Unix Access Privileges

1
Unix Access Privileges

• Chapter 5.1-5.2

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 1226
nsmail/ -rw-r--r-- 1 jdoe 945 Mar 15 1601
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
1216 old.xsession drwx------ 2 jdoe 8192 Jun
23 1321 thesis/ -rw-r--r-- 1 jdoe 1186776 Jul
13 1507 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
• All
• The set of all users, including the User, Group
  members, and Others

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 grw home.html
• chmod arw myFile
• chmod ax 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

17
chgrp

• Change the group associated with a file or
  directory
• Each user can belong to several groups
• The system administrator handles group
  permissions
• The owner of a file or direcotry can set its
  group to any group to which the owner belongs
• A file or directory can be associated with only
  one group
• Example
• chgrp cs140 myDirectory
• chgrp is not covered in our text
• How can you find out more about chgrp?

18
Viewing group association

• Use the ls -lg command

drwx------ 2 jdoe cs140 8192 Jul 12 1226
nsmail/ -rw-r--r-- 1 jdoe cs140 945 Mar 15
1601 old.cshrc -rwxrwxrwx 1 jdoe st 168 Jan
13 1998 file.exe -rw-rw-rw- 1 nobody
wheel 382 Nov 18 1998 old.profile -rw-------
1 jdoe st 652 Jul 12 1216 old.xsession drwx----
-- 2 jdoe cs140 8192 Jun 23 1321
thesis/ -rw-r--r-- 1 jdoe csclub 1186776 Jul
13 1507 win32tut.ps