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.

Practice

1. What does the exec function in Node.js primarily do?

easy

A. Runs shell commands from within a Node.js program

B. Creates a new HTTP server

C. Reads files from the filesystem

D. Starts a database connection

Solution

1. Step 1: Understand the purpose of exec

   The exec function is designed to run shell commands from Node.js code.

2. Step 2: Compare with other options

   Creating servers, reading files, or database connections are unrelated to exec.

3. Final Answer:

   Runs shell commands from within a Node.js program -> Option A

4. Quick Check:

   exec runs shell commands = B [OK]

Hint: Remember exec runs shell commands, not servers or files [OK]

Common Mistakes:

• Confusing exec with file reading functions
• Thinking exec creates servers
• Assuming exec manages databases

2. Which is the correct way to import and use exec from the child_process module in Node.js?

easy

A. import exec from 'child_process';

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

C. const exec = require('exec');

D. import { exec } from 'child_process';

Solution

1. Step 1: Recall Node.js import syntax for exec

   In Node.js CommonJS, exec is imported as const exec = require('child_process').exec;.

2. Step 2: Check other options for correctness

   import exec from 'child_process'; uses ES module default import incorrectly; const exec = require('exec'); tries to require a non-existent module; import { exec } from 'child_process'; is ES module named import but Node.js needs special config.

3. Final Answer:

   const exec = require('child_process').exec; -> Option B

4. Quick Check:

   CommonJS import for exec = A [OK]

Hint: Use require('child_process').exec for standard Node.js [OK]

Common Mistakes:

• Using import without Node.js ES module setup
• Requiring wrong module name
• Confusing default and named imports

3. What will the following Node.js code output if the current directory contains a file named test.txt?

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

medium

A. Output: test.txt

B. Error: ls command not found

C. Stderr: Permission denied

D. No output

Solution

1. Step 1: Understand exec running 'ls'

   The command ls lists files in the current directory. If test.txt exists, it will appear in the output.

2. Step 2: Analyze callback behavior

   No error or stderr means output is printed with file names, so console logs 'Output: test.txt\n'.

3. Final Answer:

   Output: test.txt -> Option A

4. Quick Check:

   exec runs ls, outputs files = A [OK]

Hint: exec callback logs stdout if no error or stderr [OK]

Common Mistakes:

• Assuming error if command runs successfully
• Ignoring stdout content
• Confusing stderr with stdout

4. Identify the error in this Node.js code using exec:

const { exec } = require('child_process');
exec('node -v', (error, stdout, stderr) => {
  if (error) {
    console.log(error);
  }
  console.log(stdout);
});

medium

A. Callback function has wrong parameters

B. Incorrect command syntax for node version

C. Missing error return causes stdout to print even on error

D. exec is not imported correctly

Solution

1. Step 1: Check error handling logic

   The code logs error but does not return or stop, so stdout logs even if error occurs.

2. Step 2: Understand impact of missing return

   This can cause confusing output mixing error and stdout, which is not ideal.

3. Final Answer:

   Missing error return causes stdout to print even on error -> Option C

4. Quick Check:

   Missing error return causes stdout to print even on error = D [OK]

Hint: Return after error to avoid printing stdout [OK]

Common Mistakes:

• Not returning after error check
• Assuming exec import is wrong
• Thinking command syntax is incorrect

5. You want to run a shell command that lists all files but only print those containing the word 'log'. Which exec usage correctly achieves this in Node.js?

hard

A. exec('ls > grep log', (err, stdout) => { if (!err) console.log(stdout); });

B. exec('ls && grep log', (err, stdout) => { if (!err) console.log(stdout); });

C. exec('ls | find log', (err, stdout) => { if (!err) console.log(stdout); });

D. exec('ls | grep log', (err, stdout) => { if (!err) console.log(stdout); });

Solution

1. Step 1: Understand shell command chaining

   The pipe | sends output of ls to grep log to filter lines containing 'log'.

2. Step 2: Evaluate each option's command

   ls | grep log correctly pipes output to grep for filtering; ls > grep log uses redirection to create a file; ls && grep log runs sequentially without piping; ls | find log pipes to filesystem search tool unsuitable for text filtering.

3. Final Answer:

   exec('ls | grep log', (err, stdout) => { if (!err) console.log(stdout); }); -> Option D

4. Quick Check:

   Pipe output to grep for filtering = C [OK]

Hint: Use pipe (|) to filter command output with grep [OK]

Common Mistakes:

• Using && instead of | for piping output
• Misusing redirection operators
• Confusing grep with find command