understand what code is doing when prepending /blog

Question

I am new to Wordpress and php coding.

We are moving from a different container system to wordpress and to keep the links intact I need to prepend /blog to all post articles.

I tried it through the individual permalink using https://www.example.com/blog/%postname%/

It works nicely for the post articles, but I realized that it seems to also impact i.e. custom layouts from Astra, making the following url: https://www.example.com/blog/astra-advanced-hook/12345

Although I am not sure this is an issue, but being an orderly person I really don't like such a strange URL convention.

So I searched the net and found the following reply from Zeth and Chaoste Change permalinks for post type 'post' only

function wp1482371_custom_post_type_args( $args, $post_type ) {
    if ( $post_type == "post" ) {
        $args['rewrite'] = array(
            'slug' => 'blog'
        );
    }
    return $args;
}

add_filter( 'register_post_type_args', 'wp1482371_custom_post_type_args', 20, 2 );

function my_change_post_link($permalink, $post, $leavename) {   
    if (get_post_type($post) == 'post') {     
        return "/blog" . $permalink;   }   
    return $permalink; 
}

add_filter('pre_post_link', 'my_change_post_link', 10, 3);

I went on and created a child theme, posted the above to function php and it is working (when I posted just the first function and add_filter, there didn't seem to be any change). But being an inquisitive person, I would really like to know what exactly this is doing?

I guess add_filter is what triggers the function when some action happens. The if checks if it is the post type.

1. But what does the first function do and what does the second one do?
2. Why do I need both functions?

Thank you for helping me learn!

Answer 1

add_filter( 'register_post_type_args', 'wp1482371_custom_post_type_args', 20, 2 );

When you register a post type, this filter is called, and wp1482371_custom_post_type_args is using this opportunity to modify the parameters for the post post type to change the slug

add_filter('pre_post_link', 'my_change_post_link', 10, 3);

Just because your posts are now available at a new URL, doesn't mean WordPress knows this and will change the permalinks! It's not psychic, it has to be told explicitly or the URLs shown in the user interface won't match and will still be missing the /blog part. pre_post_link gives you an opportunity to filter them, and if it's a post it add /blog to the beginning.

2.) Why do I need both functions?

Because if you just change the posts URL that doesn't mean WP knows what to do with /blog URLs.

And if you only add /blog URLs that doesn't mean WP knows to put /blog in front of every posts URL.

---

As an aside, just because you found this particular solution, does not mean it is the best solution. There are solutions that don't involve WordPress at all and sit deeper down at the Apache/Nginx level, you can also 301 redirect to versions that don't have /blog, as well as many other options.