scriptalias" id="scriptalias">ScriptAlias</a></h3>
60 <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
62 directive tells Apache that a particular directory is set
63 aside for CGI programs. Apache will assume that every file in
64 this directory is a CGI program, and will attempt to execute
65 it, when that particular resource is requested by a
68 <p>The <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
69 directive looks like:</p>
71 <div class="example"><p><code>
72 ScriptAlias /cgi-bin/ /usr/local/apache2/cgi-bin/
75 <p>The example shown is from your default <code>httpd.conf</code>
76 configuration file, if you installed Apache in the default
77 location. The <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>
78 directive is much like the <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> directive, which defines a URL prefix that
79 is to mapped to a particular directory. <code class="directive">Alias</code>
80 and <code class="directive">ScriptAlias</code> are usually used for
81 directories that are outside of the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> directory. The difference between
82 <code class="directive">Alias</code> and <code class="directive">ScriptAlias</code>
83 is that <code class="directive">ScriptAlias</code> has the added meaning
84 that everything under that URL prefix will be considered a CGI
85 program. So, the example above tells Apache that any request for a
86 resource beginning with <code>/cgi-bin/</code> should be served from
87 the directory <code>/usr/local/apache2/cgi-bin/</code>, and should be
88 treated as a CGI program.</p>
90 <p>For example, if the URL
91 <code>http://www.example.com/cgi-bin/test.pl</code>
92 is requested, Apache will attempt to execute the file
93 <code>/usr/local/apache2/cgi-bin/test.pl</code>
94 and return the output. Of course, the file will have to
95 exist, and be executable, and return output in a particular
96 way, or Apache will return an error message.</p>
99 <h3><a name="nonscriptalias" id="nonscriptalias">CGI outside of ScriptAlias directories</a></h3>
102 <p>CGI programs are often restricted to <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code>'ed directories for security reasons.
103 In this way, administrators can tightly control who is allowed to
104 use CGI programs. However, if the proper security precautions are
105 taken, there is no reason why CGI programs cannot be run from
106 arbitrary directories. For example, you may wish to let users
107 have web content in their home directories with the
108 <code class="directive"><a href="../mod/mod_userdir.html#userdir">UserDir</a></code> directive.
109 If they want to have their own CGI programs, but don't have access to
110 the main <code>cgi-bin</code> directory, they will need to be able to
111 run CGI programs elsewhere.</p>
113 <p>There are two steps to allowing CGI execution in an arbitrary
114 directory. First, the <code>cgi-script</code> handler must be
115 activated using the <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> or <code class="directive"><a href="../mod/core.html#sethandler">SetHandler</a></code> directive. Second,
116 <code>ExecCGI</code> must be specified in the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive.</p>
119 <h3><a name="options" id="options">Explicitly using Options to permit CGI execution</a></h3>
122 <p>You could explicitly use the <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive, inside your main server configuration
123 file, to specify that CGI execution was permitted in a particular
126 <div class="example"><p><code>
127 <Directory /usr/local/apache2/htdocs/somedir><br />
128 <span class="indent">
129 Options +ExecCGI<br />
134 <p>The above directive tells Apache to permit the execution
135 of CGI files. You will also need to tell the server what
136 files are CGI files. The following <code class="directive"><a href="../mod/mod_mime.html#addhandler">AddHandler</a></code> directive tells the server to treat all
137 files with the <code>cgi</code> or <code>pl</code> extension as CGI
140 <div class="example"><p><code>
141 AddHandler cgi-script .cgi .pl
145 <h3><a name="htaccess" id="htaccess">.htaccess files</a></h3>
148 <p>The <a href="htaccess.html"><code>.htaccess</code> tutorial</a>
149 shows how to activate CGI programs if you do not have
150 access to <code>httpd.conf</code>.</p>
153 <h3><a name="userdir" id="userdir">User Directories</a></h3>
156 <p>To allow CGI program execution for any file ending in
157 <code>.cgi</code> in users' directories, you can use the
158 following configuration.</p>
160 <div class="example"><p><code>
161 <Directory /home/*/public_html><br />
162 <span class="indent">
163 Options +ExecCGI<br />
164 AddHandler cgi-script .cgi<br />
169 <p>If you wish designate a <code>cgi-bin</code> subdirectory of
170 a user's directory where everything will be treated as a CGI
171 program, you can use the following.</p>
173 <div class="example"><p><code>
174 <Directory /home/*/public_html/cgi-bin><br />
175 <span class="indent">
176 Options ExecCGI<br />
177 SetHandler cgi-script<br />
