How can I run a shell command inside of a gulp task and detect when it's done?

Question

I'm trying to run a shell command inside of a gulp task using child_process.spawn.

I have to detect when the task is done running so I'm using stdout to check for a specific string that I emit at the end of the command, but for some reason it doesn't look like my string is being emitted:

// gulp 3.9.1
var gulp = require('gulp');
var spawn = require('child_process').spawn;

gulp.task('my-task', function(cb) {
  var command = ''; // construct my shell command here
  var end = 'end_of_command';
  var command = command + '; echo ' + end; // add special string to look for

  var cmd = spawn(command, { shell: true });

  cmd.stdout.on('data', function(data) {
    if (data.includes(end)) {
      return cb();
    }
  });
});

For some reason my echo statement isn't emitting and so the if statement is not being passed.

Where am I going wrong?

I should also note that when I run this command directly in my shell rather than through the gulp task, it runs fine and the expected output is visible.

Malnutrition

Are you doing it this way on purpose? Because waiting for end_of_command output is really weird way to do that. – Gibun

For some reason I can't find the question that I was referencing for this idea that had the solution of looking for an "end string". Is there a better way to detect the end/exiting of a shell command in a gulp task? – Malnutrition

You didn't find anything on command execution because the problem isn't specific to Gulp. – Gibun

Answer 1

Both Gulp and child_process asynchronous functions use Node-style error-first callbacks.

spawn is intended for processing streams during command execution. If all is needed is to wait until a command is finished, exec and execFile do that:

var gulp = require('gulp');
var exec = require('child_process').exec;

gulp.task('my-task', function(cb) {
  exec('cmd', cb);
});

It may be more intricate with spawn because it also allows to handle exit codes:

var gulp = require('gulp');
var spawn = require('child_process').spawn;

gulp.task('my-task', function(cb) {
  spawn('cmd', [], {})
  .on('error', cb)
  .on('close', code => code ? cb(new Error(code)) : cb());
});

Gibun