You're
starting to get the hang of CGI, but aren't too thrilled with the
fact that you have to write full-fledged CGI programs even when
you want to output a document with only a minimum amount of dynamic
information, right? For example, say you want to display the current
date and time, or a certain CGI environment variable in your otherwise
static document. You can go through the trouble of writing a CGI
program that outputs this small amount of virtual data, or better
yet, you can use a powerful feature called Server Side Includes
(or SSI).
Server Side Includes are directives which you can place into
your HTML documents to execute other programs
or output such data as environment variables and file statistics.
Unfortunately, not all servers support these directives; the CERN
server cannot handle SSI, but the servers from NCSA
and Netscape can. However, there is a CGI program called
fakessi.pl that
you can use to emulate Server Side Includes if your server does
not support them.
While Server Side Includes technically are not really CGI,
they can become an important tool for incorporating CGI-like information,
as well as output from CGI programs, into documents on the Web.
How do Server Side Includes work? When the client requests
a document from the SSI-enabled server, the server parses the specified
document and returns the evaluated document (see Figure 5.1). The
server does not automatically parse all files looking for SSI directives,
but only ones that are configured as such. We will look at how to
configure documents in the next section.
SSI sounds like a great feature, but it does have its disadvantages.
First, it can be quite costly for a server to continually parse
documents before sending them to the client. And second, enabling
SSI creates a security risk. Novice users could possibly embed directives
to execute system commands that output confidential information.
Despite these shortcomings, SSI can be a very powerful tool if used
cautiously.
Table 5.1 lists all the SSI directives. In this chapter, I'll
discuss each of these directives in detail.
Table 5.1: SSI Directives
|
Command |
Parameter |
Description |
|
echo |
var |
Inserts value of special SSI variables
as well as other environment variables |
|
include |
|
Inserts text of document into current
file |
|
|
file |
Pathname relative to current directory |
|
|
virtual |
Virtual path to a document on the server |
|
fsize |
file |
Inserts the size of a specified file |
|
flastmod |
file |
Inserts the last modification date and
time for a specified file |
|
exec |
|
Executes external programs and inserts
output in current document |
|
|
cmd |
Any application on the host |
|
|
cgi |
CGI program |
|
config |
|
Modifies various aspects of SSI |
|
|
errmsg |
Default error message |
|
|
sizefmt |
Format for size of the file |
|
|
timefmt |
Format for dates |
![[Graphic: Figure 5-1]](./figs/cgi0501.gif)
