1. Slide: 1 UNIX FILE SYSTEM By:Qing Yang ID:103968 Operating System Research Topic December, 2000

2. Slide: 2 Outline Major parts of the file system Basic components of the file system Structure of the file system Access permissions

3. Slide: 3 Major Parts of UFS Logical method for organizing and storing information in a way which is easy to manage A UNIX file system(UFS) has four major parts: boot block superblock i-node blocks data blocks

4. Slide: 4 A Simplified Module i-nodes data blocks boot block/ superblock

5. Slide: 5 Boot Block First block of every file system(block 0) Reserve for boot or initialization program Contain boot loader’s address and partition information

6. Slide: 6 Superblock Block 1 of every file system Contains following information: total size of the file system number of blocks reserved for i-nodes name of file system device identification date of last update head of the free-block list list of free i-nodes

7. Slide: 7 I-Node Blocks Group of blocks follow the superblock Each block contains a number of i-nodes An i-node describes an individual file A max. number of i-nodes in a file system  a max. number of files

8. Slide: 8 Data Blocks Contain user data or system files

9. Slide: 9 I-Node Contain the key information needed by the operating system for a particular file Contain 10 direct pointers, 1 indirect pointer, 1 double indirect pointer, and 1 triple indirect pointer By structuring the pointers in a geometric fashion, a single i-node can represent a very large file

10. Slide: 10 a0 a10 a11 a3 a4 a2 a1 a12

11. Slide: 11 I-Node (cont.) Example: type=ordinary perm=rwxr-xr-xlinks=1 user-id=2group-id=2size=3624 a0: 726 a1: 725 a2: 724 a3: 723 a4: 0 a5: 0 a6: 0 a7: 0 a8: 0 a9: 0 a10: 0 a11: 0 a12: 0 Time of last access: Fri May17 17:41:03 1989 Time of last modification: Sun Mar3 13:40:49 1989 Time of last i-node change: Sun Mar3 13:40:49 1989

12. Slide: 12 Files and Directories Two basic components: files and directories File - collection of information kept on a disk or tape Directory - list of filenames and i-node numbers

13. Slide: 13 Different Types of File Every item in a UNIX system can be defined as belonging to one of the four possible types: Ordinary files Directories Special files Pipes

14. Slide: 14 Ordinary Files Contain text, data, or program information Cannot contain another file, or directory One-dimensional array of bytes

15. Slide: 15 Directories A file that holds files and other directories Contain two pieces of information for each file: filename an i-node number - a numerical reference to the location of the file

16. Slide: 16 Special Files Represent input/output (i/o) devices Compatibility can be achieved between device i/o and ordinary file i/o, allowing for more efficient use of software Special files can be: character special files - deal with streams of characters block special files - operate on larger blocks of data

17. Slide: 17 Pipes UNIX allows user to link commands together using a pipe Pipe acts as a temporary file which only exists to hold data from one command until it is read by another

18. Slide: 18 Structure of the UFS Organized as a hierarchy tree-like directory Start from a single directory - root directory represented by a / (slash) Below the root directory are several system directories - contain information required by the operating system

19. Slide: 19 UNIX System Directories /(root) | ----------------------------------------------------------------------------- | | | | | | | | /bin /dev /etc /home /lib /tmp /usr kernel file

20. Slide: 20 A Brief Tour of UFS root - locate at top of UNIX file system bin - executable system utilities dev - contain special files etc - system configuration files and databases home - contains home directory for each user

21. Slide: 21 A Brief Tour of UFS (cont.) lib - operating system and programming libraries tmp - system scratch files (all user can write here) usr - contains system files and directories sharing with other users Kernel file - contains the kernel for the system

22. Slide: 22 Pathnames Identifies a file by specifying a path through the directory structure to the file Absolute path names start at root of the file system eg: /home/sunserv1_b/lnp5jb/bin/hello Relative path names start at the current directory eg: bin/hello

23. Slide: 23 Access Permissions Three types of permissions: r read the file or directory w write to the file or directory x execute the file or search the directory Three types of user: u the user who owns the file g members of the group to which the owner belongs o all other users The access permissions for all three types of user can be given as a string of nine characters: user group other r w x r w x r w x

24. Slide: 24 Access Control File and directory in user account can be protected from or made accessible to other users by changing its access permissions User can only change the permissions for files and directories that he owns Default value when create a file or directory: rw------- for file vs rwx------ for directory Access permissions for user home directory are usually set to rwx--x--x or rwxr-xr-x

25. Slide: 25 Summary UNIX considers any device attached to the system to be a file Files are organized in tree-structured directories Directories are files containing pair of i-node numbers and filenames File and directory can be protected by setting its access permissions