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<html>
<head>
<title>Apache Tutorial: Dynamic Content with CGI</title>
<link rev="made" href="mailto:rbowen@rcbowen.com">
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#000080"
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<!--#include virtual="header.html" -->
<h1 align="CENTER">Dynamic Content with CGI</h1>
<a name="__index__"></a> <!-- INDEX BEGIN -->
<ul>
<li><a href="#dynamiccontentwithcgi">Dynamic Content with
CGI</a></li>
<li><a href="#configuringapachetopermitcgi">Configuring Apache to
permit CGI</a>
<ul>
<li><a href="#scriptalias">ScriptAlias</a></li>
<li><a href="#cgioutsideofscriptaliasdirectories">CGI outside of
ScriptAlias directories</a>
<ul>
<li><a href="#explicitlyusingoptionstopermitcgiexecution">Explicitly using
Options to permit CGI execution</a></li>
<li><a href="#htaccessfiles">.htaccess files</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#writingacgiprogram">Writing a CGI program</a>
<ul>
<li><a href="#yourfirstcgiprogram">Your first CGI program</a></li>
</ul>
</li>
<li><a href="#butitsstillnotworking">But it's still not
working!</a>
<ul>
<li><a href="#filepermissions">File permissions</a></li>
<li><a href="#pathinformation">Path information</a></li>
<li><a href="#syntaxerrors">Syntax errors</a></li>
<li><a href="#errorlogs">Error logs</a></li>
</ul>
</li>
<li><a href="#whatsgoingonbehindthescenes">What's going on behind
the scenes?</a>
<ul>
<li><a href="#environmentvariables">Environment variables</a></li>
<li><a href="#stdinandstdout">STDIN and STDOUT</a></li>
</ul>
</li>
<li><a href="#cgimoduleslibraries">CGI modules/libraries</a></li>
<li><a href="#formoreinformation">For more information</a></li>
</ul>
<!-- INDEX END -->
<hr>
<h2><a name="dynamiccontentwithcgi">Dynamic Content with
CGI</a></h2>
<table border="1">
<tr><td valign="top">
<strong>Related Modules</strong><br><br>
<a href="../mod/mod_alias.html">mod_alias</a><br>
<a href="../mod/mod_cgi.html">mod_cgi</a><br>
</td><td valign="top">
<strong>Related Directives</strong><br><br>
<a href="../mod/mod_mime.html#addhandler">AddHandler</a><br>
<A HREF="../mod/core.html#options">Options</a><br>
<a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a><br>
</td></tr></table>
<p>The CGI (Common Gateway Interface) defines a way for a web server
to interact with external content-generating programs, which are often
referred to as CGI programs or CGI scripts. It is the simplest, and
most common, way to put dynamic content on your web site. This
document will be an introduction to setting up CGI on your Apache web
server, and getting started writing CGI programs.</p>
<hr>
<h2><a name="configuringapachetopermitcgi">Configuring Apache to
permit CGI</a></h2>
<p>In order to get your CGI programs to work properly, you'll need to
have Apache configured to permit CGI execution. There are several ways
to do this.</p>
<h3><a name="scriptalias">ScriptAlias</a></h3>
<p>The <code>ScriptAlias</code> directive tells Apache that a
particular directory is set aside for CGI programs. Apache will assume
that every file in this directory is a CGI program, and will attempt to
execute it, when that particular resource is requested by a client.</p>
<p>The <code>ScriptAlias</code> directive looks like:</p>
<pre>
ScriptAlias /cgi-bin/ /usr/local/apache/cgi-bin/
</pre>
<p>The example shown is from your default <code>httpd.conf</code>
configuration file, if you installed Apache in the default location.
The <code>ScriptAlias</code> directive is much like the
<code>Alias</code> directive, which defines a URL prefix that is to
mapped to a particular directory. <code>Alias</code> and
<code>ScriptAlias</code> are usually used for directories that are
outside of the <code>DocumentRoot</code> directory. The difference
between <code>Alias</code> and <code>ScriptAlias</code> is that
<code>ScriptAlias</code> has the added meaning that everything under
that URL prefix will be considered a CGI program. So, the example above
tells Apache that any request for a resource beginning with
<code>/cgi-bin/</code> should be served from the directory
<code>/usr/local/apache/cgi-bin/</code>, and should be treated as a CGI
program.</p>
<p>For example, if the URL
<code>http://dev.rcbowen.com/cgi-bin/test.pl</code> is requested,
Apache will attempt to execute the file
<code>/usr/local/apache/cgi-bin/test.pl</code> and return the output.
