gistfile1.js

// Build and update staging
  grunt.registerTask("sprite", "Builds sprite sheets from the src imges", function() {
    var task = this,
      // tell grunt this task is async
      done = this.async(),
      commands = [
        "montage img/_icons_blue/*.png -tile 1x -geometry '100x80>+0+0' -gravity NorthWest -background transparent img/icon_blue_sprite_2x.png",
        "montage img/_icons_white/*.png -tile 1x -geometry '100x80>+0+0' -gravity NorthWest -background transparent img/icon_white_sprite_2x.png",
        "convert img/icon_white_sprite_2x.png -negate img/icon_black_sprite_2x.png",
        "convert img/icon_white_sprite_2x.png -resize 50% img/icon_white_sprite_1x.png",
        "convert img/icon_black_sprite_2x.png -resize 50% img/icon_black_sprite_1x.png",
        "convert img/icon_blue_sprite_2x.png -resize 50% img/icon_blue_sprite_1x.png"
      ];
    var child = exec( commands.join( " && " ) ,
          function (error, stdout, stderr) {
            console.log('Sprite generation complete. Please update the CSS and documentation accordingly');
            if (error !== null) {
              console.log('exec error: ' + error);
              // Execute the callback when the async task is done
              done();
            }
        });
  } );