184 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
185 <div class="section">
186 <h2><a name="writing" id="writing">Writing a CGI program</a></h2>
189 <p>There are two main differences between ``regular''
190 programming, and CGI programming.</p>
192 <p>First, all output from your CGI program must be preceded by
193 a MIME-type header. This is HTTP header that tells the client
194 what sort of content it is receiving. Most of the time, this
197 <div class="example"><p><code>
198 Content-type: text/html
201 <p>Secondly, your output needs to be in HTML, or some other
202 format that a browser will be able to display. Most of the
203 time, this will be HTML, but occasionally you might write a CGI
204 program that outputs a gif image, or other non-HTML
207 <p>Apart from those two things, writing a CGI program will look
208 a lot like any other program that you might write.</p>
210 <h3><a name="firstcgi" id="firstcgi">Your first CGI program</a></h3>
213 <p>The following is an example CGI program that prints one
214 line to your browser. Type in the following, save it to a
215 file called <code>first.pl</code>, and put it in your
216 <code>cgi-bin</code> directory.</p>
218 <div class="example"><p><code>
219 #!/usr/bin/perl<br />
220 print "Content-type: text/html\n\n";<br />
221 print "Hello, World.";
224 <p>Even if you are not familiar with Perl, you should be able
225 to see what is happening here. The first line tells Apache
226 (or whatever shell you happen to be running under) that this
227 program can be executed by feeding the file to the
228 interpreter found at the location <code>/usr/bin/perl</code>.
229 The second line prints the content-type declaration we
230 talked about, followed by two carriage-return newline pairs.
231 This puts a blank line after the header, to indicate the end
232 of the HTTP headers, and the beginning of the body. The third
233 line prints the string "Hello, World.". And that's the end
236 <p>If you open your favorite browser and tell it to get the
239 <div class="example"><p><code>
240 http://www.example.com/cgi-bin/first.pl
243 <p>or wherever you put your file, you will see the one line
244 <code>Hello, World.</code> appear in your browser window.
245 It's not very exciting, but once you get that working, you'll
246 have a good chance of getting just about anything working.</p>
248 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
249 <div class="section">
250 <h2><a name="troubleshoot" id="troubleshoot">But it's still not working!</a></h2>
253 <p>There are four basic things that you may see in your browser
254 when you try to access your CGI program from the web:</p>
257 <dt>The output of your CGI program</dt>
258 <dd>Great! That means everything worked fine. If the output is correct,
259 but the browser is not processing it correctly, make sure you have the
260 correct <code>Content-Type</code> set in your CGI program.</dd>
262 <dt>The source code of your CGI program or a "POST Method Not
263 Allowed" message</dt>
264 <dd>That means that you have not properly configured Apache
265 to process your CGI program. Reread the section on
266 <a href="#configuring">configuring
267 Apache</a> and try to find what you missed.</dd>
269 <dt>A message starting with "Forbidden"</dt>
270 <dd>That means that there is a permissions problem. Check the
271 <a href="#errorlogs">Apache error log</a> and the section below on
272 <a href="#permissions">file permissions</a>.</dd>
274 <dt>A message saying "Internal Server Error"</dt>
276 <a href="#errorlogs">Apache error log</a>, you will probably
277 find that it says "Premature end of
278 script headers", possibly along with an error message
279 generated by your CGI program. In this case, you will want to
280 check each of the below sections to see what might be
281 preventing your CGI program from emitting the proper HTTP
285 <h3><a name="permissions" id="permissions">File permissions</a></h3>
288 <p>Remember that the server does not run as you. That is,
289 when the server starts up, it is running with the permissions
290 of an unprivileged user - usually <code>nobody</code>, or
291 <code>www</code> - and so it will need extra permissions to
292 execute files that are owned by you. Usually, the way to give
293 a file sufficient permissions to be executed by <code>nobody</code>
294 is to give everyone execute permission on the file:</p>
296 <div class="example"><p><code>
300 <p>Also, if your program reads from, or writes to, any other
301 files, those files will need to have the correct permissions
306 <h3><a name="pathinformation" id="pathinformation">Path information and environment</a></h3>
309 <p>When you run a program from your command line, you have
310 certain information that is passed to the shell without you
311 thinking about it. For example, you have a <code>PATH</code>,
312 which tells the shell where it can look for files that you
315 <p>When a program runs through the web server as a CGI program,
316 it may not have the same <code>PATH</code>. Any programs that you
317 invoke in your CGI program (like <code>sendmail</code>, for
318 example) will need to be specified by a full path, so that the