Of course, the file will have to exist, and be executable, and return
output in a particular way, or Apache will return an error message.</p>
<h3><a name="cgioutsideofscriptaliasdirectories">CGI outside of
ScriptAlias directories</a></h3>
<p>CGI programs are often restricted to <code>ScriptAlias</code>'ed
directories for security reasons. In this way, administrators can
tightly control who is allowed to use CGI programs. However, if the
proper security precautions are taken, there is no reason why
CGI programs cannot be run from arbitrary directories. For example,
you may wish to let users have web content in their home directories
with the <code>UserDir</code> directive. If they want to have their
own CGI programs, but don't have access to the main
<code>cgi-bin</code> directory, they will need to be able to run CGI
programs elsewhere.</p>
<h3><a name="explicitlyusingoptionstopermitcgiexecution">Explicitly using
Options to permit CGI execution</a></h3>
<p>You could explicitly use the <code>Options</code> directive, inside
your main server configuration file, to specify that CGI execution was
permitted in a particular directory:</p>
<pre>
<Directory /usr/local/apache/htdocs/somedir>
Options +ExecCGI
</Directory>
</pre>
<p>The above directive tells Apache to permit the execution of CGI
files. You will also need to tell the server what files are CGI files.
The following <code>AddHandler</code> directive tells the server
to treat all files with the <code>cgi</code> or <code>pl</code>
extension as CGI programs:</p>
<pre>
AddHandler cgi-script cgi pl
</pre>
<h3><a name="htaccessfiles">.htaccess files</a></h3>
<p>A <code>.htaccess</code> file is a way to set configuration
directives on a per-directory basis. When Apache serves a resource, it
looks in the directory from which it is serving a file for a file
called <code>.htaccess</code>, and, if it finds it, it will apply
directives found therein. <code>.htaccess</code> files can be permitted
with the <code>AllowOverride</code> directive, which specifies what
types of directives can appear in these files, or if they are not
allowed at all. To permit the directive we will need for this purpose,
the following configuration will be needed in your main server
configuration:</p>
<pre>
AllowOverride Options
</pre>
<p>In the <code>.htaccess</code> file, you'll need the following
directive:</p>
<pre>
Options +ExecCGI
</pre>
<p>which tells Apache that execution of CGI programs is permitted in
this directory.</p>
<hr>
<h2><a name="writingacgiprogram">Writing a CGI program</a></h2>
<p>There are two main differences between ``regular'' programming, and
CGI programming.</p>
<p>First, all output from your CGI program must be preceded by a
MIME-type header. This is HTTP header that tells the client what sort
of content it is receiving. Most of the time, this will look like:</p>
<pre>
Content-type: text/html
</pre>
<p>Secondly, your output needs to be in HTML, or some other format that
a browser will be able to display. Most of the time, this will be HTML,
but occasionally you might write a CGI program that outputs a gif
image, or other non-HTML content.</p>
<p>Apart from those two things, writing a CGI program will look a lot
like any other program that you might write.</p>
<h3><a name="yourfirstcgiprogram">Your first CGI program</a></h3>
<p>The following is an example CGI program that prints one line to your
browser. Type in the following, save it to a file called
<code>first.pl</code>, and put it in your <code>cgi-bin</code>
directory.</p>
<pre>
#!/usr/bin/perl
print "Content-type: text/html\r\n\r\n";
print "Hello, World.";
</pre>
<p>Even if you are not familiar with Perl, you should be able to see
what is happening here. The first line tells Apache (or whatever shell
you happen to be running under) that this program can be executed by
feeding the file to the interpreter found at the location
<code>/usr/bin/perl</code>. The second line prints the content-type
declaration we talked about, followed by two carriage-return newline
pairs. This puts a blank line after the header, to indicate the end of
the HTTP headers, and the beginning of the body. The third line prints
the string ``Hello, World.'' And that's the end of it.</p>
<p>If you open your favorite browser and tell it to get the address</p>
<pre>
http://www.example.com/cgi-bin/first.pl
</pre>
<p>or wherever you put your file, you will see the one line
<code>Hello, World.</code> appear in your browser window. It's not very
exciting, but once you get that working, you'll have a good chance of
getting just about anything working.</p>
<hr>
<h2><a name="butitsstillnotworking">But it's still not
working!</a></h2>
<p>There are four basic things that you may see in your browser when
you try to access your CGI program from the web:</p>
<dl>
<dt>The output of your CGI program</dt>
<dd>Great! That means everything worked fine.<br><br></dd>
<dt>The source code of your CGI program or a "POST Method Not Allowed"
message</dt>
<dd>That means that you have not properly configured
Apache to process your CGI program. Reread the section on <a
href="#configuringapachetopermitcgi">configuring Apache</a> and try to
find what you missed.<br><br></dd>
<dt>A message starting with "Forbidden"</dt> <dd>That means that there
is a permissions problem. Check the <a href="#errorlogs">Apache
error log</a> and the section below on <a
href="#filepermissions">file permissions</a>.<br><br></dd>
<dt>A message saying "Internal Server Error"</dt> <dd>If you check the
<a href="#errorlogs">Apache error log</a>, you will probably find
that it says "Premature end of script headers", possibly along with an
error message generated by your CGI program. In this case, you will
want to check each of the below sections to see what might be preventing
your CGI program from emitting the proper HTTP headers.</dd>
</dl>
<h3><a name="filepermissions">File permissions</a></h3>
<p>Remember that the server does not run as you. That is, when the
server starts up, it is running with the permissions of an unprivileged
user - usually ``nobody'', or ``www'' - and so it will need extra
permissions to execute files that are owned by you. Usually, the way to
give a file sufficient permissions to be executed by ``nobody'' is to
give everyone execute permission on the file:</p>
<pre>
chmod a+x first.pl
</pre>
<p>Also, if your program reads from, or writes to, any other files,
those files will need to have the correct permissions to permit
this.</p>
<p>The exception to this is when the server is configured to use <a
href="../suexec.html">suexec</a>. This program allows CGI programs to
be run under different user permissions, depending on which virtual
host or user home directory they are located in. Suexec has very
strict permission checking, and any failure in that checking will
result in your CGI programs failing with an "Internal Server Error".
In this case, you will need to check the suexec log file to see what
specific security check is failing.</p>
<h3><a name="pathinformation">Path information</a></h3>
<p>When you run a program from your command line, you have certain
information that is passed to the shell without you thinking about it.
For example, you have a path, which tells the shell where it can look
for files that you reference.</p>
<p>When a program runs through the web server as a CGI program, it does
not have that path. Any programs that you invoke in your CGI program
(like 'sendmail', for example) will need to be specified by a full
path, so that the shell can find them when it attempts to execute your
CGI program.</p>
<p>A common manifestation of this is the path to the script interpreter
(often <code>perl</code>) indicated in the first line of your CGI
program, which will look something like:</p>
<pre>
#!/usr/bin/perl
</pre>
<p>Make sure that this is in fact the path to the interpreter.</p>
<h3><a name="syntaxerrors">Syntax errors</a></h3>
<p>Most of the time when a CGI program fails, it's because of a problem
with the program itself. This is particularly true once you get the
hang of this CGI stuff, and no longer make the above two mistakes.
Always attempt to run your program from the command line before you
test if via a browser. This will eliminate most of your problems.</p>
<h3><a name="errorlogs">Error logs</a></h3>
<p>The error logs are your friend. Anything that goes wrong generates
message in the error log. You should always look there first. If the
place where you are hosting your web site does not permit you access to
the error log, you should probably host your site somewhere else. Learn
to read the error logs, and you'll find that almost all of your
problems are quickly identified, and quickly solved.</p>
<hr>
<h2><a name="whatsgoingonbehindthescenes">What's going on behind
the scenes?</a></h2>
<p>As you become more advanced in CGI programming, it will become
useful to understand more about what's happening behind the scenes.
Specifically, how the browser and server communicate with one another.
Because although it's all very well to write a program that prints
``Hello, World.'', it's not particularly useful.</p>
<h3><a name="environmentvariables">Environment variables</a></h3>
<p>Environment variables are values that float around you as you use
your computer. They are useful things like your path (where the
computer searches for a the actual file implementing a command when you
type it), your username, your terminal type, and so on. For a full list
of your normal, every day environment variables, type <code>env</code>
at a command prompt.</p>
<p>During the CGI transaction, the server and the browser also set
environment variables, so that they can communicate with one another.
These are things like the browser type (Netscape, IE, Lynx), the server
type (Apache, IIS, WebSite), the name of the CGI program that is being
run, and so on.</p>
<p>These variables are available to the CGI programmer, and are half of
the story of the client-server communication. The complete list of
required variables is at <a href=
"http://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a></p>
<p>This simple Perl CGI program will display all of the environment
variables that are being passed around. Two similar programs are
included in the <code>cgi-bin</code> directory of the Apache
distribution. Note that some variables are required, while others are
optional, so you may see some variables listed that were not in the
official list. In addition, Apache provides many different ways for
you to <a href="../env.html">add your own environment variables</a> to
the basic ones provided by default.</p>
<pre>
#!/usr/bin/perl
print "Content-type: text/html\n\n";
foreach $key (keys %ENV) {
print "$key --> $ENV{$key}<br>";
}
</pre>
<h3><a name="stdinandstdout">STDIN and STDOUT</a></h3>
<p>Other communication between the server and the client happens over
standard input (<code>STDIN</code>) and standard output
(<code>STDOUT</code>). In normal everyday context, <code>STDIN</code>
means the keyboard, or a file that a program is given to act on, and
<code>STDOUT</code> usually means the console or screen.</p>
<p>When you <code>POST</code> a web form to a CGI program, the data in
that form is bundled up into a special format and gets delivered to
your CGI program over <code>STDIN</code>. The program then can process
that data as though it was coming in from the keyboard, or from a
file</p>
<p>The ``special format'' is very simple. A field name and its value
are joined together with an equals (=) sign, and pairs of values are
joined together with an ampersand (&). Inconvenient characters like
spaces, ampersands, and equals signs, are converted into their hex
equivalent so that they don't gum up the works. The whole data string
might look something like:</p>
<pre>
name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
</pre>
<p>You'll sometimes also see this type of string appended to the a URL.
When that is done, the server puts that string into the environment
variable called <code>QUERY_STRING</code>. That's called a
<code>GET</code> request. Your HTML form specifies whether a
<code>GET</code> or a <code>POST</code> is used to deliver the data, by
setting the <code>METHOD</code> attribute in the <code>FORM</code>
tag.</p>
<p>Your program is then responsible for splitting that string up into
useful information. Fortunately, there are libraries and modules
available to help you process this data, as well as handle other of the
aspects of your CGI program.</p>
<hr>
<h2><a name="cgimoduleslibraries">CGI modules/libraries</a></h2>
<p>When you write CGI programs, you should consider using a code
library, or module, to do most of the grunt work for you. This leads to
fewer errors, and faster development.</p>
<p>If you're writing CGI programs in Perl, modules are available on <a
href="http://www.cpan.org/">CPAN</a>. The most popular module for this
purpose is CGI.pm. You might also consider CGI::Lite, which implements
a minimal set of functionality, which is all you need in most
programs.</p>
<p>If you're writing CGI programs in C, there are a variety of options.
One of these is the CGIC library, from <a href=
"http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a></p>
<hr>
<h2><a name="formoreinformation">For more information</a></h2>
<p>There are a large number of CGI resources on the web. You can
discuss CGI problems with other users on the Usenet group
comp.infosystems.www.authoring.cgi. And the -servers mailing list from
the HTML Writers Guild is a great source of answers to your questions.
