Ever seen 's' or 'S' in the Linux file permissions ?
'S' = The directory's setgid bit is set, but the execute bit isn't set.
's' = The directory's setgid bit is set, and the execute bit is set.
What is setgid ?
When user creates a folder or a file under a setgid folder, the new file/folder will inherit its group as the group of the folder's owner, instead of the group of the user who creates it.
So in above screenshot, all files/folders created inside /screen will have same 'screen' group.
How to change 's' permissions using chmod ?
chmod g-s /folder/file "remove" chmod g+s /folder/file "add"