You might be looking for the documentation of the built-in wp-cli command.

This is very simple php script/library for regenerating Wordpress thumbnails from command line. It can be used as standalone command line script or in third-party code.

Code isn't perfect and not well-tested, use at your own risk.

License: WTFPLv2

Put wp-regenerate-thumbnails-cli.php somewhere and run:

php wp-regenerate-thumbnails-cli.php -p /path/to/wp/root/

Options:

• -p|--path -- path to Wordpress root directory (required)
• -h|--help -- show usage info
• -s|--silent -- don't print messages (except php errors/warnings)
• -r|--remove -- remove old images (warning, experimental)
• -d|--debug -- don't really remove images, just print filenames

Removing old images must work correctly, but better to do a debug run (-d) before it.

wp-load.php, wp-admin/includes/image.php and wp-admin/includes/file.php must be included before. They are included automatically when used in Wordpress environment.

Example:

require_once('/path/to/wp/wp-load.php');
require_once('/path/to/wp/wp-admin/includes/image.php');
require_once('/path/to/wp/wp-admin/includes/file.php');

require_once('/path/to/scrip/wp-regenerate-thumbnails-cli.php');

$r = new WpRegenerateThumbnailsCli\Regenerator(false, false, false);
$r->run(); // Will take long

Constructor looks like Regenerator($remove=false, $debug=false, $silent=true), options are same as CLI args.