319 shell can find them when it attempts to execute your CGI
322 <p>A common manifestation of this is the path to the script
323 interpreter (often <code>perl</code>) indicated in the first
324 line of your CGI program, which will look something like:</p>
326 <div class="example"><p><code>
330 <p>Make sure that this is in fact the path to the
333 <p>In addition, if your CGI program depends on other <a href="#env">environment variables</a>, you will need to
334 assure that those variables are passed by Apache.</p>
338 <h3><a name="syntaxerrors" id="syntaxerrors">Program errors</a></h3>
341 <p>Most of the time when a CGI program fails, it's because of
342 a problem with the program itself. This is particularly true
343 once you get the hang of this CGI stuff, and no longer make
344 the above two mistakes. The first thing to do is to make
345 sure that your program runs from the command line before
346 testing it via the web server. For example, try:</p>
348 <div class="example"><p><code>
349 cd /usr/local/apache2/cgi-bin<br />
353 <p>(Do not call the <code>perl</code> interpreter. The shell
354 and Apache should find the interpreter using the <a href="#pathinformation">path information</a> on the first line of
357 <p>The first thing you see written by your program should be
358 a set of HTTP headers, including the <code>Content-Type</code>,
359 followed by a blank line. If you see anything else, Apache will
360 return the <code>Premature end of script headers</code> error if
361 you try to run it through the server. See <a href="#writing">Writing a CGI program</a> above for more
365 <h3><a name="errorlogs" id="errorlogs">Error logs</a></h3>
368 <p>The error logs are your friend. Anything that goes wrong
369 generates message in the error log. You should always look
370 there first. If the place where you are hosting your web site
371 does not permit you access to the error log, you should
372 probably host your site somewhere else. Learn to read the
373 error logs, and you'll find that almost all of your problems
374 are quickly identified, and quickly solved.</p>
377 <h3><a name="suexec" id="suexec">Suexec</a></h3>
380 <p>The <a href="../suexec.html">suexec</a> support program
381 allows CGI programs to be run under different user permissions,
382 depending on which virtual host or user home directory they are
383 located in. Suexec has very strict permission checking, and any
384 failure in that checking will result in your CGI programs
385 failing with <code>Premature end of script headers</code>.</p>
387 <p>To check if you are using suexec, run <code>apachectl
388 -V</code> and check for the location of <code>SUEXEC_BIN</code>.
389 If Apache finds an suexec binary there on startup, suexec will
392 <p>Unless you fully understand suexec, you should not be using it.
393 To disable suexec, simply remove (or rename) the <code>suexec</code>
394 binary pointed to by <code>SUEXEC_BIN</code> and then restart the
395 server. If, after reading about <a href="../suexec.html">suexec</a>,
396 you still wish to use it, then run <code>suexec -V</code> to find
397 the location of the suexec log file, and use that log file to
398 find what policy you are violating.</p>
400 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
401 <div class="section">
402 <h2><a name="behindscenes" id="behindscenes">What's going on behind the scenes?</a></h2>
405 <p>As you become more advanced in CGI programming, it will
406 become useful to understand more about what's happening behind
407 the scenes. Specifically, how the browser and server
408 communicate with one another. Because although it's all very
409 well to write a program that prints "Hello, World.", it's not
410 particularly useful.</p>
412 <h3><a name="env" id="env">Environment variables</a></h3>
415 <p>Environment variables are values that float around you as
416 you use your computer. They are useful things like your path
417 (where the computer searches for the actual file
418 implementing a command when you type it), your username, your
419 terminal type, and so on. For a full list of your normal,
420 every day environment variables, type
421 <code>env</code> at a command prompt.</p>
423 <p>During the CGI transaction, the server and the browser
424 also set environment variables, so that they can communicate
425 with one another. These are things like the browser type
426 (Netscape, IE, Lynx), the server type (Apache, IIS, WebSite),
427 the name of the CGI program that is being run, and so on.</p>
429 <p>These variables are available to the CGI programmer, and
430 are half of the story of the client-server communication. The
431 complete list of required variables is at
432 <a href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a>.</p>
434 <p>This simple Perl CGI program will display all of the
435 environment variables that are being passed around. Two
436 similar programs are included in the
439 directory of the Apache distribution. Note that some
440 variables are required, while others are optional, so you may
441 see some variables listed that were not in the official list.
