Internetworking (1.3): Workshop-Server-Side Includes

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WORKSHOP
Server-Side Includes
Pawan Vora, pvora@uswest.com

I prefer to include at least 2 pieces of information on a Web page: Last modified date and the Web page's URL. But because I create pages from templates, often I forget to change the last modified date and URL. This results in several pages having the same last modified date and the URL. Also, sometimes I prefer to have a left navigation bar that repeats on all the pages of the site. Although having a template helps me, it becomes a nightmare when I have to add an item to or remove an item from the navigation bar, or even reorder the items in the navigation bar. I found a solution to these problems in Server-Side Includes (SSI), which helped me solve my problems without any complicated programing or scripting--with HTML like tags.
To understand SSI better, let's break the term "Server-Side Include" into "Server-Side" and "Include." Server-side means that the processing takes place on the Web server. This also means that you won't see the results if you are not requesting the Web page from a Web server. And, Include means that you include these special tags in your HTML code of the Web page.
This is what an SSI tag looks like:


As you may notice, it looks like an HTML comment tag. What makes this comment-like tag an SSI tag is the presence of the # sign, which indicates to the Web server that this is a special tag and needs its attention. The server then processes this special tag -- and doesn't ignore it like a COMMENT tag -- and includes the results of the processing in the HTML document. In the above example, the server will include the last modified date of the HTML file being served.
Because the Web server does the processing, you want to make sure that your Web server supports Server-Side Includes before including SSI tags on your Web page. Also, you need to make sure that it is enabled on your Web server. By default, Web servers come with the processing of Server-Side Includes disabled. Not all Web servers refer to this functionality as Server-Side Includes. Some Web servers refer to them as the Parse HTML feature. This is because when SSI is enabled the server must parse (i.e., read) the HTML file before sending it to the client (for example, the user's Web browser).
Furthermore, files using SSIs may need to use a special file extension, usually .shtml, that is set up in the server configuration file. But you can always set up your server to implement SSIs even in files ending in .htm or .html. You may want to check with your server administrator whether it is necessary to change the file extension in your files that utilize SSIs.
Be careful with SSI syntax
As you saw before, an SSI command looks like:


Notice that the # sign immediately follows 
you would see the contents of navigation.shtml file included in the content.shtml file.

Including Environmental Variables
Simply stated, environmental variables store the information about the computer system and the files and directories on it. Some of the commonly used environmental variables are:

Environmental Variable What it represents

SERVER_NAME Domain name of the Web server

REMOTE_HOST User (client) computer's domain name

HTTP_USER_AGENT The user's Web browser

DATE_GMT Current date and time in Greenwich Mean Time (GMT)

DATE_LOCAL Current date and time in your local time zone (e.g., MST)

DOCUMENT_NAME File name of the Web page being displayed in the browser

DOCUMENT_URL Virtual path to the file relative to the Web server's document root

LAST_MODIFIED Date and time the current file was modified

Above SSI commands are used with the syntax . So, for example, to get the last modified date, you will write the SSI tag as follows:



Get Information About Files
There are two SSI commands to help you get information about files:

#fsize, which returns the size of the file
#flastmod, which returns the last time and date the file was modified

#fsize and #flastmode commands are particularly useful when you are allowing people to download files from your Web sites. If these files are large, it's a good practice to indicate the size of the file and when it was last modified. This way, if the file you are allowing people to download changes in some respect, you don't need to update the Web page to change the file size or last modified date information. #fsize and #flastmode can display this information for you.
Example:
Download xxx.exe, size: , last modified on 

Change the format of the information displayed by the Web server
When you use the SSI tag #flastmod, the default format is: Day, dd-Mon-yy hh:mm:ss (e.g., Sunday, 13-Dec-98 20:21:22). Now, if you want it displayed in mm/dd/yy format, you can use the config timefmt command as follows:

See Appendix B (on this page) for more on timefmt
You can also use config sizefmt command to change the format in which the file size is displayed to the user. So, the default format to display the file size using #fsize is in nK (e.g., 32K, 100K, etc.). You can configure it to show the size in bytes as follows:


Running programs from within a page itself
To run a CGI program from the page itself, you can use the SSI tag exec. A common application is to include a CGI program that shows number of hits on a page. Of course, you need to have the CGI program avilable to do so. For example, if you have a counter.cgi program in your cgi-bin directory, you can use it on a page as follows:

You can also run Shell commands using the exec tag. For example, to display the calendar on a UNIX-based Web server, you can use:
<pre></pre>
In the above example, I have used <pre> to make sure that the calendar displays properly in a mono-spaced font.
Your Webmaster may not like using exec SSIs because they can compromise the security of the Web server. Allowing use of exec makes it possible for a hacker to execute potentially destructive commands within the HTML page itself.

