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Using Server Side Includes at Martnet
A brief primer on how to set up server side includes.

Using Server Side Includes


Simply put, a Server Side Include is a command placed inside the HTML code, which is then read by our Web Server and run whenever someone calls up the Web Page. A Server Side Include can be used to embed the output of a CGI program (such as a hit counter) into a page, call external files (such as a common "footer" file) and include it in a page, and put the date and time the page was last modified on the page. There are many other possibilities, and you can use multiple commands to create truly dynamic pages.


Enabling Server-Side Includes

To use Server Side Includes in any Web Page hosted at MartNet, you MUST end your html file names with the suffix .shtml (ie. index.shtml) If you do not end your html file names with .shtml suffix, our Web Servers will not parse the document or read and execute your Server Side Include commands! There is nothing more you need to do to use Server Side Includes with your MartNet Hosted Web Site!


Some Common Uses plus sample code

Note that Server Side Includes usually expect a path to a file, NOT a URL(as in http://www...) as part of the command.

Calling a CGI
The following code Embeds a CGI (such as a hit counter) in a page, with the CGI's output displayed as part of the page:

A User-installed CGI:

<!--#include virtual="/cgi-bin/cgiwrap/USERNAME/program.cgi/" -->


A publically available CGI (Fortune Teller):

<!--#include virtual="/cgi-bin/public/fortune.cgi" --><BR>
 

Displaying document information
The following code puts the document modification date and time on the page:
Last modified: <!--#echo var="LAST_MODIFIED" -->

Adding text (such as a footer) to many documents
Add the following text to one or many documents: (view the source of this page for an example. The MartNet Support Site heavily uses this Server Side Include command, with shared header and footer files, to make managing and updating the site as easy as possible.)
<!--#include file="footer_support.shtml" -->

Hide links from external users
Use the if command and the REMOTE_ADDR CGI variable to see if the user is in the local domain:
<!--#if expr="$REMOTE_ADDR != /^1.2.3./" --> <!--#endif -->
(Where 1.2.3 is the IP address prefix of the local domain).


Other resources:

Apache mod_include Documentation We proudly use Apache Web Servers here at MartNet, and this page contains the original Server Documentation for the mod_include Apache module, the little piece of software that makes Server Side Includes actually work. A lot of the content on this page was copied from this documentation, and this page contains technical information for those interested in how everything works on the Server end.

NCSA Tutotial on Server Side Includes NCSA came up with the Server Side Include standard a long time ago, and this basic turorial explains the technical nitty-gritty of how it all works.

Apache Week - Using Server Side Includes a short, not-quite-so-technical article on using Server Side Includes to enhance your Web Site. The Apache Week Web Site contains a lot of additional information that may interest serious Web Developers, and help you take advantage of the advanced features of the Apache Web Server.


Basic Elements and Command Syntax

Note: the following section was taken from the Apache Web Server Documentation, and throughly explains the use and command syntax of available Server Side Include commands. We have modified it slightly so that it accurately reflects the features available on our Web Servers. The original document is available Here.

The document is parsed as an HTML document, with special commands embedded as SGML comments. A command has the syntax:

<!--#element attribute=value attribute=value ...-->

The value will often be enclosed in double quotes; many commands only allow a single attribute-value pair. Note that the comment terminator ( --> ) should be preceded by whitespace to ensure that it isn't considered part of an SSI token.

The allowed elements are:

config
This command controls various aspects of the parsing. The valid attributes are:
 
errmsg
The value is a message that is sent back to the client if an error occurs whilst parsing the document.
 
sizefmt
The value sets the format to be used which displaying the size of a file. Valid values are bytes for a count in bytes, or abbrev for a count in Kb or Mb as appropriate.
 
timefmt
The value is a string to be used by the strftime(3) library routine when printing dates.

echo
This command prints one of the include variables, defined below. If the variable is unset, it is printed as (none). Any dates printed are subject to the currently configured timefmt. Attributes:
 
var
The value is the name of the variable to print.

exec
The exec command executes a given shell command or CGI script. This command is obselete, and has been replaced by the include command. The exec command will not work on MartNet's Web Servers (it has been intentionally disabled for security reasons) and is included here only for the sake of completeness.
 

fsize
This command prints the size of the specified file, subject to the sizefmt format specification. Attributes:
 
file
The value is a path relative to the directory containing the current document being parsed.
 
virtual
The value is a (%-encoded) URL-path relative to the current document being parsed. If it does not begin with a slash (/) then it is taken to be relative to the current document.

flastmod
This command prints the last modification date of the specified file, subject to the timefmt format specification. The attributes are the same as for the fsize command.

include
This command inserts the text of another document or file into the parsed file. Any included file is subject to the usual access control. If the directory containing the parsed file has the Option IncludesNOEXEC set, and the including the document would cause a program to be executed, then it will not be included; this prevents the execution of CGI scripts. Otherwise CGI scripts are invoked as normal using the complete URL given in the command, including any query string.

