exec for running shell commands in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of exec with simple command

What will be the output of this Node.js code using exec to run a shell command?

Node.js

import { exec } from 'child_process';

exec('echo Hello World', (error, stdout, stderr) => {
  if (error) {
    console.error(`Error: ${error.message}`);
    return;
  }
  if (stderr) {
    console.error(`Stderr: ${stderr}`);
    return;
  }
  console.log(`Output: ${stdout.trim()}`);
});

A Output: Hello World

B Error: Command not found

C Output: echo Hello World

D Stderr: Hello World

💡 Hint

Remember that exec runs the command and returns its output in stdout.

The exec function runs the shell command 'echo Hello World'. The output of this command is 'Hello World' which is captured in stdout. The code trims and prints it with 'Output: '.

❓ Predict Output

intermediate

Handling errors in exec callback

What will this code print if the command does not exist?

Node.js

import { exec } from 'child_process';

exec('nonexistentcommand', (error, stdout, stderr) => {
  if (error) {
    console.log(`Error: ${error.message}`);
    return;
  }
  console.log(`Output: ${stdout}`);
});

A Output: nonexistentcommand

B Error: Command failed: nonexistentcommand

C Error: spawn nonexistentcommand ENOENT

D Output:

💡 Hint

If the command is not found, exec returns an error with a message about failure.

The exec function tries to run 'nonexistentcommand' which does not exist, so it returns an error. The error message includes 'Command failed: nonexistentcommand'.

❓ component_behavior

advanced

Behavior of exec with large output

What happens if you run exec with a command that produces a very large output exceeding the default buffer size?

Node.js

import { exec } from 'child_process';

exec('yes | head -n 1000000', (error, stdout, stderr) => {
  if (error) {
    console.log(`Error: ${error.message}`);
    return;
  }
  console.log(`Output length: ${stdout.length}`);
});

A Error: stdout maxBuffer exceeded

B No output and no error

C Output length: 0

D Output length: 100000

💡 Hint

exec has a default maxBuffer size for stdout and stderr.

The command produces a large output that exceeds exec's default maxBuffer size (1MB). This causes exec to return an error with message 'stdout maxBuffer exceeded'.

📝 Syntax

advanced

Correct syntax for exec import and usage

Which option shows the correct modern syntax to import and use exec from 'child_process' in Node.js ES modules?

A

import exec from 'child_process';
exec('ls', (err, stdout) => { console.log(stdout); });

B

const { exec } = require('child_process');
exec('ls', (err, stdout) => { console.log(stdout); });

C

import { exec } from 'child_process';
exec('ls').then(output => console.log(output));

D

import { exec } from 'child_process';
exec('ls', (err, stdout) => { console.log(stdout); });

💡 Hint

Node.js ES modules use import syntax with curly braces for named exports.

Option D correctly imports exec as a named export and uses the callback style. Option D uses CommonJS require which is not ES modules. Option D tries default import which is invalid. Option D treats exec as returning a promise which it does not.

🔧 Debug

expert

Why does this exec code not print output?

Consider this code snippet: import { exec } from 'child_process'; const result = exec('echo test'); console.log(result.stdout); Why does this code not print 'test'?

Node.js

import { exec } from 'child_process';

const result = exec('echo test');
console.log(result.stdout);

A Because the command 'echo test' failed to run.

B Because stdout is only available after the process exits, so result.stdout is undefined immediately.

C Because exec returns a ChildProcess object, not the command output synchronously.

D Because exec requires a callback to capture output, otherwise stdout is null.

💡 Hint

Check what exec returns and how output is accessed.

exec returns a ChildProcess object immediately. The output is provided asynchronously via a callback. Accessing result.stdout directly does not give the command output.