Slide: 1

UNIX FILE SYSTEM

By: Qing Yang

ID: 103968

Operating System Research Topic

December, 2000

Slide: 2

Outline

Major parts of the file system

Basic components of the file system

Structure of the file system

Access permissions

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

Slide: 4

A Simplified Module

i-nodes data blocksboot block/superblock

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

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

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

Slide: 8

Data Blocks

Contain user data or system files

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

Slide: 10

a0

a10a11

a3a4

a2a1

a12

Slide: 11

I-Node (cont.)

Example:

type=ordinary perm=rwxr-xr-x links=1

user-id=2 group-id=2 size=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

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

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

Slide: 14

Ordinary Files

Contain text, data, or program information

Cannot contain another file, or directory

One-dimensional array of bytes

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

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

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

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

Slide: 19

UNIX System Directories

/(root)

|-----------------------------------------------------------------------------

| | | | | | | |

/bin /dev /etc /home /lib /tmp /usr kernel file

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

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

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

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

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

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