1. UNIX File System Go Climb a Tree Weiyu Zhang (96142)
COSC513 Operating Systems
Southeastern University

2. Introduction It is true.
In UNIX, the thing we spend most of our time working with is files.
Treating files consistently is one of the greatest strengths in UNIX.
It is true.

3. What is “consistent”? UNIX use the same method to save all the files.
For example, an executable script has the same format as a text file.
Simplify the file system.
The idea is widely used later. (In DOS, Win95/98, etc.)

4. Three types of UNIX Files
Regular files
Directory files
Device files

5. Regular files Most common type Hold programs and data Example:
Source code of a program
received from friend
Executable programs and applications (such as ls, cat, etc.)

6. Directory files Basic management tools in UNIX file system
Like a cabinet in real life, the files that contain other files
Save information of files (such as location, size, etc.)

7. Devices files Each hardware is treated like a file Example Terminals
Printers
Keyboards, etc.

8. More about device file Example: Convenient to handle
Read the user input  read from a keyboard device file.
Print a text file  Copy the file to the printer device file.
Convenient to handle

9. File system organization
A bottom-up tree with root on the top
All the files must be put into directory files
Root (/), a special directory file
Exact one
Can not been deleted

10. One sample “tree” Root (/) bin/ dev/ etc/ usr/ zhang/ li/ wang/ ...
letter.txt
Hello.c

11. Find a file in UNIX Start from the root
Go down (or climb) the tree, record the path
Each file has a name

12. An example Root (/) bin/ dev/ etc/ usr/ zhang/ li/ wang/ ...
letter.txt
Hello.c
/usr/wang/letter.txt

13. File and directory permission
Only need in a multi-user OS
Keep your own secrets
Three classes of users
Three permissions

14. Three classes of users Owner: I created the file.
Group: The owner and I are in the same working group.
World (a.k.a. other): I just want to use the file.

15. Three permissions Read permission: read
Write permission: modify, delete
Execute permission: run
No exact priority, normally write permission is the most important one

16. Nine bits (3  3)

17. Nine bits (continue)
1 for Y and 0 for N. So the permission of the file is ( )2

18. Why permission Keep your secrets.
In the above example, world user can not read the file, group user can not modify or delete the file.

19. Why permission (cont) Keep your secrets.
Show more information about a file.
Since the files are saved consistently, only those which have the execute permission set can be run.

20. Why permission (cont) Keep your secrets.
Show more information about a file.
Avoid wrong operation
Setting the owner write permission to be ‘N’ (or 0) can avoid the careless delete of very important files.

21. I have no permission but I can ...

22. Conclusions Everything is a file. Bottom-up tree organization
Easy but safe

23. The End