How to call the command line in nodejs

Node.js is a highly scalable open source JavaScript platform that is widely used in web application and command line tool development. In Node.js, we can use the subprocess API to access the command line of the operating system and execute system commands. This article will introduce how to use the subprocess API to call the command line in Node.js.

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Introduction to Child Process API

Node.js provides the child_process module, which allows us to create child processes in Node.js programs and interact with them. The command line can be invoked through this module.

The two most commonly used APIs in the child_process module are spawn and exec. spawn starts a child process to execute the command line and returns an object that can interact with the process; exec is an asynchronous function that executes the command line and returns stdout and stderr.

spawn method

The spawn method is used to start a new process to execute the command line. This method accepts two parameters: the command to be executed and the parameter array.

For example:

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`子进程退出码：${code}`);
});

Here, we use the spawn method to execute the command "ls -lh /usr". This method returns a ChildProcess object whose stdin, stdout, and stderr properties can be used to interact with the child process. In this example, we listen to the stdout and stderr events of the child process in order to capture the data and error information output by the child process. When the child process exits, we also listen to its close event.

exec method

The exec method allows us to execute the command line and return the stdout and stderr output of the command line. This method accepts two parameters: the command to be executed and a callback function that takes three parameters: stdout, stderr, and the error object.

For example:

const { exec } = require('child_process');

exec('ls', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.error(`stderr: ${stderr}`);
});

Here, we use the exec method to execute the command "ls". In the callback function, we capture the stdout and stderr output, as well as any errors that may occur.

Interacting with the command line

In addition to calling the command line to execute commands, you can also interact with the command line in the Node.js program. In both the spawn and exec methods, a stdin attribute is provided through which we can send input to the child process.

For example, start a Node.js program in Windows and enter the run command on the command line:

const { spawn } = require('child_process');
const node = spawn('node');
node.stdin.write('console.log("Hello, world!");\n');
node.stdin.write('process.exit();\n');

Here, we use the spawn method to start a new Node.js process and sends a JavaScript program to the process through the stdin attribute. The program prints "Hello, world!" and exits.

Summary

This article introduces how to use the subprocess API to call the command line in Node.js. We learned the use of spawn and exec methods, and learned how to interact with the command line. The subprocess API allows us to easily execute command lines in Node.js, further enriching the application scenarios of Node.js.

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