How to send a signal from a terminal window to a webscraper running on Puppeteer/Node.js on Raspberry Pi

Question

I am running a simple web scraper written in Puppeteer / Node.js on a Raspberry Pi. It downloads data from a websites at 6pm and 6am every day. Every so often, say once a week, I'd like to send a command or signal or something telling it to download the data immediately. This presumably would be be a command from another terminal window. The question is how do I do this? I could write a small file and have the program constantly look out for the file, but this seems very crude. Is there a better way? Something simple please as I'm a bit of a novice!

Answer 1

Keeping in mind that I don't actually recommend doing this in most cases, as discussed in my previous answer, here's a contrived example to illustrate the readline approach.

const puppeteer = require("puppeteer"); // ^22.2.0
const readline = require("readline");
const {setTimeout} = require("node:timers/promises");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});

const getPageTitle = async (
  url = "https://en.wikipedia.org/wiki/Special:Random"
) => {
  let browser;

  try {
    browser = await puppeteer.launch();
    const [page] = await browser.pages();
    await page.goto(url, {waitUntil: "domcontentloaded"});
    return await page.title();
  }
  finally {
    await browser?.close();
  }
};

rl.on("line", async line => {
  try {
    console.log(`  Received command from stdin: \`${line}\``);
    console.log("  Command result:", await getPageTitle(line));
  }
  catch (err) {
    console.error(err);
  }
});

(async () => {
  for (;;) {
    try {
      console.log(await getPageTitle());
    }
    catch (err) {
      console.error(err);
    }
    await setTimeout(20_000);
  }
})();

// execute with: `cat | node script.js` to enable the below behavior
console.log(
  `to send command: echo 'https://www.stackoverflow.com' > /proc/${process.pid}/fd/0`
);
console.log("_".repeat(80));

This prints a random Wikipedia page title every 20 seconds while also listening to stdin for additional URLs to print titles for. With the script running on a single terminal, type in https://stackoverflow.com to see it print Stack Overflow - Where Developers Learn, Share, & Build Careers, for example.

Now, if you run it with cat | node script.js to provide a pipe as described here, you can send commands to it from another terminal with echo 'https://www.stackoverflow.com' > /proc/{pid}/fd/0 where {pid} is printed dynamically by the program at startup, or obtainable with pgrep node.