You can find out more at <a href=
"http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a></p>
<p>And, of course, you should probably read the CGI specification,
which has all the details on the operation of CGI programs. You can
find the original version at the <a href=
"http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a> and there is
an updated draft at the <a
href="http://web.golux.com/coar/cgi/">Common Gateway Interface RFC
project</a>.</p>
<p>When you post a question about a CGI problem that you're having,
whether to a mailing list, or to a newsgroup, make sure you provide
enough information about what happened, what you expected to happen,
and how what actually happened was different, what server you're
running, what language your CGI program was in, and, if possible, the
offending code. This will make finding your problem much simpler.</p>
<p>Note that questions about CGI problems should <strong>never</strong>
be posted to the Apache bug database unless you are sure you have found
a problem in the Apache source code.</p>
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1.1 httpd-2.0/docs/manual/howto/footer.html
Index: footer.html
===================================================================
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<H3 ALIGN="CENTER">
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<A HREF="../"><IMG SRC="../images/home.gif" ALT="Home"></A>
1.1 httpd-2.0/docs/manual/howto/header.html
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1.1 httpd-2.0/docs/manual/howto/ssi.html
Index: ssi.html
===================================================================
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head>
<title>Apache Tutorial: Introduction to Server Side Includes</title>
<link rev="made" href="mailto:rbowen@rcbowen.com">
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#000080"
alink="#FF0000">
<!--#include virtual="header.html" -->
<h1 align="CENTER">Apache Tutorial: Introduction to Server Side
Includes</h1>
<a name="__index__"></a> <!-- INDEX BEGIN -->
<ul>
<li><a href=
"#apachetutorial:introductiontoserversideincludes">Apache
Tutorial: Introduction to Server Side Includes</a></li>
<li><a href="#whataressi">What are SSI?</a></li>
<li><a href="#configuringyourservertopermitssi">Configuring your
server to permit SSI</a></li>
<li><a href="#basicssidirectives">Basic SSI directives</a>
<ul>
<li><a href="#today'sdate">Today's date</a></li>
<li><a href="#modificationdateofthefile">Modification date of the
file</a></li>
<li><a href="#includingtheresultsofacgiprogram">Including the
results of a CGI program</a></li>
</ul>
</li>
<li><a href="#additionalexamples">Additional examples</a>
<ul>
<li><a href="#whenwasthisdocumentmodified">When was this document
modified?</a></li>
<li><a href="#includingastandardfooter">Including a standard
footer</a></li>
<li><a href="#whatelsecaniconfig">What else can I config?</a></li>
<li><a href="#executingcommands">Executing commands</a></li>
</ul>
</li>
<li><a href="#advancedssitechniques">Advanced SSI techniques</a>
<ul>
<li><a href="#settingvariables">Setting variables</a></li>
<li><a href="#conditionalexpressions">Conditional expressions</a></li>
</ul>
</li>
<li><a href="#conclusion">Conclusion</a></li>
</ul>
<!-- INDEX END -->
<hr>
<h2><a name=
"apachetutorial:introductiontoserversideincludes">Apache
Tutorial: Introduction to Server Side Includes</a></h2>
<table border="1">
<tr>
<td valign="top"><strong>Related Modules</strong><br>
<br>
<a href="../mod/mod_include.html">mod_include</a><br>
<a href="../mod/mod_cgi.html">mod_cgi</a><br>
<a href="../mod/mod_expires.html">mod_expires</a><br>
</td>
<td valign="top"><strong>Related Directives</strong><br>
<br>
<a href="../mod/core.html#options">Options</a><br>
<a href="../mod/mod_include.html#xbithack">XBitHack</a><br>
<a href="../mod/mod_mime.html#addtype">AddType</a><br>
<a href="../mod/mod_mime.html#addhandler">AddHandler</a><br>
<a href=
"../mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br>
</td>
</tr>
</table>
<p>This HOWTO first appeared in Apache Today
(http://www.apachetoday.com/) as a series of three articles. They
appear here by arrangement with ApacheToday and Internet.com.</p>
<p>This article deals with Server Side Includes, usually called simply
SSI. In this article, I'll talk about configuring your server to permit
SSI, and introduce some basic SSI techniques for adding dynamic content
to your existing HTML pages.</p>
<p>In the latter part of the article, we'll talk about some of the
somewhat more advanced things that can be done with SSI, such as
conditional statements in your SSI directives.</p>
<hr>
<h2><a name="whataressi">What are SSI?</a></h2>
<p>SSI (Server Side Includes) are directives that are placed in HTML
pages, and evaluated on the server while the pages are being served.