442 In addition, Apache provides many different ways for you to
443 <a href="../env.html">add your own environment variables</a>
444 to the basic ones provided by default.</p>
446 <div class="example"><p><code>
447 #!/usr/bin/perl<br />
448 print "Content-type: text/html\n\n";<br />
449 foreach $key (keys %ENV) {<br />
450 <span class="indent">
451 print "$key --> $ENV{$key}<br>";<br />
457 <h3><a name="stdin" id="stdin">STDIN and STDOUT</a></h3>
460 <p>Other communication between the server and the client
461 happens over standard input (<code>STDIN</code>) and standard
462 output (<code>STDOUT</code>). In normal everyday context,
463 <code>STDIN</code> means the keyboard, or a file that a
464 program is given to act on, and <code>STDOUT</code>
465 usually means the console or screen.</p>
467 <p>When you <code>POST</code> a web form to a CGI program,
468 the data in that form is bundled up into a special format
469 and gets delivered to your CGI program over <code>STDIN</code>.
470 The program then can process that data as though it was
471 coming in from the keyboard, or from a file</p>
473 <p>The "special format" is very simple. A field name and
474 its value are joined together with an equals (=) sign, and
475 pairs of values are joined together with an ampersand
476 (&). Inconvenient characters like spaces, ampersands, and
477 equals signs, are converted into their hex equivalent so that
478 they don't gum up the works. The whole data string might look
481 <div class="example"><p><code>
482 name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
485 <p>You'll sometimes also see this type of string appended to
486 a URL. When that is done, the server puts that string
487 into the environment variable called
488 <code>QUERY_STRING</code>. That's called a <code>GET</code>
489 request. Your HTML form specifies whether a <code>GET</code>
490 or a <code>POST</code> is used to deliver the data, by setting the
491 <code>METHOD</code> attribute in the <code>FORM</code> tag.</p>
493 <p>Your program is then responsible for splitting that string
494 up into useful information. Fortunately, there are libraries
495 and modules available to help you process this data, as well
496 as handle other of the aspects of your CGI program.</p>
498 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
499 <div class="section">
500 <h2><a name="libraries" id="libraries">CGI modules/libraries</a></h2>
503 <p>When you write CGI programs, you should consider using a
504 code library, or module, to do most of the grunt work for you.
505 This leads to fewer errors, and faster development.</p>
507 <p>If you're writing CGI programs in Perl, modules are
508 available on <a href="http://www.cpan.org/">CPAN</a>. The most
509 popular module for this purpose is <code>CGI.pm</code>. You might
510 also consider <code>CGI::Lite</code>, which implements a minimal
511 set of functionality, which is all you need in most programs.</p>
513 <p>If you're writing CGI programs in C, there are a variety of
514 options. One of these is the <code>CGIC</code> library, from
515 <a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>.</p>
516 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
517 <div class="section">
518 <h2><a name="moreinfo" id="moreinfo">For more information</a></h2>
521 <p>There are a large number of CGI resources on the web. You
522 can discuss CGI problems with other users on the Usenet group
523 <a href="news:comp.infosystems.www.authoring.cgi">comp.infosystems.www.authoring.cgi</a>. And the -servers mailing
524 list from the HTML Writers Guild is a great source of answers
525 to your questions. You can find out more at
526 <a href="http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a>.</p>
528 <p>And, of course, you should probably read the CGI
529 specification, which has all the details on the operation of
530 CGI programs. You can find the original version at the
531 <a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a> and there is an updated draft at the
532 <a href="http://web.golux.com/coar/cgi/">Common Gateway
533 Interface RFC project</a>.</p>
535 <p>When you post a question about a CGI problem that you're
536 having, whether to a mailing list, or to a newsgroup, make sure
537 you provide enough information about what happened, what you
538 expected to happen, and how what actually happened was
539 different, what server you're running, what language your CGI
540 program was in, and, if possible, the offending code. This will
541 make finding your problem much simpler.</p>
543 <p>Note that questions about CGI problems should <strong>never</strong>
544 be posted to the Apache bug database unless you are sure you
545 have found a problem in the Apache source code.</p>
547 <div class="bottomlang">
548 <p><span>Available Languages: </span><a href="../en/howto/cgi.html" title="English"> en </a> |
549 <a href="../ja/howto/cgi.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
550 </div><div id="footer">
551 <p class="apache">Copyright 1999-2004 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
552 <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>