I have a share directory with another user, say
Bill. The directory has permission as
775. I create my own file with the same permission in the directory. My file has owner and group as my user account. If I log in as user Bill, and use vim, I see
read-only warning. However, if Bill write to the file, he still can save it. And now the file owner and group belong to user Bill.
This is not what I expected. How can I prevent Bill to edit my file, and only allow to read it?