ch11.htm

美国Macmillan出版社编写的Perl教程《Perl CGI Web Pages for WINNT》

intelligence for their toasters.
<P>
But, as with most of C's features, there is a &quot;very good&quot;
and &quot;very bad&quot; side to each. The tie to UNIX makes it
a natural for UNIX boxes, and UNIX programmers. Those of us who
are working in different environments, such as Windows NT, have
special concerns that do not mesh easily with the UNIX world,
some of which have already been mentioned in this book. With UNIX,
the operating system works without the additional level of the
graphical interface that Windows NT has, making UNIX closer to
the actual operating system. This is ideal for C, which also works
closely with the operating system. With Windows NT, the graphical
interface level creates a layer of difficulty for C that has to
be overcome.
<P>
There is also no file I/O or OS interfacing in a low-level language
such as C. Many of the other features expected from a higher-level
language (such as Perl) are available only in procedural script
libraries with C. Each OS will have its own specifics when it
comes to these libraries.
<P>
There is a great debate raging, as there always is, as to whether
or not C or Perl is the superior CGI language. I have a sneaking
suspicion that those people who learned to program with C will
favor C, and those who learned with other languages will favor
those languages. Perl is a language that excels when it deals
with text manipulation. The CGI environment's biggest demand is
the ability to deal with text smoothly, making Perl an obvious
best choice.
<P>
C++ is a superset of C, meaning that its functions and operators
are built on C, so the concerns that are raised with C also apply
to C++ as far as the scope of this book is concerned.
<H4>REXX</H4>
<P>
REXX is a language that was developed at IBM in the late 1970s.
Mike Cowlishaw developed REXX as a procedural language.
<P>
One problem with REXX is that there is still no formal standard.
Although most REXX scripts are portable, there is no standard
specification that applies to REXX, so there can be irregularities
that need to be addressed when using REXX scripts from another
operating system. It is also not as popular as Perl or C for CGI
programming, so there is much less &quot;out there&quot; for REXX
in the way of support and script libraries. REXX, however, has
been ported to just about every platform, including OS/2, Macintosh,
Amiga, AS/400, and mainframes such as VM and MVS.
<P>
This is not to make the claim that Perl is the be all and end
all of CGI programming. The more a Web Master knows, the more
tricks he or she has to solve problems. Complex sites will often
contain some Perl and C scripts. In addition, the CGI languages
mentioned here are only a few of the those currently available.
Because CGI is only a specification, not a language itself, there
are no real restrictions on what can be used for it. For more
information on REXX, read M.F. Cowlishaw's book, <I>The REXX Language,
A Practical Approach to</I> <I>Programming</I> (Prentice-Hall,
1985). This is the closest thing there is to a formal REXX standard.
You can also try these Web sites:
<BLOCKQUOTE>
<PRE>
<A HREF="javascript:if(confirm('http://www.pvv.unit.no/RexxLA/index.htmlhttp:/www2.hursley.ibm.com/rexx/ftp:/rexx.uwaterloo.ca/pub/rexxfaq.txt  \n\nThis file was not retrieved by Teleport Pro, because it is addressed on a domain or path outside the boundaries set for its Starting Address.  \n\nDo you want to open it from the server?'))window.location='http://www.pvv.unit.no/RexxLA/index.htmlhttp:/www2.hursley.ibm.com/rexx/ftp:/rexx.uwaterloo.ca/pub/rexxfaq.txt'" tppabs="http://www.pvv.unit.no/RexxLA/index.htmlhttp:/www2.hursley.ibm.com/rexx/ftp:/rexx.uwaterloo.ca/pub/rexxfaq.txt">http://www.pvv.unit.no/RexxLA/index.html
http://www2.hursley.ibm.com/rexx/
ftp://rexx.uwaterloo.ca/pub/rexxfaq.txt
</A></PRE>
</BLOCKQUOTE>
<H4>Python</H4>
<P>
Python was developed in Amsterdam, The Netherlands, by Guido van
Rossum for a company called Stichting Mathematisch Centrum. It
is a new language that goes even further in being readable than
Perl.
<P>
Python, like Perl, is an interpreted language, so like Perl the
Python interpreter must be installed on your server to use it.
