In the early 2000s, we used to write a lot of CGI programs.
This was the primary way to make websites dynamic at the time. These CGI programs were usually written in Perl, but sometimes in C or other languages to increase performance.
The CGI mechanism is conceptually simple but powerful. When the web server receives an incoming request for a CGI script (e.g. /~jakegold/cgi-bin/guestbook.cgi), it:
Sets up environment variables containing request metadata (HTTP headers, query parameters, request method, etc.) Spawns a new process to execute the CGI program Passes the request body (if any) to the program via stdin Captures the program’s stdout as the HTTP response Sends any error output from stderr to the error log The CGI program reads the environment variables to understand the request, processes it, and writes an HTTP response to stdout, starting with headers.

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