How web servers provide HTTP GET/POST to CGI apps

 HTTP client---(HTTP GET/POST)---web server---(Environment variables/stdin)---CGI app

• For an HTTP GET request, web servers receive a query string from the client, then put it as Environment variable QUERY_STRING into the CGI app's process.
• For an HTTP POST request, web servers receive form data from the client, then write to the CGI app's standard input, and denote the data length by Environment variable CONTENT_LENGTH.

Below is a demonstration.

CGI app

As environment variables and standard input are used by any process, any application can work as a CGI app.

Below is a simple bash script CGI app that prints out HTTP GET/POST data.

$ cat ./cgi-bin/app.sh

#!/bin/bash

# http header

echo -n 'Cotent-Type: text/plain'

echo -ne '\r\n\r\n'

# http GET query string becomes cgi app's environment variable $QUERY_STRING

echo $QUERY_STRING

# http POST form data becomes input to cgi app's stdin

# the data length is saved in $CONTENT_LENGTH

if [[ ! -z  $CONTENT_LENGTH ]]; then

    read -n $CONTENT_LENGTH post_data

    echo $post_data

fi

Web server

For simplicity, python's http.server module is used as our Web server.

$ python -m http.server --cgi 8000

HTTP client

Any web client works but here we use curl for demonstration.

$ curl --data 'form data or json' 'http://192.168.0.31:8000/cgi-bin/app.sh?x=2&y=2'

x=2&y=2

form data or json