CGI Startup Guide for COSI developers

CGI mechanics


HTML Syntax

A CGI script is referred to using a URL like any HTML document. The difference is that CGI scripts normally need some parameters in its standard input, based on which they generate something meaningful (in HTML format) to their standard output. This input is usually generated by using a "front end" HTML page with an input form.

In its input form the "front end" HTML page specifies an action to take when the client submits the input [for further processing]. E. g.:
<form name="PrimTextForm" action="" method="Post">
<form name="PrimTextForm" action="/cgi-bin/cgi_prim/cgi_prim.cgi" method="Post">
How does the webserver understand that the CGI script needs to be executed rather than displayed like a regular HTML page? CGI scripts reside in a special directory ("cgi-bin" in the above example). When a webserver receives a request for a page from this directory it will not display it, but rather run it and send back what the program printed to its standard output.


CGI syntax

A CGI script can be anything as long as the webserver can execute it: a binary program, a set of shell commands, a script in any language, etc. The first step to make sure it works is to log into the shell of the webserver and try running it in command line. Unless special modifications are made, only "normal" standard output will be displayed to the client browser: error messages will be seen in shell only.


Exchange format

Data sent and received by the CGI script has different formats.

Text input

The client browser replaces all spaces with '+' and a good deal of non-alphanumeric characters with their hexadecimal values. Besides, each input is prepended by its input field name followed by '='.

File input

The client browser converts the input into [Quoted-Printable] MIME format.


The client browser expects the output to begin with "Content-type: text/html" followed by data in HTML format.



The program displays what the web browser sends to its standard input


<html><head><title>CGI primer</title>
<script language="JavaScript"> <!--
 function ChkBlank(FormField) {
  if(FormField.value == "") {
   alert("The input field is empty");
  else {

<body><h1>CGI primer</h1>

<h2>Cut'n'paste input</h2>
 <form name="PrimTextForm" action="" method="Post" onSubmit="return ChkBlank(document.PrimTextForm.InText);">
  <textarea name="InText" rows="5" cols="80"></textarea><br>
  <input type="reset" value="Reset"> <input type="submit" value="Analyse">

<h2>Input by filename</h2>
 <form name="PrimFileForm" action="" method="Post" enctype="multipart/form-data" onSubmit="return ChkBlank(document.PrimFileForm.InFile);">
  <input type="file" name="InFile">
  <input type="submit" value="Analyse">



gdragon@sc8-pr-shell1:/home/groups/c/co/cosi-nms/cgi-bin/cgi_prim$ ls -al
total 12
drwxr-sr-x    2 gdragon  cosi-nms     4096 Mar 14 23:59 .
drwxrwsr-x    4 dummy    cosi-nms     4096 Mar 14 23:28 ..
-rwxr-xr-x    1 gdragon  cosi-nms      234 Mar 14 23:33 cgi_prim.cgi
gdragon@sc8-pr-shell1:/home/groups/c/co/cosi-nms/cgi-bin/cgi_prim$cat cgi_prim.cgi

echo Content-type: text/html
echo "<html><head><title>CGI primer</title></head>"
echo "<body><h1>CGI primer</h1>"
echo "("
echo | date
echo ")<hr>"
echo "<pre>"
echo "</pre>"
echo "</body>"
echo "</html>"
gdragon@sc8-pr-shell1:/home/groups/c/co/cosi-nms/cgi-bin/cgi_prim$ echo "Test input string" | cgi_prim.cgi
Content-type: text/html

<html><head><title>CGI primer</title></head>

<body><h1>CGI primer</h1>
Sat Mar 15 02:06:06 PST 2003
Test input string

See Example in action
          * *
        *     *
     ||*       *||
     || *     * ||
    ||||  * *  ||||

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