File Permissions

File permissions and ownership

The sole reason for having multiple users is to be able to enforce access control i.e. allowing each user to only a subset of the files which exist on the system. A Linux system administrator should be able to:

• Examine the permissions of an existing file
• Changing permissions
• Understand special permissions (e.g. suid, sticky bit)
• Use extended file attributes (e.g. immutable)
• Understand umask
• Changing the owner and group of a file
• Use Access Control Lists (ACLs) (sparingly…)
• etc.

When allowing multiple users to share the same files, it is sometimes useful to be able to create either hard or symbolic links. This is important e.g. when trying to save space or to simplify paths. A Linux system administrator should be able to:

• Create hard links
• Create symbolic links
• Identify broken symbolic links
• Understand the importance of links e.g. in the lib directory