From the start Python was developed as an object-oriented language,
so it is a strong, effective language for programming CGI.
<P>
Python's most interesting feature is its huge library of functions.
It can use these to communicate over networks or to access system-specific
functions. There is also a CGI library that is available to programmers.
Some of the functions in the library include parsing, printing
the defined environmental variables (to aid in debugging), and
printing the contents of an HTML form. To attest to Python's usefulness
it was used to create Infoseek's search engine, and all of its
other programs. More information about Python can be found at
<BLOCKQUOTE>
<PRE>
<A HREF="javascript:if(confirm('http://www.python.org/  \n\nThis file was not retrieved by Teleport Pro, because it is addressed on a domain or path outside the boundaries set for its Starting Address.  \n\nDo you want to open it from the server?'))window.location='http://www.python.org/'" tppabs="http://www.python.org/">http://www.python.org/</A>
comp.lang.python
</PRE>
</BLOCKQUOTE>
<H2><A NAME="ServerSideIncludes"><FONT SIZE=5 COLOR=#FF0000>
Server-Side Includes</FONT></A></H2>
<P>
There is one other area of CGI that warrants attention before
concluding this section. To provide greater usefulness to your
users, you might want to have Server Side Includes as a service
available on your Web server. It is important to note that although
the NSCA and Netscape HTTP servers do support Server Side Includes,
or SSIs, at the time this book goes to press, the CERN HTTP server,
EMWAC HTTP server, and IS do not provide such support. There are,
however, several other HTTP servers available, which also support
SSIs. CERN has announced plans to make SSIs a part of their server
in the future.
<P>
Although SSIs are not strictly in the realm of the CGI, they are
included here because they may solve some problems you may have
with some of your Web pages. Remember, you want to have as many
problem-solving strategies as possible available because many
problems require more than one strategy to be solved successfully.
<P>
SSIs denote the handling of special extensions to HTML tags. Resembling
HTML documents, SSI files are very similar to HTML files. SSI
files differ from HTML in their use of a superset of the CGI environmental
variables. This does not make SSI files particular to the CGI,
because SSI files do not have to have a gateway to operate.
<P>
SSIs do not run automatically. You have to enable them on your
server before they will work. Please check your HTTP server's
documentation for the way in which to enable SSIs.
<P>
It is important to know that although SSIs add a lot to a Web
site, they also place a greater demand on your server's resources.
For SSIs to run, the server has to read, or parse, every line
of the SSI file to find the special SSI commands. If you find
your server is becoming overworked, one quick way to deal with
the traffic overload is to disable the SSI capacity on your server.
Be very careful not to identify every HTML file as an SSI extension,
or then your server will parse every HTML file that is accessed.
In turn, this creates a huge drain on your server's resources
and a time lag in satisfying the client's request. Typical SSI
files have .shtml as their file extension. 
<P>
Comparing SSIs with HTML will give you a clearer picture. Because
SSIs are parsed by the server into HTML before they go to the
browser, an SSI file looks very similar to an HTML file. This
is file jazz.shtml.
<BLOCKQUOTE>
<PRE>
&lt;HTML&gt;
&lt;HEAD&gt;
&lt;TITLE&gt;Jazz on the Web&lt;/TITLE&gt;
&lt;/HEAD&gt;
&lt;BODY&gt;
&lt;H1&gt;Jazz on the Web!&lt;/H&gt;&lt;BR&gt;
&lt;H2&gt;
This site was last modified on &lt;!--#echo var=&quot;LAST_MODIFIED&quot; --&gt;
&lt;/H&gt;
&lt;HR&gt;
&lt;A HREF=&quot;http://town.hall.org/Archives/radio/Kennedy/Taylor/&quot;&gt;&lt;H1&gt;Jazz Styles&lt;/A&gt;&lt;/H&gt;
&lt;A HREF=&quot;http://www.yahoo.com/Entertainment/Music/Artists/By_Genre/Jazz/&quot;&gt;
&lt;H1&gt;Jazz Musicians&lt;/A&gt;&lt;/H&gt;
&lt;A HREF=&quot;http://www.yahoo.com/Entertainment/Music/Genres/Jazz/Labels/&quot;&gt;
&lt;H1&gt;Jazz Labels&lt;/A&gt;&lt;/H&gt;&lt;BR&gt;&lt;BR&gt;
&lt;H2&gt;The jazz quote of the day is:&lt;/H&gt;&lt;BR&gt;
&lt;!--#include virtual=&quot;quotes&quot; file=&quot;jazzquotes.html&quot; --&gt;
&lt;/BODY&gt;
&lt;/HTML&gt;
</PRE>
</BLOCKQUOTE>
<P>
This file looks like a regular HTML document, except for two lines.
