Installing Drush - permission denied error
You can follow following two links, they are simple two steps and make the Drush running perfectly with no issue, even you can upgrade drush with that. 

(1) How to install the Drush using Homebrew. And after that follow following steps to Symlink the php file location between your servers and MAMP. 

(2) MAMP: Symlink PHP to /usr/bin/php

I was facing the same issue and re-installed using above steps and this set of two links worked perfectly.