file.php

<?php
// Get all the categories first
$cat_args=array(
	        'orderby' => 'order',
	        'order' => 'ASC',
	        //'include' => array(2,3,4,5,6,7,8,9,10,11) // use this to display fixed cat only
	    );
$categories = get_categories( $cat_args );

// Now query for posts of each category
foreach( $categories as $category ) {
	    $args = array(
	        'posts_per_page' => 6,
	        'category__in' => array( $category->term_id ),
	        'ignore_sticky_posts' => 1,
	        'post_status' => 'publish'
	    );

	    $posts = get_posts( $args );
      var_dump($posts);
} //endforeach