They let you add dynamically generated content to an existing HTML
page, without having to serve the entire page via a CGI program, or
other dynamic technology.</p>
<p>The decision of when to use SSI, and when to have your page entirely
generated by some program, is usually a matter of how much of the page
is static, and how much needs to be recalculated every time the page is
served. SSI is a great way to add small pieces of information, such as
the current time. But if a majority of your page is being generated at
the time that it is served, you need to look for some other
solution.</p>
<hr>
<h2><a name="configuringyourservertopermitssi">Configuring your
server to permit SSI</a></h2>
<p>To permit SSI on your server, you must have the following directive
either in your <code>httpd.conf</code> file, or in a
<code>.htaccess</code> file:</p>
<pre>
Options +Includes
</pre>
<p>This tells Apache that you want to permit files to be parsed for SSI
directives.</p>
<p>Not just any file is parsed for SSI directives. You have to tell
Apache which files should be parsed. There are two ways to do this. You
can tell Apache to parse any file with a particular file extension,
such as <code>.shtml</code>, with the following directives:</p>
<pre>
AddType text/html .shtml
AddHandler server-parsed .shtml
</pre>
<p>One disadvantage to this approach is that if you wanted to add SSI
directives to an existing page, you would have to change the name of
that page, and all links to that page, in order to give it a
<code>.shtml</code> extension, so that those directives would be
executed.</p>
<p>The other method is to use the <code>XBitHack</code> directive:</p>
<pre>
XBitHack on
</pre>
<p><code>XBitHack</code> tells Apache to parse files for SSI directives
if they have the execute bit set. So, to add SSI directives to an
existing page, rather than having to change the file name, you would
just need to make the file executable using <code>chmod</code>.</p>
<pre>
chmod +x pagename.html
</pre>
<p>A brief comment about what not to do. You'll occasionally see people
recommending that you just tell Apache to parse all <code>.html</code>
files for SSI, so that you don't have to mess with <code>.shtml</code>
file names. These folks have perhaps not heard about
<code>XBitHack</code>. The thing to keep in mind is that, by doing
this, you're requiring that Apache read through every single file that
it sends out to clients, even if they don't contain any SSI directives.
This can slow things down quite a bit, and is not a good idea.</p>
<p>Of course, on Windows, there is no such thing as an execute bit to
set, so that limits your options a little.</p>
<p>In its default configuration, Apache does not send the last modified
date or content length HTTP headers on SSI pages, because these values are
difficult to calculate for dynamic content. This can prevent your
document from being cached, and result in slower perceived client
performance. There are two ways to solve this:</p>
<ol>
<li>Use the <code>XBitHack Full</code> configuration. This tells
Apache to determine the last modified date by looking only at the date
of the originally requested file, ignoring the modification date of
any included files. </li>
<li>Use the directives provided by <a
href="../mod/mod_expires.html">mod_expires</a> to set an explicit
expiration time on your files, thereby letting browsers and proxies
know that it is acceptable to cache them. </li>
</ol>
<hr>
<h2><a name="basicssidirectives">Basic SSI directives</a></h2>
<p>SSI directives have the following syntax:</p>
<pre>
<!--#element attribute=value attribute=value ... -->
</pre>
<p>It is formatted like an HTML comment, so if you don't have SSI
correctly enabled, the browser will ignore it, but it will still be
visible in the HTML source. If you have SSI correctly configured, the
directive will be replaced with its results.</p>
<p>The element can be one of a number of things, and we'll talk some
more about most of these in the next installment of this series. For
now, here are some examples of what you can do with SSI</p>
<h3><a name="today'sdate">Today's date</a></h3>
<pre>
<!--#echo var=DATE_LOCAL -->
</pre>
<p>The <code>echo</code> element just spits out the value of a
variable. There are a number of standard variables, which include the
whole set of environment variables that are available to CGI programs.
