spawn in batches with async/await and stuff.

test.js

const { spawn } = require('child_process');

// Search for each the following terms in a file called 'words.txt'
const SEARCH_TERMS = "at quis risus sed vulputate odio";

// Split the terms into an array.
const searchArray = SEARCH_TERMS.split(" ");

// Create an array of 'batches' with 2 terms each.
let batches = [];
while ( searchArray.length > 0) {
	batches.push(searchArray.splice(0,2));
}

// A function to process the batch concurrently. 
// Returns a Promise that is resolved when all spawned processes have finished.
function processBatch(batch) {

	// Get an array of promises for each spawned search process.
	return Promise.all(batch.map( term => {
		
		// Return a promise that resolves when term is searched.
		return new Promise( (resolve, reject) => {

			let grep = spawn("grep",[term,'words.txt']);

			grep.stdout.on('data', (data) => {
				console.log("Search term: ", term, "\r\n__________");
				console.log(data.toString());
				resolve();
			});

			grep.stderr.on('data', (data) => {
				reject(`grep stderr: ${data}`);
			});

		});

	}));
	
}

// Asynchronously process each batch.
batches.forEach( async ( batch, i ) => {
	console.log("Queueing Batch ", i);

	// Each batch resolves (finishes) only when all spawned processes have resolved or rejected.
	await processBatch(batch);	
	
	console.log('Processed Batch: ', i+1, "\r\n");
});

words.txt

Lectus
urnaduis
convallis
convallis
tellus
id.
Eget
nullam
non
nisi
est.
Lacinia
quis
vel
eros
donec
ac.
Diam
ut
venenatis
tellus
in
metus
vulputate.
Malesuada
proin
libero
nunc
consequat
interdum
varius.
Vitae
ultricies
leo
integer
malesuada
nunc
vel
risus.
Leo
in
vitae
turpis
massa.
Auctor
augue
mauris
augue
neque
gravida
in
fermentum
et
sollicitudin.
Tempor
orci
dapibus
ultrices
in
iaculis
nunc
sed
augue
lacus.
Ut
placerat
orci
nulla
pellentesque
dignissim.
Interdum
posuere
lorem
ipsum
dolor
sit
amet
consectetur
adipiscing
elit.
At
lectus
urna
duis
convallis
convallis
tellus
id.
Tristique
magna
sit
amet
purus
gravida
quis
blandit
turpis.
Sollicitudin
aliquam
ultrices
sagittis
orci
a
scelerisque
purus.
Fusce
ut
placerat
orci
nulla
pellentesque
dignissim
enim
sit
amet.
Ac
odio
tempor
orci
dapibus
ultrices
in
iaculis.
Nec
sagittis
aliquam
malesuada
bibendum.
Arcu
non
sodales
neque
sodales
ut
etiam
sit
amet
nisl.