Unix file systems support, separately or in combination, read, write, execute and execute, using another user's id, file access modes. These permissions are available for the individual owner of the file, for the group and for the public. The Unix file access mechanism was designed to support a wide variety of security policies, including arbitrary security policies implemented by individual users. For example, individual users may decide what permissions to give themselves (preventing write access would protect a file from accidental deletion), members of their group and general public access. Since file system directories are files, access modes on directories control whether or not the names of files are publicly known and are changeable as well as whether the contents of files are publicly known or changeable. The owner of the file also may change these permissions for members of the group associated with that file as well as the group to which the file belongs.
The system administrator exercises the same control mechanism for files owned by administrative accounts. This flexible security mechanism allows implementation of a variety of different security policies for different groups of users within the same computing system. Other types of computer operating systems (except for most personal computer systems) provide a similar file security mechanism.