Common mistakes with the use of SSI

Forgetting the pound (#) sign. If you forget the # sign, the Web server interprets it as a regular HTML comment and ignores it.

Using "smart" or "curly" quotes

Upper and lowercase problems. Make sure that you type the commands as they appear in the list below. Otherwise, you will either get an error or the server may treat the SSI command as the comment (especially when the first character after <-- is not a #.

Typos and spaces. The most obvious and the most common mistake is because of the typos and missing or extra spaces. Often, extra spaces show up before and after the '='.

Missing double-quotes. Make sure that the right side of the SSI command (after =) is enclosed in double-quotes.

HTML parsing not enabled. Make sure that your Web server supports Server-Side Includes (or HTML parsing) and that it is enabled. Otherwise, the Web server will treat it as an HTML comment and ignore it. If your SSI command is correct, and you don't get an error message, the very likely reason is that the SSI is not enabled on the Web server.

Not making a server request when requesting a page with SSI commands. If you simply open the page with SSI commands in a browser without making the server request, you will not see any errors since the browser will treat the SSI commands as comments. Remember, SSI means Server-Side Include, so the page must be served by a Web server.

One mistake many people make in using SSI is when embedding the SSI tags such as LAST_MODIFIED and DOCUMENT_URI in the "included" file. It's better to explain this with an example. Suppose you had an HTML file called document_info.shtml, in which you wanted to include some copyright information, last modified date, and the URL. So, you created a file, footer.shtml, as follows:
© XYZ Corporation  
Although it seems to be okay and you don't get any errors, what you will see included in the document_info.shtml file is the last modified date of the file footer.shtml and the path to the file footer.shtml insted of that of document_info.shtml. So, note that these tags are file specific and should be included in the main file, in this case in document_info.shtml file.

List of commonly used SSI Commands


To include a file with a relative path. Can not refer to a file above the current directory.


To include a file with an absolute path from the document root. The paths must begin with a "/".


Displays the date and time the file was last modified.


Displays the file name.


Displays the user (client) computer's domain name (e.g., www.sandia.gov).


Displays the IP address (e.g., 209.181.87.94).


Displays the user (client) browser information (e.g., Mozilla/4.03 [en]C-USWC0810 (WinNT; U).


Displays the Web server's domain name (www.uswest.com).


Displays the current date and time in Greenwich Mean Time (GMT).


Displays the current date and time in the local time zone.


Displays the path of the file from the Web server's document root (e.g., /itg/internetworking/dec98/ssi_workshop.html).


Displays the domain name of the Web server (e.g., www.sandia.gov).


Displays the server port number (e.g., 80).


Displays the Web server software (e.g., Netscape-Enterprise/3.5.1).


Displays the base URL of the Web server including the protocol (e.g., http://www.sandia.gov).


Displays the last modified date of the file. The file path is shown as a relative path.


Displays the last modified date of the file. The file path is specified as an absolute path from the Web server's document root.


Displays the size of the specified file. The file path is specified as a relative path. (e.g., 3K)


Displays the size of the specified file. The file path is specified as an absolute path from the Web server's document root. (e.g., 3K)


Displays the time in mm/dd/yy format.


Displays the time format in Day, month date, year format.


Displays the size of the file in bytes (instead of Kilobytes).


Displays the size of the file either in kilobytes (K) or megabytes (MB).

<pre></pre>
Displays the results of the UNIX command cal. PRE is used to format the displays in mono-spaced font.

Appendix B: List of formating modifiers for Displaying Date
(to be used with config timefmt)

Modifier What does it display? Example

%A Day of the week Wednesday

%a Day of the week (abbreviated) Wed

%B Month of the year January

%b Abbreviated month of year Jan

%c Date & time,
formatted as per local preferences Sun Dec 13 01:10:16 1998

%d Day of the month 07

%H Hour of the day (24 hour clock) 22

%I Hour of the day (12-hour clock) 10

%j Day of year 350

%M Minutes past the hour 10

%m Month of year (01-12) 12

%p AM or PM PM

%S Seconds past the minute 43

%U Week of the year (00-53)
(with Sunday as the first day of the week) 50

%W Week of the year (00-53)
(With Monday as the first day of the week) 49

%w Weekday as a decimal number (0-6)
(with Sunday as the first day of the week)
06

%X Time, as formatted per local preferences 01:10:16

%x Date, as formatted per local preferences 12/07/98

%Y Year, 4 digits 1998

%y Year, 2 digits 98

%Z Time zone (abbreviated) EST

References

NCSA Server Side Includes (SSI). http://hoohoo.ncsa.uiuc.edu/docs/tutorials/includes.html
How To: Server-Side Includes (SSIs). http://www.sps.edu/Academics/AIS/Webweaver/help/HowTo_SSI.shtml

© Internet Technical Group
Last update: December 5, 1998
URL: http://www.sandia.gov/itg/newsletter/dec98/ssi.html
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