# Executing files or commands with Child Processes

# Spawning a new process to execute a command

To spawn a new process in which you need unbuffered output (e.g. long-running processes which might print output over a period of time rather than printing and exiting immediately), use child_process.spawn().

This method spawns a new process using a given command and an array of arguments. The return value is an instance of ChildProcess, which in turn provides the stdout and stderr properties. Both of those streams are instances of stream.Readable.

The following code is equivalent to using running the command ls -lh /usr.

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

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

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

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Another example command:

zip -0vr "archive" ./image.png

Might be written as:

spawn('zip', ['-0vr', '"archive"', './image.png']);

# Spawning a shell to execute a command

To run a command in a shell, in which you required buffered output (i.e. it is not a stream), use child_process.exec. For example, if you wanted to run the command cat *.js file | wc -l, with no options, that would look like this:

const exec = require('child_process').exec;
exec('cat *.js file | wc -l', (err, stdout, stderr) => {
  if (err) {
    console.error(`exec error: ${err}`);
    return;
  }

  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

The function accepts up to three parameters:

child_process.exec(command[, options][, callback]);

The command parameter is a string, and is required, while the options object and callback are both optional. If no options object is specified, then exec will use the following as a default:

{
  encoding: 'utf8',
  timeout: 0,
  maxBuffer: 200*1024,
  killSignal: 'SIGTERM',
  cwd: null,
  env: null
}

The options object also supports a shell parameter, which is by default /bin/sh on UNIX and cmd.exe on Windows, a uid option for setting the user identity of the process, and a gid option for the group identity.

The callback, which is called when the command is done executing, is called with the three arguments (err, stdout, stderr). If the command executes successfully, err will be null, otherwise it will be an instance of Error, with err.code being the exit code of the process and err.signal being the signal that was sent to terminate it.

The stdout and stderr arguments are the output of the command. It is decoded with the encoding specified in the options object (default: string), but can otherwise be returned as a Buffer object.

There also exists a synchronous version of exec, which is execSync. The synchronous version does not take a callback, and will return stdout instead of an instance of ChildProcess. If the synchronous version encounters an error, it will throw and halt your program. It looks like this:

const execSync = require('child_process').execSync;
const stdout = execSync('cat *.js file | wc -l');
console.log(`stdout: ${stdout}`);

# Spawning a process to run an executable

If you are looking to run a file, such as an executable, use child_process.execFile. Instead of spawning a shell like child_process.exec would, it will directly create a new process, which is slightly more efficient than running a command. The function can be used like so:

const execFile = require('child_process').execFile;
const child = execFile('node', ['--version'], (err, stdout, stderr) => {
  if (err) {
    throw err;
  }

  console.log(stdout);
});

Unlike child_process.exec, this function will accept up to four parameters, where the second parameter is an array of arguments you'd like to supply to the executable:

child_process.execFile(file[, args][, options][, callback]);

Otherwise, the options and callback format are otherwise identical to child_process.exec. The same goes for the synchronous version of the function:

const execFileSync = require('child_process').execFileSync;
const stdout = execFileSync('node', ['--version']);
console.log(stdout);

# Syntax

  • child_process.exec(command[, options][, callback])
  • child_process.execFile(file[, args][, options][, callback])
  • child_process.fork(modulePath[, args][, options])
  • child_process.spawn(command[, args][, options])
  • child_process.execFileSync(file[, args][, options])
  • child_process.execSync(command[, options])
  • child_process.spawnSync(command[, args][, options])

# Remarks

When dealing with child processes, all of the asynchronous methods will return an instance of ChildProcess, while all the synchronous versions will return the output of whatever was run. Like other synchronous operations in Node.js, if an error occurs, it will throw.