10.2 File System Permissions
10.2.3 Permission types
Every file or directory has a set of permissions that determines who can do what with it. Permissions are represented by characters that control who may read, write, and execute the contents of a file or directory.

The set of permissions for each category of users (Users, Group or Other) consists of three possible permission types. These are used to control access to the file or directory with which the permissions are associated. The first position in each set is the Read (r) permission, the second is Write (w) and the third position is Execute (x). You may see a dash (-) in place of r, w, or x. This indicates that the permission (r, w or x) where the dash is located is denied.

Rolling over Figure shows the permissions for the .profile file in the /home/user2 directory. This is a text file that is usually placed in the user's home directory by the system administrator. When the user logs in, this file is read, and the environment is tailored to the user's needs. This file can be read (viewed) and written to (modified) by the user. The set of permissions for User access to this file is r w -. The presence of the r allows the User (Owner) to Read the file and the w allows the User to Write to or modify the file. The absence of an x means the User cannot Execute the file since this is not an executable file. A dash is used in place of a permission that is not allowed. The group and other categories of users can read the file but cannot write to it or execute it.

Figure also shows the permissions for the mkdir file in the /bin directory. This is an executable UNIX command file which you have used previously to create new directories. Since all users need to be able to use the mkdir command, all three categories of users have the permission to Read the file and to Execute it. The set of permissions for User, Group and Other access to this file is r - x. The presence of the r and x allows everyone (User, Group and Other) to Read and Execute the file. The dash in place of the Write permission prevents anyone from Writing to or changing the file.

The meaning of the r, w, and x permissions will vary depending on whether you are working with a file or a directory. Figure shows the three possible permissions and the differences in their effect on files and directories. In order for a directory to be of any practical use, at least read and execute permissions must be set. 

Note -  To copy (cp) a file from a directory or move (mv) a file into or out of a directory, you must have the execute (x) permission for the directory. You must also have the read permission for the file being copied or moved.  Additionally, changing the permissions on a folder does NOT change the permissions for that folder's subfolders.  To change permissions on a folder and all of its subfolders, the -R (recursive) command needs to be used.

Figure shows several examples of the different permissions for files and directories.