Give access to a specific user to a specific folder within a group? [Terminal]

Question

I have created a file, admin, that I want to assign to a group, admingroup, and later on give a specific user, adminuser, exclusive access to that file but I have no idea how.

Answer 1

This is very easy in POSIX based systems.

First

chown adminuser:admingroup admin

That will make the owner of the file specificly adminuser and the entire group admingroup.

Then you will likely want to:

chmod a-rwx admin
chomd ug+rwx admin

That will remove all permissions from admin and then give user and group access to read write and execute the admin file.

This is a very decent tutorial on file permissions, though there are better ones. This should get you started though.