Sockets in shell

Recently, I read an article on Sockets in your shell and I thought it was amazing find for me.

Lot of times especially working with docker, I had to install tools like telnet, nc and curl to check basic connectivity.

Little did I realize that this functionality is inbuilt into shell.

For ex, let’s say we have a basic http server running in local as follows:

const http = require('http');

const requestListener = function (req, res) {
    console.log("Request recieved");
    res.writeHead(200);
    res.end('Hello, World!');
}

const server = http.createServer(requestListener);
server.listen(8080);

How would you send the request to this port without telnet, browser, curl and nc from terminal? Answer is:

printf "GET /\n\n" > /dev/tcp/localhost/8080

Its as simple as that. The only downside is that it wont be possible to know if the data was successfully submitted to port 8080 or not. However, there is a simple workaround for that:

#!/bin/bash
if exec 3>/dev/tcp/localhost/8000 ; then
	echo "server up!"
else
	echo "server down."
fi

Quoting from the article:

If you’re unfamiliar, exec without any arguments is used to redirect file descriptors and files. By associating fd 3 with /dev/tcp/localhost/4000, it attempts to create a file there and thus a connection. We use > to open the socket > for writing, although we don’t need to write anything in this case.