How do I send a message to my socket.io websocket from the command line in linux?

You can write a simple client like this (let's name it client with no extension):

#!/usr/bin/env node
const socket = require('socket.io-client')('http://localhost:3000');
const someDelay = 10;
socket.on('connect', function () {
    console.log('connected...');
    if (process.argv[2] && process.argv[3]) {
        console.log('sending ' + process.argv[2] + ': ' + process.argv[3]);
        socket.emit(process.argv[2], process.argv[3]);
        setTimeout(() => {
            process.exit(0);  
        }, someDelay);
    } else {
        console.log('usage: ./client.js <event> <data>');
        process.exit(1);
    }
});

with a very basic package.json

{
  "name": "client",
  "version": "0.0.1",
  "private": true,
  "scripts": {
    "start": "client"
  },
  "dependencies": {
    "socket.io-client": "^1.4.6"
  }
}

Then run npm install, give client permissions to be executed and you can run it with (for example) ./client message testControl

What do you think? ;)

You can use iocat cli client from npm which is based on Socket.IO.

$ iocat --socketio ws://127.0.0.1:8081
< Hello there

For a more low-level (socket.io-independent) solution you can use my tool websocat.

Example session for communicating with socket.io's chat example:

$ websocat 'wss://socket-io-chat.now.sh/socket.io/?transport=websocket'
< 0{"sid":"yGVElkNCXVgc5w_JOVtc","upgrades":[],"pingInterval":25000,"pingTimeout":60000}
< 40
> 2
< 3
> 42["add user", "websocat"]
< 42["login",{"numUsers":15}]
> 42["new message", "Hello from websocat"]
< 42["typing",{"username":"1223"}]
< 42["new message",{"username":"1223","message":"Reply from browser."}]
< 42["stop typing",{"username":"1223"}]
> 42["new message", "OK, may flood again."]

> denotes messages to be typed.

I was able to work around this by doing the following:

1. Create a new endpoint in your server

   app.post('/sendSocketMessage', function(req, res){
     console.log("Request is "+JSON.stringify(req.body));
     socketManager.parse(req.body); //send message directly to your socket parser
     res.status(200).send("OK");
   });
   

2. Send data using curl

   curl --noproxy localhost, 
        -X POST 
        --data "data=testControl,loadModels,start," 
        http://localhost:8081/sendSocketMessage