The first line shows a very common use of SSI, which is to keep
a running update on when the page was last modified.
<P>
The server reads the line
<BLOCKQUOTE>
<PRE>
This site was last modified on &lt;!--#echo var=&quot;LAST_MODIFIED&quot; --&gt;
</PRE>
</BLOCKQUOTE>
<P>
and sees that it has to resolve the variable LAST_MODIFIED, which
it then echoes to the client in HTML. The second line not standard
to HTML is
<BLOCKQUOTE>
<PRE>
&lt;!--#include virtual=&quot;quotes&quot; file=&quot;jazzquotes.html&quot; --&gt;
</PRE>
</BLOCKQUOTE>
<P>
which the server parses and then sees it has to add the file jazzquotes.html
to the page before it sends the data to the browser. The end result
looks something like Figure 11.2.
<P>
<A HREF="f11-2.gif" tppabs="http://210.32.137.15/ebook/PC%20Magazine%20Programming%20Perl%205.0%20CGI%20Web%20Pages%20for%20Microsoft%20Windows%20NT/f11-2.gif"><B>Figure 11.2 :</B> <I>An example SSI file parsed in HTML</I>.</A>
<P>
It is interesting to note that when the server returns the called
SSI file, jazz.quotes.html, it not only attaches the file to the
new, parsed HTML file, but in the exact place where the SSI line
of code is in the unparsed .shtml file.
<P>
If your haven't already noticed, SSI commands are an adaptation
of the HTML comment tags. This was intentional, so that if you
move your HTML documents, which contain SSIs, to another server,
they will look the same, regardless of whether that server supports
SSIs or not. Inside the HTML comment form the SSI syntax looks
like this:
<BLOCKQUOTE>
<PRE>
&lt;!--#command cmd_argument=&quot;argument_value&quot; --&gt;
</PRE>
</BLOCKQUOTE>
<P>
where the command is a special SSI command, the cmd_argument is
related to the SSI command, and the argument_value is based on
the cmd_argument.
<H3><A NAME="SSISyntax">
SSI Syntax</A></H3>
<P>
There are six commands for SSI. They are config, echo, exec, fsize,
flastmod, and include. Their functions are listed in Table 11.1.
<BR>
<P>
<CENTER><B>Table 11.1 SSI Syntax</B></CENTER>
<P>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD WIDTH=108><CENTER><B>Command</B></CENTER></TD><TD WIDTH=468><B>Function</B>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>config</TD><TD WIDTH=468>This sets the format of the size, time, or error messages.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>echo</TD><TD WIDTH=468>This will place the value of the SSI variables into an HTML document.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>exec</TD><TD WIDTH=468>Will execute a CGI program or system command, and output the result into the HTML document.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>Fsize</TD><TD WIDTH=468>Places the size of the file in the HTML document.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>Flastmod</TD><TD WIDTH=468>Places the date of the last modification of an HTML document into that HTML document.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=108>Include</TD><TD WIDTH=468>Places the contents of other HTML documents into the HTML document, or specified data file.
</TD></TR>
</TABLE></CENTER>
<P>
<P>
When working with SSI commands, it is important to remember these&nbsp;rules:
<UL>
<LI>SSI syntax is based on UNIX commands, which are case-sensitive.
Config is not the same as config.
<LI>SSI requires the right file extension if it is to be recognized
and parsed by the server. Make sure your SSIs are .shtml. You
can also turn parsing on for all documents on your server, or
set the file's attributes, like the execute or archive bit. You
also must make sure that your server is aware of this file type
by associating the extension using File Manager.