Use wget to find used images

Your asking for command line solution, I don’t have one – but for PHP you can cycle through all posts, build an array of files, compare it to an array of the media library, find unused and delete.

Cycle through Posts

use get_posts() for pages andor posts, and build your foreach loop

Extract Media From Posts

while in the loop find the media items and build a big array of used media urls

function myplugin_find_media($content) {

    $mediaRegex = "/(src|href)=\"(.+?).(jpg|png|pdf|gif)\"/i"; // your support filetypes
    $mediaFind = preg_match_all($mediaRegex, $content, $media);

    if (isset($media[2]) && count($media[2]) > 0)
        return $media[2];

    return false;
}

Get Library Items

You can get all library items with somthing like

// get the media library for comparison
$library = array();
$args = array(      
    'post_type' => 'attachment',
    //'post_mime_type' => 'image', // if theres only one
    'numberposts' => -1,
    'post_status' => null,
    'post_parent' => null,
);
$attachments = get_posts($args);
foreach ($attachments as $post)
    $library[$post->ID] = wp_get_attachment_url($post->ID); // or add more info about thumbnails, etc

Compare library and Used Media Items

You can then loop through your library and compare it to your extracted used media items.

Note you’ll need to factor in the use of thumbnails and resized -###-### images.

Deleting

With a library media item not being in any posts, and you’ve got it’s post ID as the key in $library, you can use wp_delete_post() to remove it completely.

Cleaning

If this site has a ton of extra thumbnails from different plugins and themes that have created their own sizes, a plugin like this can clean it for you.

UPDATE:

Looks like there’s a plugin that does this already: https://wordpress.org/plugins/media-cleaner/