exec for running shell commands in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the exec function in Node.js?

The exec function runs shell commands from a Node.js script, letting you execute system commands and get their output.

beginner

Which Node.js module provides the exec function?

The child_process module provides the exec function to run shell commands.

intermediate

What are the three main arguments of exec(command, options, callback)?

1. command: The shell command string to run.
2. options: Optional settings like environment variables.
3. callback: Function called with error, stdout, and stderr after command finishes.

intermediate

How do you handle errors when using exec?

Check if the error argument in the callback is set. If yes, the command failed or had issues. You can then handle or log the error.

beginner

What is a simple example of using exec to list files in the current directory?

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

Which module must you import to use exec in Node.js?

A http

B fs

C os

D child_process

The exec function is part of the child_process module.

What does the exec callback receive as arguments?

A error, stdout, stderr

B stdout, stderr, error

C command, options, callback

D input, output, error

The callback receives error, stdout, and stderr in that order.

If a shell command run by exec fails, where will the error info appear?

A In the <code>error</code> argument of the callback

B In the <code>stdout</code> argument

C In the <code>options</code> argument

D In the <code>command</code> string

Errors are passed as the first argument error in the callback.

Which of these is a correct way to run the pwd command using exec?

A exec('pwd', (stdout, stderr) => { console.log(stdout); });

B exec('pwd', (err) => { console.log(err); });

C exec('pwd', (err, stdout) => { console.log(stdout); });

D exec('pwd', (error, output) => { console.log(output); });

The callback receives error first, then stdout. Option C uses correct argument order.

What is a common use case for exec in Node.js?

A Manipulating arrays

B Running system shell commands and getting their output

C Creating HTTP servers

D Reading files from disk

exec is used to run shell commands and capture their output.

Explain how to use exec to run a shell command and handle its output and errors.

Think about the three arguments in the callback and how to safely use them.

Describe a simple example where exec is used to list files in a directory and print them.

Remember the common 'ls' command and how to handle callback arguments.

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