Saturday April 19 2014
information technology central services at the university of michigan

Server Side Includes

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What is a Server Side Include?

An SSI is a command embeded in an HTML comment in a web page. This allows you to have dynamic web pages without having to write your own CGI software. When the web server is asked for your home page (i.e. if someone opens up your page in her browser), the server reads through (or parses) your page for commands that look like this:

<!--#element name="test" value="1" -->

These commands are interpreted by the web server and their output is shown to the person viewing your page. Note: Only files ending in ".html" or ".shtml" are currently parsed by the server. If your file ends in ".htm" the server will not parse it for SSIs.

An example of an SSI is the line at the beginning of this FAQ which begins "This page last updated." If you view the source of this page, all you see is "This page last updated:" followed by the date and time of last modification. However, were you to look at the actual .html file, you would see this command:

<!--#echo var="LAST_MODIFIED" -->

What SSIs are available, and how do I use them?

This text is from Apache's mod_include documentation, but has been modified for use at UM.

SSIs that will work on the UM Web servers 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.

var

The value is the name of the variable to print.

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 including the document would cause a program to be executed, then it will not be included; this prevents the execution of CGI scripts.

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 -->

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" -->

Working with variables

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, à 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="test_condition" -->
<!--#elif expr="test_condition" -->
<!--#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

The operators "=" 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,

string1    string2  results in string1 string2
'string1    string2' results in string1    string2