Force Javascript (node) to wait on exec

Question

I want to make my main process wait on some output from an exec.

I want to force the program to perform the steps in sequence, waiting for each step before going onto the next step. I feel this should be easy, yet I cannot see how to do it.

With other languages it is the default to wait on things, in Perl for example the following 5 lines would do exactly what I want.

print("START")
print(`sleep 3; echo 1`)
print(`sleep 2; echo 2`)
print(`sleep 1; echo 3`)
print("END")

How to do it in Node JavaScript?

Here is my code so far

const util = require('util');
const exec = require('child_process').exec;

function sleepy1() {
  exec("sleep 3; echo 1", function( stderr, stdout ) {
    console.log( stdout.trim() )
  })
}

function sleepy2() {
  exec("sleep 2; echo 2", function( stderr, stdout ) {
    console.log( stdout.trim() )
  })
}

function sleepy3() {
  exec("sleep 1; echo 3", function( stderr, stdout ) {
    console.log( stdout.trim() )
  })
}

console.log("START")
sleepy1()
sleepy2()
sleepy3()
console.log("END")

Here is the output from the code:

$ node make_it_wait.js 
START
END
3
2
1

This is a Duplicate, please search StackOverflow before asking a question. — Jabster28
– Jabster28, Commented Jun 16, 2019 at 16:29

georg · Accepted Answer · 2019-06-16 16:15:36Z

6

You can promisify exec and await until each command finishes:

const util = require('util');
const exec = require('child_process').exec;

const execa = util.promisify(exec);

function sleepy1() {
  return execa("sleep 3; echo 1")
}

function sleepy2() {
  return execa("sleep 2; echo 2")
}

function sleepy3() {
  return execa("sleep 1; echo 3")
}

async function main() {
    console.log((await sleepy1()).stdout)
    console.log((await sleepy2()).stdout)
    console.log((await sleepy3()).stdout)
}


console.log("START");
main().then(() => console.log("END"))

As pointed out in another answer, there's also the sync variant of exec, execSync. If what you do is a script or another one-pass program, you might find that easier to use. In a server/multiprocess environment it's better to use async/await.

edited Jun 16, 2019 at 16:15

answered Jun 16, 2019 at 16:10

georg

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