File access permissions

Because the UNIX operating system provides a multiuser environment, permissions are assigned to files.

• Each user is identified primarily by a user ID (UID), and secondarily by a group ID (GID).

• Any process started by a user is associated with that user's UID and GID.

• Within the filesystem, files have ownership expressed through a combination of an attached UID, GID, and access permission flags. All these are actually specified in the inode, so that references from several directories have the same ownership and access restrictions from all points. The access permissions are specified by read, write, and execute permissions for each of:

  • owner (process UID matches inode UID)

  • group (process GID matches inode GID)

  • other (no match)

Note that all applications running on embedded clients have root permissions.