Also, you can define your own variables with the <code>set</code>
element.</p>
<p>If you don't like the format in which the date gets printed, you can
use the <code>config</code> element, with a <code>timefmt</code>
attribute, to modify that formatting.</p>
<pre>
<!--#config timefmt="%A %B %d, %Y" -->
Today is <!--#echo var=DATE_LOCAL -->
</pre>
<h3><a name="modificationdateofthefile">Modification date of the
file</a></h3>
<pre>
This document last modified <!--#flastmod file="index.html" -->
</pre>
<p>This element is also subject to <code>timefmt</code> format
configurations.</p>
<h3><a name="includingtheresultsofacgiprogram">Including the
results of a CGI program</a></h3>
<p>This is one of the more common uses of SSI - to output the results
of a CGI program, such as everybody's favorite, a ``hit counter.''</p>
<pre>
<!--#include virtual="/cgi-bin/counter.pl" -->
</pre>
<hr>
<h2><a name="additionalexamples">Additional examples</a></h2>
<p>Following are some specific examples of things you can do in your
HTML documents with SSI.</p>
<hr>
<h2><a name="whenwasthisdocumentmodified">When was this document
modified?</a></h2>
<p>Earlier, we mentioned that you could use SSI to inform the user when
the document was most recently modified. However, the actual method for
doing that was left somewhat in question. The following code, placed in
your HTML document, will put such a time stamp on your page. Of course,
you will have to have SSI correctly enabled, as discussed above.</p>
<pre>
<!--#config timefmt="%A %B %d, %Y" -->
This file last modified <!--#flastmod file="ssi.shtml" -->
</pre>
<p>Of course, you will need to replace the <code>ssi.shtml</code> with
the actual name of the file that you're referring to. This can be
inconvenient if you're just looking for a generic piece of code that
you can paste into any file, so you probably want to use the
<code>LAST_MODIFIED</code> variable instead:</p>
<pre>
<!--#config timefmt="%D" -->
This file last modified <!--#echo var="LAST_MODIFIED" -->
</pre>
<p>For more details on the <code>timefmt</code> format, go to your
favorite search site and look for <code>ctime</code>. The syntax is the
same.</p>
<hr>
<h2><a name="includingastandardfooter">Including a standard
footer</a></h2>
<p>If you are managing any site that is more than a few pages, you may
find that making changes to all those pages can be a real pain,
particularly if you are trying to maintain some kind of standard look
across all those pages.</p>
<p>Using an include file for a header and/or a footer can reduce the
burden of these updates. You just have to make one footer file, and
then include it into each page with the <code>include</code> SSI
command. The <code>include</code> element can determine what file to
include with either the <code>file</code> attribute, or the
<code>virtual</code> attribute. The <code>file</code> attribute is a
file path, <em>relative to the current directory</em>. That means that
it cannot be an absolute file path (starting with /), nor can it
contain ../ as part of that path. The <code>virtual</code> attribute is
probably more useful, and should specify a URL relative to the document
being served. It can start with a /, but must be on the same server as
the file being served.</p>
<pre>
<!--#include virtual="/footer.html" -->
</pre>
<p>I'll frequently combine the last two things, putting a
<code>LAST_MODIFIED</code> directive inside a footer file to be
included. SSI directives can be contained in the included file, and
includes can be nested - that is, the included file can include another
file, and so on.</p>
<hr>
<h2><a name="whatelsecaniconfig">What else can I config?</a></h2>
<p>In addition to being able to <code>config</code> the time format,
you can also <code>config</code> two other things.</p>
<p>Usually, when something goes wrong with your SSI directive, you get
the message</p>
<pre>
[an error occurred while processing this directive]
</pre>
<p>If you want to change that message to something else, you can do so
with the <code>errmsg</code> attribute to the <code>config</code>
element:</p>
<pre>
<!--#config errmsg="[It appears that you don't know how to use SSI]" -->
</pre>
<p>Hopefully, end users will never see this message, because you will
have resolved all the problems with your SSI directives before your
site goes live. (Right?)</p>
<p>And you can <code>config</code> the format in which file sizes are
returned with the <code>sizefmt</code> attribute. You can specify
<code>bytes</code> for a full count in bytes, or <code>abbrev</code>
for an abbreviated number in Kb or Mb, as appropriate.</p>
<hr>
<h2><a name="executingcommands">Executing commands</a></h2>
<p>I expect that I'll have an article some time in the coming months
about using SSI with small CGI programs. For now, here's something else
that you can do with the <code>exec</code> element. You can actually
have SSI execute a command using the shell (<code>/bin/sh</code>, to be
precise - or the DOS shell, if you're on Win32). The following, for
example, will give you a directory listing.</p>
<pre>
<pre>
<!--#exec cmd="ls" -->
</pre>
</pre>
<p>or, on Windows</p>
<pre>
<pre>
<!--#exec cmd="dir" -->
</pre>
</pre>
<p>You might notice some strange formatting with this directive on
Windows, because the output from <code>dir</code> contains the string
``<<code>dir</code>>'' in it, which confuses browsers.</p>
<p>Note that this feature is exceedingly dangerous, as it will execute
whatever code happens to be embedded in the <code>exec</code> tag. If
you have any situation where users can edit content on your web pages,
such as with a ``guestbook'', for example, make sure that you have this
feature disabled. You can allow SSI, but not the <code>exec</code>
feature, with the <code>IncludesNOEXEC</code> argument to the
<code>Options</code> directive.</p>
<hr>
<h2><a name="advancedssitechniques">Advanced SSI techniques</a></h2>
<p>In addition to spitting out content, Apache SSI gives you the option
of setting variables, and using those variables in comparisons and
conditionals.</p>
<h3><a name="caveat">Caveat</a></h3>
<p>Most of the features discussed in this article are only available to
you if you are running Apache 1.2 or later. Of course, if you are not
running Apache 1.2 or later, you need to upgrade immediately, if not
sooner. Go on. Do it now. We'll wait.</p>
<hr>
<h2><a name="settingvariables">Setting variables</a></h2>
<p>Using the <code>set</code> directive, you can set variables for
later use. We'll need this later in the discussion, so we'll talk about
it here. The syntax of this is as follows:</p>
<pre>
<!--#set var="name" value="Rich" -->
</pre>
<p>In addition to merely setting values literally like that, you can
use any other variable, including, for example, environment variables,
or some of the variables we discussed in the last article (like
<code>LAST_MODIFIED</code>, for example) to give values to your
variables. You will specify that something is a variable, rather than a
literal string, by using the dollar sign ($) before the name of the
variable.</p>
<pre>
<!--#set var="modified" value="$LAST_MODIFIED" -->
</pre>
<p>To put a literal dollar sign into the value of your variable, you
need to escape the dollar sign with a backslash.</p>
<pre>
<!--#set var="cost" value="\$100" -->
</pre>
<p>Finally, if you want to put a variable in the midst of a longer
string, and there's a chance that the name of the variable will run up
against some other characters, and thus be confused with those
characters, you can place the name of the variable in braces, to remove
this confusion. (It's hard to come up with a really good example of
this, but hopefully you'll get the point.)</p>
<pre>
<!--#set var="date" value="${DATE_LOCAL}_${DATE_GMT}" -->
</pre>
<hr>
<h2><a name="conditionalexpressions">Conditional expressions</a></h2>
<p>Now that we have variables, and are able to set and compare their
values, we can use them to express conditionals. This lets SSI be a
tiny programming language of sorts. <code>mod_include</code> provides
an <code>if</code>, <code>elif</code>, <code>else</code>,
<code>endif</code> structure for building conditional statements. This
allows you to effectively generate multiple logical pages out of one
actual page.</p>
<p>The structure of this conditional construct is:</p>
<pre>
<!--#if expr="test_condition" -->
<!--#elif expr="test_condition" -->
<!--#else -->
<!--#endif -->
</pre>
<p>A <em>test_condition</em> can be any sort of logical comparison -
either comparing values to one another, or testing the ``truth'' of a
particular value. (A given string is true if it is nonempty.) For a
full list of the comparison operators available to you, see the
<code>mod_include</code> documentation. Here are some examples of how
one might use this construct.</p>
<p>In your configuration file, you could put the following line:</p>
<pre>
BrowserMatchNoCase macintosh Mac
BrowserMatchNoCase MSIE InternetExplorer
</pre>
<p>This will set environment variables ``Mac'' and ``InternetExplorer''
to true, if the client is running Internet Explorer on a Macintosh.</p>
<p>Then, in your SSI-enabled document, you might do the following:</p>
<pre>
<!--#if expr="${Mac} && ${InternetExplorer}" -->
Apologetic text goes here
<!--#else -->
Cool JavaScript code goes here
<!--#endif -->
</pre>
<p>Not that I have anything against IE on Macs - I just struggled for a
few hours last week trying to get some JavaScript working on IE on a
Mac, when it was working everywhere else. The above was the interim
workaround.</p>
<p>Any other variable (either ones that you define, or normal
environment variables) can be used in conditional statements. With
Apache's ability to set environment variables with the
<code>SetEnvIf</code> directives, and other related directives, this
functionality can let you do some pretty involved dynamic stuff without
ever resorting to CGI.</p>
<hr>
<h2><a name="conclusion">Conclusion</a></h2>
<p>SSI is certainly not a replacement for CGI, or other technologies
used for generating dynamic web pages. But it is a great way to add
small amounts of dynamic content to pages, without doing a lot of extra
work.</p>
</body>
</html>