Bulk updates to post_content

<?php

/**
 * Plugin Name: Bulk Post Update
 * Version:     1.0.0
 *
 * @author Nabil Kadimi <[email protected]>
 * $link   http://wordpress.stackexchange.com/a/242499/17187
 **/

add_action( 'init', function() {

    global $wpdb;

    $table            = $wpdb->posts;
    $post_type="custompost";
    $new_post_content="Text I want to update all my post content to";

    $affected = $wpdb->query(
        $wpdb->prepare( "UPDATE {$table} SET post_content = %s, WHERE post_type = %s"
            , $new_post_content
            , $post_type
        )
    );

    echo $affected . ' rows modified, please disable code.' . "\n";
    echo 'File: ' . __FILE__ . "\n";
    echo 'Line: ' . __LINE__ . "\n";
    die();
} );

---

Edit

Based on our discussion, I believe overriding content on the fly with code will be less intensive on PHP and MySQL, here is how I would do it:

<?php

/**
 * Plugin Name: Override Post Content
 * Version:     1.0.0
 *
 * @author Nabil Kadimi <[email protected]>
 * $link   http://wordpress.stackexchange.com/a/242499/17187
 **/

add_filter( 'the_content', function( $content ) {

    global $post;
    $custom text="blah [blah]";

    /**
     * Ignore other post types.
     */
    if ( 'custompost' !== $post->post_type ) {
        return $content;
    }

    /**
     * Return the value I want to override with.
     */
    return do_shortcode( $custom_text );
} );