Unix Access Privileges

1. Unix Access Privileges Chapter 6.15-6.17

2. Access privileges • All UNIX files have privileges associated with them • These privileges determine who can access the file • These privileges determine how people can access the file

3. Viewing access privileges • Use the ls -l command drwx------ 2 jdoe 8192 Jul 12 12:26 nsmail/ -rw-r--r-- 1 jdoe 945 Mar 15 16:01 old.cshrc -rwxrwxrwx 1 jdoe 168 Jan 13 1998 file.exe -rw-rw-rw- 1 nobody 382 Nov 18 1998 old.profile -rw------- 1 jdoe 652 Jul 12 12:16 old.xsession drwx------ 2 jdoe 8192 Jun 23 13:21 thesis/ -rw-r--r-- 1 jdoe 1186776 Jul 13 15:07 win32tutorial.ps

4. Types of file access • Read • Allows viewing at the contents of the file • Allows listing the contents of a directory • Write • Allows changing or deleting the file • Allows creation of files within a directory • Execute • Allows execution of a program • Allows traversal of a directory • You can't access files or directories inside a directory without execute permissions on that directory

5. Categories of access rights • User/owner • The person who owns/created the file • Group • Each file and directory belongs to a group • Each user is a member of 1 or more groups • Others • Everyone else that has access to the computer

6. To change permissions • chmod • changes the access mode of a file • Two methods exist • symbolic • absolute

7. chmod - symbolic • chmod requires • which permissions to add or subtract • the category of user to change • chmod [who] [operation] [permission] [file]

8. chmod - symbolic • chmod [who] [operation] [permission] [file] • Who • u = user/owner • g = group • o = others/world • a = all • Note, these are all lower-case letters

9. chmod - symbolic • chmod [who] [operation] [permission] [file] • Operation + add permission - remove permission = set permission

10. chmod - symbolic • chmod [who] [operation] [permission] [file] • Permission r = read permission w = write permission x = execute permission • Note these are lower case letters

11. chmod - examples • chmod o-w temp • chmod u-r bar • chmod g+rw home.html • chmod a=rw myFile • chmod a+x homework

12. chmod - absolute • Absolute • specify a numeric equivalent for a set of permissions • You specify all permissions at once • This is not for the faint of heart

13. chmod - absolute • chmod [xxx] [file] • Where each x is some number from 0 - 7 • Each number specifies a level of privileges for a specific group

14. chmod - absolute • e.g., chmod 644 moocow.txt User permission World permission Group permission

15. chmod - absolute • Permissions: • Read = 4 • Write = 2 • Execute = 1 • Set permissions by adding the values of all the permissions you wish to set

16. chmod - examples • To give yourself read & write permission and no permission to anyone else: • chmod 600 foobar.txt • To give yourself read & write permission and everyone else read permission only: • chmod 644 index.html • To give yourself full access to a directory, and everyone else read & execute permission only: • chmod 755 homeDirectory