Handling security for your files

Each user has user ID (UID) and group ID (GID) numbers that are set when the user is defined to the system. A user always belongs to at least one group—for example, a department—and each group that uses the system is assigned a GID. The system uses the UID and GID to identify the files and processes that a user may use. When you create a directory or a file, it is automatically associated with your UID, and its GID is set to the owning GID for the parent directory (the directory it is in).

There are three classes of users whose access you can control with the permission bits (ACLs allow access control for any user or group):

Owner (the owner of the file or directory whose UID matches the UID for the file)
Group (a member of the group whose GID matches the GID for the file)
Other (anyone else)

You control access to a file and directory that you own through its permission bits. (Taken together, the permission bits are often called the mode.)

In this topic, we discuss:

Default permissions set by the system
Changing permissions for files and directories
Using the sticky bit on a directory to control file access
Auditing file access
Displaying file and directory permissions
Setting the file mode creation mask for programs
Changing the owner ID or group ID associated with a file
Temporarily changing the user ID or group ID during execution
Displaying extended attributes
Using access control lists (ACLs) to control access to files and directories