Chapter 6. Controlling Access to Files with Linux File System Permissions.

1. Linux file system permissions
   
   Access to files by users are controlled by file permissions.
   
   r (read)    = 4
   w (write)  = 2
   x (exec)   = 1
    
2. Viewing file/directory per missions and ownership
   
   # ls -l
    
    
3. Changing file/directory permissions
   # chmod whowhatwhich file|directory
   who is u, g, o, a (for user, group, other, all)what is +, -, = (for add, remove, set exactly)which is r, w, x (for read, write, executable)# chmod ### file|directory
   # is sum of r = 4, w = 2, and x = 1
    
4. Changing file/directory user or group ownership
   
   # chown owner:group file|directory
   chown can be used with the -R option to recursively change the
   ownership of an entire directory tree.
    
5. Special  permissions
   
   
    
6. Default file permissions
   
   # umask
   0002 
   
   後面三個數字與一般權限有關，
   第一個數字表示特殊權限(參考上面第5點說明)。
        0          0           0           2
   special + user + group + other
   此 umask 表示：使用者新增檔案或目錄時，針對 other 的
   權限要扣除 2，也就是不開放寫入權限。
   
   ※ 關於預設 umask 的設定可以參考 /etc/bashrc 這個檔案的內容