1. Introduction to Unix – CS 21
Lecture 3

2. Lecture Overview Lab review More on files and listings
Disk usage and compressing files
Permissions and ownership

3. How Did Lab Go? What was missing?
What did I want you to get out of it?
1. Basic differences between Windows and Unix
2. Difference between a good design and a bad design
A, b, x don’t really make good descriptions

4. Lab Continued 3. Getting used to navigating the system
4. rmdir can only remove empty directories
5. A lot of environment variables are automatically set, but not every one will be used every time
6. Difference between an environment variable and a local variable
7. Get used to reading man pages
8. Think about the future

5. What ls Output Means

6. Properties Of Files All files have these properties Permissions Links
For directories, this lists the number of subdirectories
Owner
Owner group
Size
Timestamp
Name

7. Disk Space And Quota
Every file takes some amount of space, even an empty file
Space for the name, and information about the file
Your account has limited space
quota can be used to check available space
Du shows how much space you are using and where it is located
“du –s” summarizes and just gives you the total

8. The du Command

9. Saving Disk Space Compressing files Decompressing files Zip gzip bzip2
GNU version of zip
bzip2
Different algorithm
Decompressing files
unzip, gunzip, bunzip2

10. Running zip, gzip, And bzip2

11. A Special Type of File: A Symbolic Link
Not an actual file, but a pointer to another file
Acts as a shortcut
Can act as a shortcut to a directory
Provides a quick link to any file
Often used so files can be changed without disturbing other programs

12. Graphical Representation
/home/csmajs/user
/home/csmajs/user/cs21
/home/csmajs/user/cs21/link
/home/csmajs/user2/otherFile

13. Creating Symbolic Links With ln
ln –s TARGET NAME
-s signifies a symbolic link
Without the –s, a hard link is created
Difference between a hard link and a symbolic link?
A hard link’s target must exist
A symbolic link can point to nothing (broken link)

14. Overview Of Permissions
-rwxrwxrwx -
rwx
rwx
rwx
File Type
Owner
Group
World

15. Owner, Group, The World Owner Group The world The creator of the file
A set of users grouped together
The world
Every other account not in the group

16. Read, Write, Execute Read Write Execute
File can be read, but not modified
Write
Permission is granted to modify the file
Execute
Run directly as if the file is a program
All programs should be executable (/bin)

17. What Permissions Mean On A Directory
Read
Users can get a listing of that directory
Write
Users can create and remove files in that directory
Execute
Users can examine files in that directory

18. Changing Permissions With chmod
chmod SETTINGS FILENAME
u = user
g = group
o = other
a = all
+ (add)
(remove)
= (set)
r = read
w = write
x = execute
chmod a-x testFile

19. More Examples Set read and write access for all?
chmod a=rw FILE
Add executable access for others?
chmod o+x FILE
Remove all access for owner?
chmod u-rwx FILE

20. Advanced chmod Usage Most Unix hackers don’t use this form
They prefer the more direct approach
Set permissions for owner, group, and others all with one number
Unfortunately, this approach requires a little bit of information

21. How Decimal Numbers Work
1234
1 * * * * 1
1 * * * * 10
10 distinct values
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Why is this system so comfortable? 3 2 1

22. Binary Numbers Only two distinct values
0 and 1
On and off
Binary numbers work the exact same way as decimal numbers

23. Conversion 1010 111 1 * 2 + 0 * 2 + 1 * 2 + 0 * 2 8 + 0 + 2 + 0 = 10
1 * * * 2
= 7 3 2 1 2 1

24. Powers Of Two x 2
Decimal Value 1 4 8 16 32 64 1 2 3 4 5 6

25. Octal Numbers Eight distinct numbers
0, 1, 2, 3, 4, 5, 6, 7
Binary numbers can be split into groups of three and represented by an octal number exactly

26. Conversion Table Binary Number Octal/Decimal Number 000 001 1 010 2
001 1
010 2
011 3
100 4
101 5
110 6
111 7

27. Relationship Between Binary Numbers And Permission
Each permission can be represented by a binary number
Each slot is either on or off
= 000
rwx = 111
rw- = 110
r-x = 101
Each group of three corresponds exactly to an octal number

28. Conversion Examples Read, write, and execute permission for everybody?
rwx rwx rwx = = 777
Read and execute permission for everyone?
r-x r-x r-x = = 555
Read permission for owner, write permission for group, and execute permission for everyone else?
r-- -w- --x = = 421

29. More Examples
All permission for owner, read and execute for everyone else?
rwx r-x r-x = = 755
Read, write for owner, and read only for everyone else?
rw- r-- r-- = = 644

30. Back To chmod
The advanced way to use chmod is to use octal numbers and set all permissions at once

31. chgrp And chown
chgrp will change the group setting of a file to another that you are a member of
You probably don’t belong to more than one group
chown will change the owner of a file
Only the owner may chown a file
Once chown’ed, the new owner is the only one who can grant permission back

32. Using umask To Set Default Permissions
Every time you create a file, the permissions are set to a default
umask will set this default
Unfortunately, it does it the exact opposite way than you would think

33. How umask Works
If you want default permissions to be 770, you set your umask with the opposite: 007
Permissions = 770 =
umask =
Unix tries to be helpful and clears the executable flags, though

34. Examples
Read, write, execute permission for owner, nothing for anyone else?
Permissions = 700 =
umask =
All permissions for owner and read, write for group?
Permissions = 760 =
umask =

35. More Examples Read and write permissions for everybody?
umask =
No permissions for anybody?
Permissions = 000 =
umask =

36. A Couple Simple Commands
echo
Simply prints out whatever it is given
Example:

37. Using echo To Make A Simple File

38. Timestamps touch command
Without any flags, it will make the modification time of that file the current time
Can be used to create a new file, but the file will be empty (0 byte size)

39. Final Notes For Today
We have covered up through chapter 5 in the book, so you will be expected to have read these chapters
Next time we will delve deeper into creating files and moving them around in the file system