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?
File permissions and vim (ro) warnings are not the same thing.
If vim is giving you are ro warning on a file with 755 permissions, then, my guess is either the file is in use of was not saved.
See https://superuser.com/questions/218262/how-to-get-rid-of-the-warnings-when-opening-a-file-that-has-a-swp-file or give details of the vim error.
File permissions are detailed here https://help.ubuntu.com/community/FilePermissions
You probably want file permissions of 644 or 640
See also acl How do I set up a folder so that anything created in it inherits permissions?
