HOWTO Permissions in Linux

Use Case: 
A number of users (ex. www-data, ivailom, etc) need to access a directory (ex. /var/www/media) securely. 

Steps:
1. Create a new group that would govern the group access to the directory.
  sudo groupadd varwwwmedia
  
  NOTE: Using the default group of the user that created the directory may have unintended consequences elsewhere
  
2. Add the user that needs access to the group.
  sudo adduser www-data varwwwmedia

3. Recursively, set the the newly created group for the directory
  sudo chgrp -R varwwwmedia /var/www/media

4. Recursively, set the group permissions for the directory and its subdirectories to (rwxrws---) 
  sudo find /var/www/media -type d -exec chmod -v 2770 {} \;
  
  NOTE: instead of 770 (rwxrwx---) we are using the sticky bit set on group ownership, so that each file created in it
        inherits its group ownership from the directory group ownership (and not from the user's group ownership)
  
5. Recursively, set the group permissions for the files in the directory and its subdirectories to (rwxrw----)
  sudo find /var/www/media -type f -exec chmod -v 760 {} \;
  
  
  Sources:
  https://www.adamerispaha.com/2016/12/14/file-permissions-for-django-media-uploads/
  https://stackoverflow.com/questions/21797372/django-errno-13-permission-denied-var-www-media-animals-user-uploads