Drush update: How to keep file permissions? Following scenario happens to be in place. There is a server administrator (root) who doesn't know a thing about Drupal. Then, there two (or more) web developers (userA, userB), who do not have root access but are all in the group "web".

The server has several Drupal instances which have to be updated on a regular basis. Each Drupal instance is set with permissions 2775 (dirs) or 2664 (files) and belongs to either userA or userB, group in all cases is web.

When using drush to update an instance, all files that are overwritten (due to updates) now belong to the user running drush and get the permissions 644 (instead of 664). Thus, when userA updates e.g. a module, this module can't be changed or updated by userB afterwards. userA always has to chmod the files after an update.

How can this be rendered unnecessary, or how can the file permissions be kept when using Drush?
Write a script that correct permissions, you can find an example in this article:

Securing file permissions and ownership

Execute this script manually every time a drush update is made. 

Or you can write a simple drush command that executes this script when a drush commands that cnahges files executes. Check the drush_hook_post_COMMAND:
http://drupalcontrib.org/api/drupal/contributions%21drush%21docs%21drush.api.php/function/drush_hook_post_COMMAND/7