Ubuntu Security :: Understanding Unix Style Permissions

On Windows, you can go to a file's permissions and it's clearly stated who can do what. You can choose between individual users or groups such as 'everyone' or certain types of users such as 'domain users'. You could create a clear cut list of every single user/group on the system and what their permissions for a file are and have it neatly displayed in a list.On Unix, we have octal permissions and sticky bits. I understand the whole concept of rwxrwxrwx (777). The first three are what the file owner can do, the second is what the main group the user belongs to can do, and the third is what other users can do.

But, when you view a file's permissions you are only getting the permissions as they apply to the user that owns the file. For example, as I understand it, if I viewed a file that only the root user had rwx permissions on and everyone else could only read. The permissions would show up as rwxr--r-- (744). But, those same permissions would show up to any user as 744 as well. Since the last 3 characters are what applies to "other users" (pretty vague). How would someone know what users in particular those permissions apply to? There could be one "other user" that can rwx that file and another "other user" that can't.Also, why just stop with the main group? What about other groups? A the user Foo's main group he belongs to might be Foo. But he could also belong to the groups Boo and Zoo, which belong to other users and would give him full rwx permissions over Boo and Zoo's files just as if he were Boo or Zoo.

Then you have the whole sticky bit thing that makes it so that files can be owned by the same person and at the same time be made use of (to varying degrees) by other users. To chmod the UID you'd chmod 2777 or for GID 4777 (just an an example). I did this for a file and it allowed a standard user account who was previously unable to run the command to be able to run it. But, how can that work when I didn't anywhere specify what particular user (or groups of users) that sticky bit applies to?

I'm confused about this whole thing to the point that I'm not even sure exactly what questions I should be asking or even if my examples are even 100% correct. I just sort of ranted about some specific things that floated to the top of my head. Permissions are easy to understand when your running a Unix-like system on a single user desktop. Because the only users/groups you have are root, the single user, and various system users/groups that you don't really need to worry about. So a file with rwxr--r-- means that only the Root user (not even members of his group) can edit the file and you can't unless you use sudo. Because the "other user" in the last 3 characters always just means you. But, things seem to get a whole lot more complicated when you start adding in multiple users. Can someone explain this or link to a "for dummies" article that can explain all of this to me in a way that someone who's used to Windows style permissions can make a connection between the two OS families and their way of handling these things?