1. Output

   CGI program works as a filter. It gets input from client and output the processed result. Usually CGI program outputs a HTML file. It is possible to return binary data or send HTTP status codes. _cgi command is on the header of HTTP of a CGI program. The simplest header is 'Content-type: text/html'. It specifies that the output has a HTML format. The 'Content-length' header specifies the length (in bytes) of a binary data. 'Expires' header specifies the expire date and time of a document for reloading.

   In addition, 'Status' specifies the status of a request. It includes three-digit numerical status code. Usually, value 200 indicates success; 204 is no response; 301 means document moved; 400 has a bad request; 401 is unauthorized; 403 is forbidden; 404 represents not found; 500 has internal server error; 501 means not implemented, and so on.

   The rarely used 'Location' header specifies the server redirection; 'Pragma' header controls document cache; and 'Refresh' header lets client reload document. Finally, 'Set-Cookie' header stores a tag data on the client for tracing. The 'Location' and 'Status' must be used in a complete header specification.

   When a CGI program returns a complete HTTP header, there is no overhead for server. Its output is directly sent to the client. Therefore, server can not intercept the output errors. In the NCSA server, CGI program must have a file name with nph- prefix for complete header.

   #!/usr/local/bin/snaml
   _cgi "Content-type: text/html"
   _html
   _head
     _title 
       _quote "CGI Environment Variables" 
     title_ 
   head_
   _body
     # list environment variables 
     foreach key [array::names ::env] {
       _quote "env($key) = $::env($key)"
       _br
     }
   body_
   html_
       

2. Example

   The status of Neatware web site is:

   env(GATEWAY_INTERFACE) = CGI/1.1 
   env(DOCUMENT_ROOT) = /home/neatware/HTML 
   env(SERVER_PORT) = 80 
   env(HTTP_HOST) = www.neatware.com 
   env(REMOTE_ADDR) = 209.148.144.12 
   env(HTTP_CONNECTION) = Keep-Alive 
   env(HTTP_PRAGMA) = no-cache 
   env(SERVER_NAME) = www.neatware.com 
   env(SERVER_PROTOCOL) = HTTP/1.0 
   env(REMOTE_PORT) = 1215 
   env(REQUEST_METHOD) = GET 
   env(SERVER_SOFTWARE) = Apache/1.2.6 FrontPage/3.0.4 
   env(REMOTE_HOST) = 209.148.144.12 
   env(QUERY_STRING) = 
   env(HTTP_USER_AGENT) = Mozilla/3.0Gold (Win95; I) 
   env(PATH) = /usr/local/bin:/usr/bin:/bin 
   env(SCRIPT_NAME) = /cgi/status.cgi 
   env(SCRIPT_FILENAME) = /home/neatware/HTML/cgi/status.cgi 
   env(HTTP_ACCEPT) = image/gif, image/x-xbitmap, 
     image/jpeg, image/pjpeg, */* 
       

Output

Example