When you need to execute a shell command in Node.js and capture its entire output as a string, child_process.exec is the simplest approach. It buffers the command's output and passes it to a callback once the process finishes. This is great for commands that return a manageable amount of data, such as getting the current Node.js version or listing files in a directory.

In contrast, child_process.spawn is designed for handling large amounts of data or streaming output. Instead of buffering everything, spawn provides streams for stdout and stderr, making it better for real-time data processing or when output size could exceed system limits. Use exec for short, simple commands where you want the whole output at once, and choose spawn for long-running processes or when you need to process output as it arrives.