An attribute defines the location of the document; the inclusion is done for each attribute given to the include command. The valid attributes are:

 
file
The value is a path relative to the directory containing the current document being parsed. It cannot contain ../, nor can it be an absolute path. The virtual attribute should always be used in preference to this one.
 
virtual
The value is a (%-encoded) URL relative to the current document being parsed. The URL cannot contain a scheme or hostname, only a path and an optional query string. If it does not begin with a slash (/) then it is taken to be relative to the current document.

A URL is constructed from the attribute, and the output the server would return if the URL were accessed by the client is included in the parsed output. Thus included files can be nested.


printenv
This prints out a listing of all existing variables and their values. No attributes.
For example: <!--#printenv -->
Apache 1.2 and above.

set
This sets the value of a variable. Attributes:
 
var
The name of the variable to set.
 
value
The value to give a variable.

For example:

<!--#set var="category" value="help" -->
Apache 1.2 and above.


Include Variables

In addition to the variables in the standard CGI environment, these are available for the

echo command, for if and elif, and to any program invoked by the document.

DATE_GMT
The current date in Greenwich Mean Time.
 
DATE_LOCAL
The current date in the local time zone.
 
DOCUMENT_NAME
The filename (excluding directories) of the document requested by the user.
 
DOCUMENT_URI
The (%-decoded) URL path of the document requested by the user. Note that in the case of nested include files, this is not then URL for the current document.
 
LAST_MODIFIED
The last modification date of the document requested by the user.


Variable Substitution

Variable substitution is done within quoted strings in most cases where they may reasonably occur as an argument to an SSI directive. This includes the config, exec, flastmod, fsize, include, and set directives, as well as the arguments to conditional operators. You can insert a literal dollar sign into the string using backslash quoting:

<!--#if expr="$a = $test" -->

If a variable reference needs to be substituted in the middle of a character sequence that might otherwise be considered a valid identifier in its own right, it can be disambiguated by enclosing the reference in braces, la shell substitution:

<!--#set var="Zed" value="${REMOTE_HOST}_${REQUEST_METHOD}" -->

This will result in the Zed variable being set to "X_Y" if REMOTE_HOST is "X" and REQUEST_METHOD is "Y".

EXAMPLE: the below example will print "in foo" if the DOCUMENT_URI is /foo/file.html, "in bar" if it is /bar/file.html and "in neither" otherwise:

<!--#if expr=""$DOCUMENT_URI" = "/foo/file.html"" --> in foo <!--#elif expr=""$DOCUMENT_URI" = "/bar/file.html"" --> in bar <!--#else --> in neither <!--#endif -->


Flow Control Elements

The basic flow control elements are:

<!--#if expr="<EM>test_condition</EM>" --> <!--#elif expr="<EM>test_condition</EM>" --> <!--#else --> <!--#endif -->

The

if element works like an if statement in a programming language. The test condition is evaluated and if the result is true, then the text until the next elif, else. or endif element is included in the output stream.

The

elif or else statements are be used the put text into the output stream if the original test_condition was false. These elements are optional.

The

endif element ends the if element and is required.

test_condition is one of the following:

string
true if string is not empty
string1 = string2
string1 != string2
string1 < string2
string1 <= string2
string1 > string2
string1 >= string2
Compare string1 with string 2. If string2 has the form /string/ then it is compared as a regular expression. Regular expressions have the same syntax as those found in the Unix egrep command.
( test_condition )
true if test_condition is true
! test_condition
true if test_condition is false
test_condition1 && test_condition2
true if both test_condition1 and test_condition2 are true
test_condition1 || test_condition2
true if either test_condition1 or test_condition2 is true

"=" and "!=" bind more tightly than "&&" and "||". "!" binds most tightly. Thus, the following are equivalent:

<!--#if expr="$a = test1 && $b = test2" --> <!--#if expr="($a = test1) && ($b = test2)" -->

Anything that's not recognized as a variable or an operator is treated as a string. Strings can also be quoted: 'string'. Unquoted strings can't contain whitespace (blanks and tabs) because it is used to separate tokens such as variables. If multiple strings are found in a row, they are concatenated using blanks. So,

<EM>string1 string2</em> results in <EM>string1 string2</em> <EM>'string1 string2'</em> results in <EM>string1 string2</em>

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