php exec 中的 purifycss 不能正常工作

$purifyCSS = "purifycss %s %s --info --out %s";

$match = "/usr/share/nginx/example/media/css_html/main.html";

$input = "/usr/share/nginx/example/media/css_html/input.css";

$output = "/usr/share/nginx/example/media/css_html/ouput.css";

exec(sprintf($purifyCSS, $input, $match, $output), $result);