shell: expose promises variant

src/shell.js

@@ -1,18 +1,24 @@
 'use strict';
 
-exports = module.exports = {
-    spawn,
-    exec,
-    sudo
-};
-
-var assert = require('assert'),
+const assert = require('assert'),
     BoxError = require('./boxerror.js'),
     child_process = require('child_process'),
     debug = require('debug')('box:shell'),
     once = require('once'),
     util = require('util');
 
+exports = module.exports = {
+    spawn,
+    exec,
+    sudo,
+
+    promises: {
+        exec: util.promisify(exec),
+        spawn: util.promisify(spawn),
+        sudo: util.promisify(sudo)
+    }
+};
+
 const SUDO = '/usr/bin/sudo';
 
 function exec(tag, cmd, callback) {
@@ -88,7 +94,7 @@ function sudo(tag, args, options, callback) {
     if (options.preserveEnv) sudoArgs.push('-E'); // -E preserves environment
     if (options.ipc) sudoArgs.push('--close-from=4'); // keep the ipc open. requires closefrom_override in sudoers file
 
-    var cp = spawn(tag, SUDO, sudoArgs.concat(args), options, callback);
+    const cp = spawn(tag, SUDO, sudoArgs.concat(args), options, callback);
     cp.stdin.end();
     return cp;
 }