Traditional UNIX file permissions can assign ownership to three classes of users:
user – The file or directory owner, which is usually the user who created the file. The owner of a file can decide who has the right to read the file, to write to the file (make changes to it), or, if the file is a command, to execute the file.
group – Members of a group of users.
others – All other users who are not the file owner and are not members of the group.
The owner of the file can usually assign or modify file permissions. Additionally, the root account can change a file's ownership. To override system policy, see Example 1–2.
A file can be one of seven types. Each type is displayed by a symbol: