Code fails in plugin file but works in functions.php

You need to use a hook and a function in your plugin file.
For example:

function sher_posts() {

    $pages = get_posts($args);

    foreach($pages as $page) {

        $out = get_permalink($page->ID);
    }
}
add_action('wp', 'sher_posts'); 

Of course this example doesn’t know what $args are or what to do with $out.