Node.js Executing files or commands with Child Processes Spawning a shell to execute a command


Example

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}`);