Unix Linux Permissions

1  chmod - modify file access rights
2  chown - change file ownership
3  chgrp - change a file's group ownership
4  su - temporarily become the superuser
5  sudo - temporarily become the superuser
File Permissions
On a Linux system, each file and directory is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program).To see the permission settings for a file, we can use the ls command. As an example, we will look at the bash program which is located in the /bin directory:
[root@prakash]# ls -l /bin/bash
-rwxr-xr-x 1 root root  325213 Feb 23  2007 /bin/bash
Here we can see:
*  The file "/bin/bash" is owned by user "root"
*  The superuser has the right to read, write, and execute this file
*  The file is owned by the group "root"
*  Members of the group "root" can also read and execute this file
*   Everybody else can read and execute this file
In the diagram below, we see how the first portion of the listing is interpreted. It 
consists of a character indicating the file type, followed by three sets of three 
characters that convey the reading, writing and execution permission for the owner, group, and everybody else.

1 chmod  (modify file access rights)
The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify.There are two ways to specify the permissions. In this lesson we will focus on one of these, called the octal notation method. It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here's how it works:
 rwx rwx rwx = 111 111 111
rw- rw- rw- = 110 110 110
rwx --- --- = 111 000 000
and so on...
rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r-- = 100 in binary = 4
Now, if you represent each of the three sets of permissions (owner, group, and other) as a single digit, you have a pretty convenient way of expressing the possible permissions settings. For example, if we wanted to set some_file to have read and write permission for the owner, but wanted to keep the file private from others, we would:
  [root@prakash]# chmod 600 some_file
Here is a table of numbers that covers all the common settings. The ones beginning with "7" are used with programs (since they enable execution) and the rest are for other kinds of files.
Value         Meaning

777      (rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not  a desirable setting.                                    

Value         Meaning

755          (rwxr-xr-x) The file's owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users
Value         Meaning                

700          (rwx------) The file's owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.                                                                 

Value         Meaning

666         (rw-rw-rw-) All users may read and write the file.

Value         Meaning

644         (rw-r--r--) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.                                              

Value         Meaning

600          (rw-------) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.                     

Directory Permissions
The chmod command can also be used to control the access permissions for directories. Again, we can use the octal notation to set permissions, but the meaning of the r, w, and x attributes is different:
     r - Allows the contents of the directory to be listed if the x attribute is also set.
    w - Allows files within the directory to be created, deleted, or renamed if the x attribute is also  set.               
    x - Allows a directory to be entered (i.e. cd dir).
Here are some useful settings for directories:
Value            Meaning
  777               (rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new  files in the directory and delete files in the directory. Generally not a good setting.                                            
Value            Meaning
755                   (rwxr-xr-x) The directory owner has full access. All others may list the directory,  but  cannot create files nor delete them. This setting is common for directories that you wish to share with other users.                          
                                                    
Value            Meaning
700                 (rwx------) The directory owner has full access. Nobody else has any rights. This  setting is useful for directories that only the owner may use and must be kept  private from others.                          
                         
 2 chown   (Changing File Ownership)                     
You can change the owner of a file by using the chown command. Here's an example: Suppose I wanted to change the owner of some_file from "me" to "you". I could:
[root@prakash]$ su
Password:
[root@prakash]# chown you some_file
[root@prakash]# exit
[root@prakash]$
Notice that in order to change the owner of a file, you must be the superuser. To do this, our example employed the su command, then we executed chown, and finally we typed exit to return to our previous session chown works the same way on directories as it does on files.
3  chgrp  (Changing Group Ownership)
The group ownership of a file or directory may be changed with chgrp. This command is used like this:
[root@prakash]#  chgrp new_group some_file
In the example above, we changed the group ownership of some_file from its previous group to "new_group". You must be the owner of the file or directory to perform a chgrp. 
Becoming The Superuser For A Short While
4  su  (temporarily become the superuser)
It is often necessary to become the superuser to perform important system administration tasks, but as you have been warned, you should not stay logged in as the superuser. In most distributions, there is a program that can give you temporary access to the superuser's privileges. This program is called su (short for substitute user) and can be used in those cases when you need to be the superuser for a small number of tasks. To become the superuser, simply type the su command. You will be prompted for the superuser's password: 
[root@prakash]$ su
Password:
[root@prakash]#
After executing the su command, you have a new shell session as the superuser. To exit the superuser session, type exit and you will return to your previous session.
5  sudo  (temporarily become the superuser)
In some distributions, most notably Ubuntu, an alternate method is used. Rather than using su, these systems employ the sudo command instead. With sudo, one or more users are granted superuser privileges on an as needed basis. To execute a command as the superuser, the desired command is simply preceeded with the sudo command. After the command is entered, the user is prompted for the user's password rather than the superuser's:
[root@prakash]$ sudo some_command
Password:
[root@prakash]#

	Warning Beware 666 and 777
	Setting permissions to 666 or 777 will allow everyone to read and write to a file or directory. These permissions could allow tampering with sensitive files, so in general, it is not a good idea to use these settings.

Here is a list of some common settings, numerical values and their meanings:

• -rw------- (600) — Only the owner has read and write permissions.
• -rw-r--r-- (644) — Only the owner has read and write permissions; the group and others can read only.
• -rwx------ (700) — Only the owner has read, write and execute permissions.
• -rwxr-xr-x (755) — The owner has read, write and execute permissions; the group and others can only read and execute.
• -rwx--x--x (711) — The owner has read, write and execute permissions; the group and others can only execute.
• -rw-rw-rw- (666) — Everyone can read and write to the file. (Be careful with these permissions.)
• -rwxrwxrwx (777) — Everyone can read, write and execute. (Again, this permissions setting can be hazardous.)

Here are some common settings for directories:

• drwx------ (700) — Only the user can read, write in this directory.
• drwxr-xr-x (755) — Everyone can read the directory, but its contents can only be changed by the user.           

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