How to detect custom post type inside admin_init hook callback?

Question

Or should I use different hook? Mainly I'm trying to perform some actions only when in admin and only for some of custom posts.

Answer 1

This method is an easy one:

/**
 * Based on http://wordpress.stackexchange.com/a/59999/12615
 */

add_action( 'admin_head', 'wpse_59652_list_terms_exclusions' );

function wpse_59652_list_terms_exclusions() {
    global $current_screen;

    if( 'post' != $current_screen->post_type )
        return;

    // Do your stuff
}

Another:

/**
 * Based on http://wordpress.stackexchange.com/a/54279/12615
 */

add_action( 'admin_notices', 'wpse_54258_display_error_message' );

function wpse_54258_display_error_message() { 
    global $post;
    if( !isset( $post ) || 'page' != $post->post_type )
        return;

    ?>
        <div class="error fade">This is a "page" post type</div>
    <?php
}

And a complex one:

/***
  * Conditional enqueue of scripts according to Admin page
 * Based on http://wordpress.stackexchange.com/a/9095/12615
 */

add_action('admin_init', 'wpse_9080_admin_init');

function wpse_9080_admin_init() {
    global $pagenow;
    global $firephp; // Using FirePHP for debugging - Remove if library not included
    if ( 'edit.php' == $pagenow) {
        if ( !isset($_GET['post_type']) ) {
            $firephp->log('I am the Posts listings page');  
        }
        elseif ( isset($_GET['post_type']) && 'page' == $_GET['post_type'] ) {
            // Will occur only in this screen: /wp-admin/edit.php?post_type=page
            $firephp->log('I am the Pages listings page');  
        }
    }
    if ('post.php' == $pagenow && isset($_GET['post']) ) {
        // Will occur only in this kind of screen: /wp-admin/post.php?post=285&action=edit
        // and it can be a Post, a Page or a CPT
        $post_type = get_post_type($_GET['post']);
        $firephp->log($post_type);
        if ( 'post' == $post_type ) {
            $firephp->log('I am editing a post');   
        }
        elseif ( 'page' == $post_type) {
            $firephp->log('I am editing a page');   
        }
        elseif ( 'movie' == $post_type) {
            $firephp->log('I am editing a custom post type');   
        }
    }

    if ('post-new.php' == $pagenow ) {
        // Will occur only in this kind of screen: /wp-admin/post-new.php
        // or: /wp-admin/post-new.php?post_type=page
        if ( !isset($_GET['post_type']) ) {
            $firephp->log('I am creating a new post');  
        }
        elseif ( isset($_GET['post_type']) && 'page' == $_GET['post_type'] ) {
            $firephp->log('I am creating a new page');  
        }
        elseif ( isset($_GET['post_type']) && 'movie' == $_GET['post_type'] ) {
            $firephp->log('I am creating a new custom post type');  
        }
    }   
}