exclude custom post type from running a function

Question

I have the following code in my functions.php file to display post shortlink in my posts (I prefer not to edit theme files if possible)...

function add_post_content($content) {
    if ( is_single() ) {
        $content .= '<p>Shortlink: ' .wp_get_shortlink(); '</p>';

    } elseif ( 'portfolio' == get_post_type() ) {
        $content .= '<p></p>';

}
    return $content;
}
add_filter('the_content', 'add_post_content');

The wp_get_shortlink function doesn't work with custom post types (I understand). I have 1 custom post type and so I am trying to stop it running on that post type but is does - just displays 'Shortlink: ' (because the function doesn't work it doesn't actually display anything else). Can anyone help me with either a way to display the custom post shortlink or edit my code to stop it showing the words 'Shortlink: '?

As an aside, would it be possible to edit the code so that the shortlink is displayed in a form textbox? I tried but couldn't get it working. Many thanks.

I've left what I have active. You'll see the output at the bottom of the pages. The second link is is the custom post type...

http://skipology.com/iphoneography-feature-puddles-featured-by-eyeem/

http://skipology.com/project/iphoneography-exhibition-hip-2b-square/

Answer 1

Reorder the logic, and fix the minor syntax error in your appended string:

function add_post_content($content) {
    if ( 'portfolio' == get_post_type() )
        return $content;

    if ( is_single() )
        return $content . '<p>Shortlink: ' . wp_get_shortlink() . '</p>';

    return $content;
}

Answer 2

This really helped me, I combined this with other code I had - I needed to give image links their own class for Magnific Popup, BUT only for my Blog/news section and not custom posts.

Hope this helps others:

add_filter('the_content', 'my_addlightboxrel');
function my_addlightboxrel($content) {
     if ( 'craft' == get_post_type() )
        return $content;

    if ( is_single() )
       global $post;
       $pattern ="/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
       $replacement = '<a$1href=$2$3.$4$5 class="inline-image"$6>';
       $content = preg_replace($pattern, $replacement, $content);
       return $content;
}